Fixed three-dimensional holographic images

Clint Wood, Gregory J . Salamo, John Goff, Gary L. Wood, Richard J . Anderson, and David J. McGee

Three-dimensional holograms were recorded in a cerium-doped, strontium barium niobate (SBN:751 photorefractive crystal. These holograms are shown to not degrade after more than one week of con- tinuous readout and to reconstruct reproductions of the original object with a n observable field of view of approximately 35". O 2002 Optical Society of America

OCZS codes: 090.0090, 100.0100, 190.0190.

1. Introduction

In the current era of high-speed computers, the need for an ultrahigh-speed, large-capacity data storage system becomes greater than ever. Holographic storage in general and photorefractive crystals in particular offer the potential to meet this need. Pho- torefractive crystals have the unique property of al- lowing optical interference patterns to redistribute charge among traps, which then generate local space- charge fields that replicate the original optical inter- ference patterns.'-4 The resulting space-charge field induces an index of refraction grating that can be used to reconstruct a holographic image of the original object. Theoretical predictions indicate that a crystal's storage capacity is of the order of -V/h3 bits, where V is the volume of the crystal and A is the wavelength of light.5 For a typical crystal volume of 1 cm3 and green light, this prediction is of the order of eight trillion bits or approximately 4 orders-of-magnitude times larger than the storage capacity of a typical compact disk.2 Not only does this technology promise large storage capacity, recent results demonstrate that a randomly chosen data el- ement could be accessed in approximately 100 ys, a figure expected to decrease to around 10 ys in the

C. Wood and G. J . Salamo (salamo@eomp.uark.edu) are with the Department of Physics, University of Arkansas, Fayetteville, Ar- kansas 72701. J. Goff and G. L. Wood are with the U.S. Army Research Laboratory, 2800 Powder Mill Road, Adelphi, Maiyland 20783-1197. R. d. Anderson is with the National Science Foun- dation, 4201 Wilson Boulevard, Arlington, Virginia 22230. D. J . McGee is with the Department of Physics, Drew University, Mad- ison, New Jersey 07940.

Received 17 October 2001; revised manuscript received 22 July 2002.

0003-6935/02/326796-06$15.00/0 O 2002 Optical Society of America

foreseeable future.6 That figure is also several orders-of-magnitude faster than conventional mag- netic disk drives. These two attributes of hoto ore- fractive crystals offer substantial advantages over conventional devices and make photorefractive holo- graphic storage an attractive alternative for next- generation computing.

In addition to computing applications, photorefrac- tive crystals also offer the possibility of storing a library of three-dimensional (3-D) holographic im- ages in a relatively small volume.7-14 Of the many 3-D image display techniques, holography provides one of the most aesthetically pleasing (possibly be- cause of the different ~ers~ect ives observed as the viewer moves from sid;! to side) 3-D images for the human eye-brain system. As a result, applications of holography to cinematography, , artificial intelli- gence, and security have long been the pursuit of scientists and engineers. Recently, Ketchel et ~1.15 reported storing 3-D holograms in cerium-doped, strontium barium niobate (SBN:60). The ex~eri- ment demonstrated that high-resolution 3-D oGects could be stored and retrieved in real time, meaning that no processing is required to view the image. The stored 3-D image was characterized by a large depth of field, high resolution, and a wide field of view of -35". To take advantage of the large storage ca- pacity of these crystals, Ketchel et a1.'5 used angle multiplexing of images to store several 3-D color ho- lograms in one crystal. In this technique one image is stored, and the crystal is rotated by an angle of 0.082" to allow another distinct image to be written. By use of angle, wavelength, and other multiplexing techniques, many 3-D images can be written into a single crystal. This ability to store large numbers of high-quality 3-D images allows one to imagine a wide area of applications for photorefractive holography

6796 APPLIED OPTICS / Vol. 41, No. 32 / 10 November 2002

Three-dimensional color holographic display

Brian P. Ketchel, Christy A. Heid, Gary L. Wood, Mary J. Miller, Andrew G. Mott, Richard J. Anderson, and Gregory J. Salamo

Three-dimensional (3D) color holograms are recorded in a cerium-doped, strontium barium niobate (SBN:60) photorefractive crystal. These holograms are shown to reconstruct true color reproductions of the original object with an observable field of view of 37". Angle multiplexing of two or more 3D color holograms is also demonstrated with angle tuning of the reference beam corresponding to a separation angle between stored images of 0.082". Each of these results is compared with corresponding theoretical predictions. O 1999 Optical Society of America

OCIS codes: 090.2870, 100.6890, 190.5330, 090.4220.

1. Introduction

The possibility of storing a three-dimensional (3D) image library holographically in a small crystal has long attracted the attention of researchers. This at- traction is driven by the fact that, among the many 3D image display techniques, holography provides the most pleasing 3D images for the human eye- brain system. Applications of holography to cinema- tography, artificial intelligence, and security have long been a goal of scientists and engineers. Al- though progress toward this goal has been slow, re- cent experiments have clearly demonstrated the potential of photorefractive crystals for real-time storage and retrieval of 3D images.1 Moreover, the potential for storage and reconstruction of 3D images has been demonstrated with freedom from distor- t i~n,~>%igh resolution,4 large depth of field,"7 and wide field of view (FOV).8 In this paper we report the demonstration of the corresponding storage and retrieval of multiple 3D color images. These images are shown to have a wide FOV as demonstrated by movement of one's head back and forth during view- ing of the hologram or by use of an imaging lens with a color CCD camera to record the 3D image from

B. P. Ketchel, C. A. Heid, G. L. Wood, M. J. Miller ~mn~iller@arl.mil), and A. G. Mott are with the U.S. Army Research Laboratory, 2800 Powder Mill Road, Adelphi, Maryland 20783- 1197. R. J. Anderson is with the National Science Foundation, 4201 Wilson Boulevard, Arlington, Virginia 22230. G. J. Salamo is with the Department of Physics, University of Arkansas, Fay- etteville, Arkansas 72701.

Received 23 February 1999; revised manuscript received 18 June 1999.

0003-6935/99/296159-08$15.00/0 O 1999 Optical Society of America

different perspective views. The observed FOV, ac- curacy of color, resolution of multiplexed images, and storage time are found to be in excellent agreement with theory.

Our approach to color 3D holographic storage is based on four-wave mixing in photorefractive crys- tals.9 The term photorefractive is used to describe a special kind of optically induced refractive-index change that can occur in electro-optic materials. The microscopic details of the photorefractive mech- anism are normally described by use of a band trans- port mode1,lo-12which assumes the existence of a pool of charges residing in low-lying traps. When a spa- tially varying intensity pattern is produced at a pho- torefractive medium, photoexcitation of the trapped charges occurs at the maxima of the spatially varying intensity pattern. The photoexcited charges mi- grate by drift or diffusion out of the illuminated re- gions and are eventually retrapped in the dark regions of the crystal. The charge transport then results in a spatially varying charge distribution that is balanced by a strong space-charge field according to Poisson's equation. This strong electrostatic field (E, lo4 V/cm) then produces a change in the re- fractive index (An = 0.0001) through the electro-optic effect, and a phase hologram is written.

In the case of holography the spatially varying in- tensity pattern is produced when a reference beam, Ere, is interfered with light scattered off of the object, Eobj, which itself may be thought of as a summation of plane waves. As long as these plane waves and the reference beam are mutually coherent and the photorefractive storage crystal has sufficient photore- fractive response at the laser wavelength along with low dark current, interference of Ere, and Eobj in the crystal will yield a refractive-index grating propor-

10 October 1999 / Vol. 38, No. 29 / APPLIED OPTICS 6159

Three-dimensional image reconstruction using strontium barium niobate Brian P. etche el^) and Gary L. Wood US. Army Research Laboratory, AATTN: AMSRL-SE-EO, Adelphi, Maryland 20783-1197

Richard J. Anderson National Science Foundation, Arlington, Virgittia 22230

Gregory J. Salamo Department of Physics, Uttiversity of Arkansas, Fayetteville, Arkansas 72701

(Received 27 December 1996; accepted for publication 29 April 1997)

A definitive demonstration of the use of a photorefractive crystal to project a three-dimensional image in space is reported on. The image is bright and different perspective views of the object appear as the viewing direction is changed. Q 1997 American Institute of Physics. [SOOO3-G95 1 (97)00427-01

Although four-wave mixing in photorefractive crystals has been used extensively to store and project two- dimensional holographic images, we report a definitive dem- onstration of true three dimensional (3D) image reconstruc- tion using a photorefractive crystal. The use of an inorganic photorefractive crystal as a storage medium allows:

(1) simultaneous recording and read out of three dimen- sional (3D) holograms possessing easily observable par- allax;

(2) the entire holographic process to occur at low light levels (e.g., milliwatt levels for the object beam);

(3) the entire process to occur without processinglfixing the material; and

(4) thousands of holograms to be stored in a relatively small crystal volume via wavelength andlor angle multiplex- ing.

Our 3D imaging technique employs a Ce-doped, stron- tium barium niobate crystal [(SBN):60 20X 20X 1.3 mm] as the storage medium. Figure 1 is a schematic diagram of the experimental setup used to record and project 3D images. The summation of light beams scattered off of the object, E l , and the reference beam, E,, write a hologram in the form of transmission gratings in the photorefractive crystal. The read beam, E,, counterpropagating to E,, is produced by phase conjugation of E , by a 13.5 X 12.2X 6 rnm, Ce- doped, SBN:60 photorefractive crystal acting as a double phase conjugate mirror (DPCM). The counterpropagating beam diffracts to form beam Ed that recreates the recorded 3D image at a distance from the crystal equal to that between the object and crystal (e.g., -40-80 mm). A plate beam splitter, placed between the object and crystal, is used to view the real 3D image, produced by E d . Viewing is accom- plished by:

(1) using the eye, just as one views a conventional holo- gram ;

(2) projecting the image onto a screen or into a scattering cell; or

(3) using an imaging lens in conjunction with a charge- coupled device (CCD) or video camera to magnify and

record the 3D image. In the latter case, different perspec- tives are observed by placing the camera and imaging lens on a goniometer that is rotated about a fixed point (e.g., the location of the 3 0 image).

Since the recording medium is a photorefractive crystal, unlike conventional holography where photographic film is employed, the hologram can occur in real time with continu- ous recording and display. The geometry employed in the current experiment is readily recognized as being typical of degenerate four-wave mixing, a technique which has been compared in the literature to conventional holography by Pepper and ~ a r i v . ' One major difference, however, is the use of a DPCM to provide the read beam, E, . The use of the DPCM has three well-documented advantages over other methods:

(1) distortion introduced by inhomogeneities in the photore- fractive crystal are eliminated;,.'

(2) high resolution holograms are possible;4 and (3) as reported here, the 3D image can be observed over a

large perspective range.

One can readily examine the impact of the DPCM on the perspective range viewed by replacing it with a plane mirror (PM) or a separate counterpropagating beam. Analysis shows that, when this substitution is made, the ability of the nonphase conjugate counterpropagating beam to fulfill the Bragg match conditions over the crystal area is extremely

Ar hrer beam SBN:Ce i - 488 nm P m 100 rnW

al Image plan. DPCM

To CCD, video cemera, or eye

a'~lectronic mail: bketchel@arl.mil FIG. I . Schematic diagram of experimental apparatus

Appl. Phys. Lett. 71 (I), 7 July 1997 0003-6951/97/71(1)/7/3/$10.00 O 1997 American Institute of Physics 7 Downloaded~l3~Ma~2008fo~l30.184.237.6.~Redistribution~su bject-rto-rAIP~license~o~copy right;iseeihttp:Ilapl.aip.orglapI/copyright.jsp

Image transfer by mutually pumped phase conjugators

Richard J. Anderson, Edward J. Sharp, Gary L. Wood, and Gregory J. Salamo

Cross talk is observed during the transient time of the photorefractive grating formation in a mutually pumped phase conjugator. We show that this feature can be used to transfer pictorial information from one location to another. The transfer is instantaneous and is demonstrated at a resolution of 6 lineslmm. 0 1996 Optical Society ofAmerica

1. Introduction

Multiwave mixing in photorefractive crystals has been used for a number of applications in optical image processing. The photorefractive properties of energy exchange and phase conjugation have led to demonstrations of image amplification, edge en- hancement, addition and subtraction of images, res- toration of aberrated images, correlation and convo- lution of images, conversion of incoherent images into coherent images, and image transfer from one location to another.' Sending spatial information from one location to another can be accomplished by many different techniques. One technique is to use photorefractive crystals as a medium for the transfer of images between two laser beams or between an incoherent source and a laser beam."I For ex- ample, in the case of the photorefractive incoherent- to-coherent converter described in Ref. 6, which is based on a self-pumped phase conjugator, the image transfer was demonstrated for both low-power cw- laser beams and for white light. This particular device exhibited a spatial resolution of -40 line pairs/mm, and the typical time to transfer a single, two-dimensional image was 140 ms a t 1 W/cm2. This slow response time is typical for photorefractive materials and corresponds to the time to write or erase a photorefractive grating.

R. J. Anderson is with the National Science Foundation, Wash- ington, D.C. 20550. E. J. Sharp and G. L. Wood is with the U.S. Army Research Laboratory, Fort Belvoir, Virginia 22060-5838. G. J. Salamo is with the Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701.

Received 21 February 1995; revised manuscript received 26 September 1995. 0003-6935/96/050854-06$06.00/0 o 1996 Optical Society of America

While the concept of image transfer between light sources by use of photorefractive crystals can be used to transfer information from one location to another, it is both alignment sensitive and slow. In this paper we present a new technique to accomplish this transfer that is based on a mutually pumped phase conjugator (MPPC) that is self-aligning and instanta- neous. Previous studiess have shown that temporal information at one location could instantaneously be transferred to another location by the MPPC. In this paper we show, for the first time to our knowl- edge, that pictorial information can also be trans- ferred. In addition we characterize the quality of the image transfer.

2. Double Phase Conjugation

Anew type of phase conjugator, unique to photorefrac- tive crystals, called the mutually pumped phase conjugator (MPPC), has been demonstrated in a variety of g e o m e t r i e ~ . ~ ~ ~ In these devices, two beams are incident (usually on opposite faces of the crystal) and overlap in some region of the crystal. The beams may be derived from different lasers as long as the laser wavelength is nominally the same (i.e., two He-Ne lasers, for example). These conjugators can be classified by the number of internal reflec- tions the beams experience before conjugation: none,sJOJ1 one,12 two,13 or three.14 There have also been MPPC's demonstrated that are based on the semilinear mirror desien.15J6 In all these devices. ', two phase-conjugate outputs (double phase conjuga- tion) are produced simultaneously by the interaction of the two mutually incoherent beams of the same wavelength within the photorefractive crystal. Since MPPC's differ in geometry rather than in their physical mechanism, they have been shown to share many common characteristics: (1) the conjugation of two beams occur simultaneously, (2) the conjugat-

854 APPLIED OPTICS / Vol. 35. No. 5 / 10 February 1996

986 OPTICS LETTERS / Vol. 18, No. 12 / June 15, 1993

Mutually pumped phase conjugator as a moving-object correlator

Richard J. Anderson

National Science Foundation, Washington, D.C. 20550

Edward J. Sharp, Gary L. Wood, William W. Clark III, and Quochien Vuong

U.S. Army Research Labomtory, Fort Belvoir, Virginia 22060-5028

Gregory J. Salamo

Department of Physics, University of Arkansas, Fayeiteville, Arkansas 72701

Ratnakar R. Neurgaonkar

Rockwell International Science Center, Thousand Oaks, California 91360

Received November 23, 1992

Cross talk is observed in a photorefractive bridge mutually pumped phase conjugator during the transient time of photorefractive grating formation and is utilized to construct a moving-object correlator. The correlation of various input images has been demonstrated and compared with calculated results. The device is currently capable of a resolution of approximately 4 to 6 lines/mm.

Photorefractive crystals have been used for a number of optical processing applications arising from multi- wave mixing.' For example, with images spatially impressed on one or more of the beams in four- wave-mixing geometries, correlation/convolution op- erations have been demon~trated.~-~ Recently a new class of phase conjugator, called the mutually pumped phase conjugator (MPPC), has been demonstrated in a variety of materials and geometries. These con- jugators can be classified by the number of internal reflections the beams experience before conjugation; zero:-? one,8 two: or three.1° In these devices, two phase-conjugate outputs (double phase conjugation) are produced simultaneously by the interaction of two mutually incoherent beams of the same wavelength within the photorefractive crystal.

In this Letter we describe how the occurrence of cross talk in MPPC's can be used to demonstrate a correlator. Whereas there are similarities between the device discussed here and the concept of a photorefractive novelty filter," the distinction is that the MPPC device not only detects moving objects but identifies them as well. In addition, unlike other four-wave-mixing correlators, this device selectively identifies only moving objects and ignores stationary objects.

The no-cross-talk criterionu established for MPPCJs refers to steady-state conjugate signals (a time that is long compared with the photorefractive response time of the particular crystal being used). For example, suppose that the double phase-conjugate mirror is formed by a plane wave and an image-bearing beam. After steady state is reached there is no observed cross talk between the two beams, i.e., there is no evidence of an image present in the phase-conjugate

signal of the plane wave. If, however, while the MPPC is operating in the steady state, the ampli- tudes of the input beams are spatially modulated in a time that is short compared with the photorefractive response time, then cross talk between the input beams is observed.13 After a time corresponding to the photorefractive response time of the crystal, the cross talk is observed to disappear. If the object is suddenly moved, however, the image is again instan- taneously present on the phase-coqiugate signal of the plane-wave beam.

To understand the correlation process in the bridge MPPC consider the diagram shown in Fig. 1, which schematically shows the bridge MPPC operating in the steady state. The two input beams are of

Fig. 1. Optical correlator that uses the bridge MPPC: L1, Lz, Fourier-transform lenses; S's, shutters; CP1, CP2, correlation planes; ul, uz, objects; BS's, beam splitters; A, B, regions of dynamic holography. Re- gion A is expanded in the inset.

O 1993 Optical Society of America

Electron impact excitation of the 3 p ( S P ) state of atomic oxygen G. A. Germany, R. J. Anderson, and G. J. Salamo Uniwmity of Ankonsa& Fayetteville. A r k a m 72701

(Received 21 March 1988; accepted 9 May 1988)

The 157.6 nm output from a fluorine excimer laser is focused in a vacuum chamber containing 0, gas at a pressure of 20 mTorr. Laser photodissociation of the 0, target gas produces ~P*('P) and 2p4('D) oxygen atoms with unit quantum efficiency. A low-energy electron beam is crossed with the laser beam to produce 3p('P) excited states of 0 1 via electron impact excitation of the 2p4PP) ground state. Intensity measurements of the 777.4 nm radiation, corresponding to the 3p('P)-3s('S) transition, are used to calculate the excitation cross section of the 3p('P) state of atomic oxygen. Absolute optical cross sections are reported for a range of incident electron energies less than 18 eV.

INTRODUCTION

Because of its prominence as an excitation mechanism for the aeronomically important states of atomic oxygen (011, electron impact excitation has been of continuing in- terest to atmospheric researchers. The present experiment reflects this interest and the continuing need for additional electron excitation cross section data. It is based on the pre- vious work of Germany et al. ' in which (i) vacuum ultravio- let photodissociation of 0, was used to produce the O('D) state and (ii) time-resolved analysis of the resulting 2~*( 'D)-2p~(~P) transition at 630.0 nm yielded its colli- sional deactivation cross section. Here, the W V photodis- sociation technique is used to produce the 2p4('D) metasta- ble state as well as the ~ P * ( ~ P ) ground state which is subsequently excited by low-energy electron excitation to produce a 3p('P) atomic oxygen gas sample (Fig. 1).

The O( 'P) sample is monitored to determine the optical cross section for the emission of the 777.4 nm radiation cor- responding to the 3pOP)-3s('S) transition of atomic oxy- gen. In addition, the radiative decay of the metastable 0 ( ID) state is detected as a monitor of the atomic density of the O(3P) ground state target sample. Using this two-step photon-dissociation, electron-excitation technique, absolute values of the 777.4 nm optical cross sections for atomic oxy- gen are determined. Although Doering el 0 1 . ~ recently per- formed an energy-loss measurement of the 3p('P) excitation cross section for 30 eV incident electrons, the 777.4 nm opti- cal data reported here is believed to be the first measurement of the excitation function for this important transition.

TECHNIQUE Apparatus

The present apparatus (Fig. 2) u t i l i i a fluorine ex- cimer laser ( 157.6 nm; 2nd & 10 Hz) to excite the parent oxygen molecule, with unit quantum dfi~iency,~ to a d b ciative state ( B 'L; that results in the ground ~ P * ( ~ P ) and metastable 2p4('D) states of atomic oxygen. Because the photoabsorption process provides excess energy beyond that needed for the production of O( ID) and O('P), each of the products share 0.8 eV of kinetic energy4 (corresponding to a velocity of about 2x 10' d s ) . Because time-resolved ex-

perimental measurements indicated that significant field-of- view signal lossor occur in the present optical measurements )3 p after the exciting laser pulse, the experiment is per- formed within 1.5 p s after the incident laser pulse. There- fore, field-of-view losses are not significant.

Due to high molecular oxygen absorption of the laser WV radiation at atmospheric pressures, the laser output is coupled to the collision chamber via a flexible tube through which helium is flowed. In addition to the WV emissions, the laser also produces visible fluorescent emissions which are discriminated against by a series of optical stops, aper- tures, and light traps.

The experiment is housed in a stainless steel high-vacu- um collision chamber pumped by two liquid-nitrogen- trapped diffusion ppmps. Acalibrated leak valve is used to

FIG. 1. Atomic o x y p energy d i a m . Thm manifolds of stata arc fonncd from the p u n d md excited atate configurations. Eminsion cross mtiona for the 777.4 om mnsitim arc repow.

'161BgBM02.10 @ 1988 American Institute of Physlcs 1989

Downloaded 13 Mar 2008 to 130.104.237.6. Redistribution subject to AIP llcense or copyright; see http:iljcp.alp.orgljcplcopyright.jsp

BooI( reviews

607 University Physics (George B. Arfken, David F. Griffing, Donald C. -Kelly, and Joseph Priest), Richard J. Anderson and Susan McLaughlin

608 A Hundred Billion Stars (Mario Rigutti), Hzlzne R. Dickel

608 Energy: Insights from Physics (Philip DiLavore), Howard Hayden

608 Physics, 1st ed. (Edward R. McCliment), Walter C. Connolly

609 Astronomy: Fundamentals and Frontiers, 4th ed. (Robert Jastrow and Malcolm H. Thompson), Richard Berendzen

610 The Invisible World of the Infrared (Jack R. White), Ted Bredderman

610 Engineers and Electrons (John D. Ryder and Donald G. Fink), Alfred Romer

A STUDENT'S VIEWPOINT

University Physics. George B. Arfken, David F. Griffing, Donald C. Kelly, and Joseph Priest. Academic Press, Orlando, FL, 1984. 905 pp. $31.00.

Since their subject matter and format are dictated by convention and the competition, calculus-based intro- ductory physics texts can be viewed as among the "generic foodstuffs" of the textbook industry. University Physics falls easily into this category. Thirty- nine of its forty-three chapters cover topics which normally comprise a two- semester introductory course, e.g., Newtonian mechanics, electricity and magnetism, optics, etc. The remaining four chapters are devoted to brief descriptions of special relativity, rela- tivistic mechanics, topics in quantum physics, and nuclear structure/technol- ogy. In accordance with current pub- lishing practices a student study guide, a solutions manual, overhead trans- parencies, and a limited amount of CAI software also accompany the text. Since most competing texts follow similar formats and offer com- parable supplementary materials, the author's claim of "sensitivity to stu- dents" becomes an important dis- tinguishing factor. To test this claim I enlisted the aid o f Susan McLaughlin, a sophomore physics major and future

high school physics teacher. Her impressions of Univer~ity Physics are given below.

"The text has several helpful characteristics which provide ease in reading. At the beginning of each chapter is a table of contents listing the sections of the chapter, therefore the student knows exactly what will be covered in the chapter and the number of sections. In addition, the first item in each chapter is a preview, which gives a short explanation of the material covered in the chapter, and the last is a summary which again gives an explanation of the material, as well as equations with which the reader should then be familiar. There- fore, this book follows a very impor- tant speech technique which facilitates remembering: Tell them what you're going to say, say it, and tell them what you've said. Furthermore, the two- column style of print seems to be faster-reading than other styles, and the numerous pictures and examples also seem to make the complex ideas easier to understand. One of the most helpful characteristics of the text is that it ties each consecutive equation to the one before it. While working the end-ofchapter problems, I noted three other good points. First, there

is more than one problem of each type. This provides practice, helping the student become more familiar with the equations used and the processes of working each problem. Second, the problems themselves seem to parallel the text. This eliminates the frustra- tion a student feels when a problem does not seem to relate to the chapter. Third, the difficulty of the problems is directly related to the amount of information given to solve them.

"The text, however, also has some undesirable qualities. For example, the placement of the pictures, which have explanations with them, is often in the middle of a sentence of the text. This is confusing to the reader, making it rather difficult to read continuously. Another charac- teristic which does not facilitate .easy reading is the size of the print, which is fairly small and becomes worrisome when many of the symbols contain subscripts.

"Ail in all, I would say that this book is well written, clear, easy to understand, and has problems which can be worked by the average student."

Richard J. Anderson and Susan McLaughlin, University of Arkansas, Fayetteville, Arkansas 72701

T H E PHYSICS TEACHER DECEMBER 1984 607

problem the author is to be commend- ed for trying to weave astronomy into some semblance of order. As any teach- er of the subject knows, the material is so interrelated that almost any approach demands bringing in ideas from many different areas at once.

Henry Albers, Vassar College, Pough- keepsie, New York 12601

THE SET IS COMPLETE

Physics for Everyone: Book 3 - Elec- trons, Book 4 - Photons and Nuclei, A. I. Kitaigorodsky. MIR Publishers, Moscow, U.S.S.R., 1981. Book 3 - 248 pp., Book 4 - 235 pp. S6.60each.

Electrons and Photons and Nuclei represent the concluding works in the Physics for Everyone series. The first two books Physical Bodies and Molecules, published in 1980 were pre- viously reviewed [Phys. Teach. 19, 138 (198111 and were found by this author to be appropriate as a supplement to a standard text or as a physics primer for teachers and students. Although the same may be safely said about the re- maining two volumes of the series, it might also be interesting to try using the entire series as the text for a one- semester introductory course. This course could provide students with a valuable overview of the subject before they embark on their venture through the usual encyclopedic version of calculus-based introductory physics.

The Physics for Everyone series presents the basic concepts of physics via illustrations of the physical laws. The facts derived from the laws are then skillfully woven together to pro- vide an integrated discussion of the subject. The material is presented with a minimum of mathematical clutter and with the author's strong personal perspective of physics clearly evident.

The contents of Electrons includes sections on electric fields and poten- tials, the electrical structure of matter, and a "summary of electrical engineer- ing" (i.e., ac circuits). Also included is a discussion of electromagnetic fields; their production, propagation and application to communications (i.e., "radio," etc.). The latter section includes an interesting aside describing the accomplishments of A.S. Popov, the "unpretentious" Russian scientist, who in the spring of 1895 invented the radio and who on March 12, 1896 trans- mitted the first radiogram over a dis-

tance of 250 m. We are told by Kitaigo- rodsky that Popov's contributions to the field of communications were later overshadowed by the Italian inventor, engineer and entrepenebr G. Marconi because the Russian scientist "refused to put his knowledge and research at the disposal of any other country except his native land." Observations such as this, which appear frequently throughout the material, serve to remind the reader of the author's strong personal involvement with phys- ics and its history.

The topics covered in the final volume of the series, Photons and Nuclei, include the usual topics such as electromagnetic radiation, optical instruments (from the prism to laser devices), hard electromagnetic radia- tion (e.g., x rays), relativistic and wave mechanics and the structure of atomic nuclei. In addition, there are discus- sions of energy sources such as electric and nuclear power plants, thermonucle- ar energy and solarlwind energy. Book four of the series ends with a discussion of the physics of the universe, including brief introductions to cosmology, the general theory of relativity, radio as- tronomy and cosmic rays.

The fact that the Physics for Everyone series is not a formal textbook written for a specific audience may ad- versely affect its acceptance by the physics teaching community. This would be unfortunate, however, be- cause it could be used by both teachers and students to provide a coherent overview of the subject. It should find a place in a school library or a personal book collection as a valuable supple- ment to the usual collection of intro- ductory textbooks.

Richard J. Anderson, Universiv of Ar- kansas, Fayetteville, Arkansas 72701

40 READ Review(1) 50 IF NOT Review(1) THEN 40 60 CONTINUE Physics Problems for Programmable Calculators: Wave Motion, Optics and Modern Physics, J. Richard Christman. Wiley, New York, 1982. 309 pp. $7.95, paper.

This book is the sequel to Physics Problems for Programmable Calcula- tors: Mechanics and Electromagnetism by the same author. The same com- ments made in an earlier review of that text [Phys. Teach. 19, 504 (1981)l with regard to level, style, organization and

usefulness apply equally well to this book and will not be restated.

From the topical coverage men- tioned in the title, one would expect that roughly one-third of the book would be devoted to each topic. In- stead, one finds over one-half of the content and programs devoted solely to the Modern Physics material. Many readers will find no problem with the author's material mix. I would rather have had more coverage of wave phe- nomena, perhaps including damping and energy-loss analysis, loaded strings, coupled strings with differing linear mass densities, etc.

One of the chapters which will probably generate high student interest is the chapter dealing with matrices and their application to lens systems. Subse- quent use of matrix techniques in the relativistic kinematics and dynamics treatments serves to reinforce the stu- dents' confidence in this mathematical tool.

The author has once again chosen problems where the students gain insight into the meaning of the physical laws by graphing numerical so- lutions or by studying the effects of parameter variations on the numerical solutions. In developing concepts, a few more figures would have been useful to helpclarify the notation.

Together, the two volumes make useful supplements to any calculus-based introductory physics course. Even if the instructor does not spend class time discussing computers or programmable calculators, many of the students will spend time doing the problems if they are made aware of the availability of these books.

Donald F. Kirwan, Department of Physics, University of Rhode Island, Kingston, Rhode Island 02881

PHYSICS FOR THE SOCIALLY AWARE

Physics and Human Affairs. Art Hobson. Wiley, New York, 1982. 418 pp. $19.95.

Much of the general public is scientifically illiterate. Science- and-Society courses may alleviate this problem. Art Hobson's text, Physics and Human Affairs,very nicely presents physics together with its social context, and thereby may attract previously rebellious and uninterested students into an acquaintance with science.

This textbook completely inte-

134 T H E PHYSICS T E A C H E R F E B R U A R Y 1983

Production of O(1D) and O(3P) by vacuum ultraviolet photodissociation of molecular oxygen

G. A. Germany, G. J. Salamo, and R. J. Anderson Department of Physics, University of Arkansas, Fayetteuille, Arkansas 72701

(Received 3 1 January 1983; accepted for publication 29 April 1983)

A pulsed fluorine excimer laser operating at 1576 A wavelength, 10 ns pulse width, 10 Hz repetition rate, and -0.2 MW average output power is used to photodissociate ground state molecular oxygen, O,(X ,Z[ ). The dissociation products, O( 'D) and O(,P), are produced with unit quantum efficiency and -0.4 eV translation energy. Time-resolved techniques are used to monitor the O('D,)-O('P2) radiative transition over the 0, pressure range 1C400 mTorr. The O( lD 14, quenching rate constant isdetermined to be (3.7 + 0.3) X lo-" cm3 s- I. The O( 'D )/0, dissociation fraction is estimated to be 0.4% to - 50% corresponding to O['D ) densities as high as 3 x lOI5 cm-'.

PACS numbers: 79.20.Ds, 33.20.Ni

I. INTRODUCTION laser photolysis of a gas such as 0, through the process

Because of its 1.97 eV of internal energy and 147 s radia- tive lifetime, the O( 'D j metastable state of atomic oxygen plays an important role in the free radical chemistry of the atmosphere. Therefore, much effort has been devoted to lab- oratory analysis of O( 'D ) gas kinetic' and photochemical re- action rates2 and to the roleof theO( 'D) state in intermediate reactions3 of atmospheric interest. For example, the auroral and airglow characteristics ascribed to the 0 2 ( b '2: -Xi - ) emissions appear to have their origin in the energy transfer process O( 'D ) + 02(X3Z; )-02(b lZ,+ )? Similarly the O('P) ground state atom also has received considerable at- tention in the analysis of the photochemistry of the terrestri- al a t m o ~ ~ h e r e . ~ Its production by solar ultraviolet photoly- sis of O,, 0 , , NO,, etc., and its role in subsequent gas kinetic collisional processes also make it an important factor in at- mospheric models. Thus, laboratory studies of reaction rates for three body collision processes such as O('P] + 0, + M-0, + M have received considerable attention from

atmospheric scientists.' In addition, the possibility of the production of significant densities of electronically excited oxygen atoms by charged particle impact or optical pumping of metastable or ground state atoms introduce yet another factor into the analysis of observed atmospheric emissions.'

Unambiguous laboratory analyses of these and other collisional excitation processes require the use of experimen- tal techniques capable of producing a high density, well characterized sample of atomic oxygen. The most popular of these have been dissociation by (i) a rf or microwave dis- charge,' (ii) an oven or f ~ r n a c e , ~ or (iii) photolysis of 0 , , NO,, etc.l0 Each, however, is not without experimental drawbacks. For example, although rf or microwave dis- charge sources have the capability of producing intense beams of atomic oxygen, their very nature makes the excited state distribution of these atomic samples ambiguous. An oven source, meanwhile, yields an atomic sample which is comprised of nearly 100% ground state atoms at a typical operating temperature of 2300 K. However, its density is a rather low - lo9 cmP3 and the accompanying background oven radiation is quite intense. On the other hand, although

0, + hv-O('D) + 02(a'A,) has been used to produce workable densities of metastable oxygen for quenching rate experiments, it requires the presence of a high pressure back- ground 0, carrier gas such as He or SF,.

This work describes a different approach to the prob- lem of atomic oxygen production. It reports the details asso- ciated with the production and measurement of a high den- sity sample of O(,P) and O( 'D) states produced by laser photodissociation of 0, in the absence of a high pressure buffer gas. Although our present research plan is to use this technique to provide target atoms in future low-energy elec- tron impact experiments, it is also suitable for use in a variety of other photochemical, quenching, or reaction rate experi- ments.

II. EXPERIMENT A. Technique

Our approach to the production of high density, well characterized samples of atomic oxygen is based upon the previous work of Lee and Slangerl and utilizes 1576 A laser radiation to photodissociate 0, via the process

where A€ = 0.8 eV is the kinetic energy shared by the disso- ciation fragments.I2 This process is characterized by several features which make it an especially attractive source of oxy- gen target atoms for excitation experiments involving charged particles or photons. Among these are that:

(i) every photon absorbed results in a photodissociation event,"

(ii) both O(,P) and O(lD ) are produced in equal quanti- ties and measurement of the O('D2)+0(3P,) radiative transi- tion at 6300 A can be used to directly monitor the densities of the atomic states (Fig. l) ,

(iii) only O('P), O('D ) and Oz(X3Z, )exist as initial ex- perimental conditions for subsequent excitation experi- ments,

(iv) given sufficient laser intensity (either through high laser power, focusing of the laser beam, or multiple laser

4573 J. Appl. Phys. 54 (8). August 1983 0021-89791831084573-05802.40 @ 1983 American Institute of Physics 4573

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Heavy-ion source using a laser-generated plasma transported through an axial magnetic field

L. G. Gray,") R. H. Hughes, and R. J. Anderson Physics Department. UniversityofArkansas, Fayetteuille. Arkansas 72701

(Received 26 April 1982; accepted for publication 14 June 1982)

Results of transporting a laser-generated plasma through magnetic fields are reported. Plasma plumes have been generated in strong magnetic fields, in directions both transverse and parallel to the field. Collective effects are demonstrated by the plasma while in the high-density state near the laser target. The formation of the plasma and its transport through an axial magnetic field enhances the relative amount of highest charge states and the lowest charge states. The focusing properties of the magnetic field near the extractor gap can prove useful in enhancing ion density a t the anode aperture of the extractor gap. It is suggested that the duty cycle of laser ion sources can be extended by simply increasing the ion flight time through the magnetic field from the laser target to the extractor gap without appreciable loss of ions. Further, it is suggested that energy spread in a given ion species can be made small by using an extractor potential programmed to increase in time relative to the laser fire time.

PACS numbers: 29.25.Cy, 52.50.Jm

I. INTRODUCTION

Experimental results were reported in a previous publi- cation' for extraction of heavy ions from a freely expanding plasma generated by 20-MW bursts of 1.06-p radiation from an active Q-switched Nd:Yag laser at a target power density near 10" W/cm2. Ions were extracted from the plasma by applying a 15-kV potential across the extractor gap. Poten- tially useful properties of the laser-generated plasma plume as a source of ions were pointed out. It seems appropriate to again enumerate these properties: ( 1) a copious supply of ions per laser pulse, (2) high states of ionization, (3) short plasma generation times, (4) highly directional plasma plumes which can be efficiently directed along a beam axis, (5) versatility in producing not only a multiplicity of charge states but also a variety of nuclear species, (6) a freeze in charge states with little recombination effects after the initial plasma genera- tion, (7) simplicity in plasma generation since the target a t high potential is optically connected to the laser at ground potential, and (8) the source contributes essentially no gas load to the vacuum system.

However, the most obvious drawback to the laser source is its duty cycle since the repetition rate of a high- power laser is not particularly high. Our laser, a Holobeam 5050Q, has a maximum repetition rate of 50 pps. The length of a typical magnetically separated ion pulse under the con- ditions of Ref. 1 (30-cm drift distance from the target to the extractor) was - 10 psec. This would produce a maximum duty cycle of only 0.05% with our laser. Somewhat higher repetition rates are now available in present day lasers, but it is clear that an important concept in making the laser ion source more useful is increasing the duty cycle by other means.

One possibility of increasing the duty cycle is the use of magnetic confinement of the ions. However, past attempts in this direction have not been Haught et al.' demonstrated some containment of a LiH plasma in a simple

"Present address: Physics Department, Rice University, Houston, Texas.

magnetic bottle where the plasma generated by a laser fo- cused on a pellet target, situated near the center of the bottle, was allowed to expand in the presence of a magnetic field. The experimental evidence bore out the theoretical predic- tion that the plasma expansion transverse to the field could be slowed and even stopped, the lost kinetic energy going into heating the plasma, resulting in a redirected expansion along the magnetic axis. The ions then escape through the bottle necks first, Li"+ followed by the other ions with H + the last to leave. Escape times varied from a few micro- seconds to about 150psec for the H + component. Sucov er al.' performed a similar experiment with aluminum pellets. However, they concluded that there was no evidence of any appreciable slowing of the motion transverse to the field. Their explanation was that the transverse motion separates the charge producing an electric field across the plasma which in turn leads to an E X B drift across the field lines. They further concluded that at most 5% of the plasma was trapped for a very few tens of microseconds. This motion transverse to the field is in qualitative agreement with the experimental results reported by Bruneteau et

II. APPARATUS Several modifications were made on the basic apparatus

described in Ref. 1. The magnetic separation of charge states used the same basic technique. The ions are accelerated across an extraction gap giving the ions a known velocity large compared with the initial plasma velocity. The anode aperture diameter was increased from 1/4" to 1/2" while the extraction gap was also increased from 1/4" to 1/2". The laser target to extractor distance was increased 30 cm to between 60 and 70 cm. This increased the ion drift time and, hence, the temporal ion pulse length.

Figure 1 is a schematic diagram of the experimental apparatus. The laser, laser target assembly, and Einzel lens, are all similar to those used in Ref. 1 . Figure 1 shows a large Faraday cup that measures the ion current that comes di- rectly out of the Einzel lens.

6628 J. Appl. Phys 53(10), October 1982 0021 -8979/82/106628-06$02 40 @ 1982 American Institute of Phys~cs 6628

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Day e t al. Vol. 71, No. 71July 1981lJ. Opt. Soc. Am. 851

Radiative-lifetime measurements of the 4p 5P, 4p 3P, and 4d multiplets of O I

R. L. Day, R. J. Anderson, and G . J. SaIamo

Department of Physics, University of Arkansas, Fayetteville. Arkansos 72701

Received September 21,1980: revised manuscript received January 22,1981

Time-resolved spectroscopy is used to observe the 4p 5P -' 3s 5 S 0 , 4p 3P 3s 3S0, and 4d bDO 7 3 p 6P multiplet transitions of the 0 I spectrum occurring at X = 3947,4368, and 6157 A, respectively. The exclted atomic states are produced through dissociative excitation of an Oz target gas by a pulsed electron beam of -0.5-2-psec pulse width and 100-eV incident energy. The mean radiative lifetimes of the 4p 5P, 4p 3P, and 4d 5D0 multiplets are obtained from analysis of the resulting radiative decay over the pressure range -20-100 mTorr and are reported as 194,161, and 95 nsec, respectively. The corresponding collisional deactivation cross sections for the multiplets are also obtained from the lifetime-versus-pressure measurements and are reported as 3.2 X 10-15, 7.7 X 10-lb, and 1.6 X 10-15 cm2, respectively.

1. INTRODUCTION slotted stainless-steel plates covered with 90%-transmission

Determination of radiative-transition probabilities for strong spectral lines of the atomic-oxygen spectrum is an important problem that has received the attention of numerous inves- tigators. These previous efforts include both theoretical calculations and experimental measurements with a wide variety of research techniques used to obtain values of the radiative-transition probabilities, oscillator strengths, or mean lifetimes. A catalog of the research effort during the period 1914-1977 is available in the National Bureau of Standards (NBS) bibliography by Fuhr et al.,' whereas Wiese2 recently published a new survey of the atomic-transition probability literature covering the period 1975-1978. Many of the more recent lifetime measurements have been obtained by using pulsed-electron excitation and delayed-coincidence photon- counting techniques. These methods include the high-fre- quency-deflection (HFD) in which excitation is produced by thousand-electron-volt electron impact of low-density beams of target atoms or molecules, and sup- pressor grid pulsing, in which excitation is produced by low- energy (e.g., 5500-eV) electron impact over a range of tar- get-gas pressures, electron-beam energies, and excitation- pulse widths. The present work employs the latter technique to c o n f m the lifetime measurements of the 4p 5P, 4p 3P, and 4d 5D0 multiplets of 0 I previously carried out by Bromander et al.? using the HFD technique, and to obtain estimates of the 0 I*-02 collisional quenching cross section of these levels.

2. EXPERIMENT Detailed descriptions of the construction and operation of the present time-resolved apparatus have been given in previous paper^^;^ and will not be repeated here. However, use of a modified electron gun is required in the present work to ensure its extended operation in an O2 atmosphere. There'fore our experiment employs a new triode electron gun, which uses

tungsten mesh as excitation-tube grids and a 0.1-cm X 2-cm iridium-coated tungsten ribbon as a directly heated cathode. The cathode, operating a t -6 A and 10 V, produces a sheet electron beam of --5-mA/cm2 current density at 100-V ac- celerating voltage. A positive square-wave voltage pulse applied to the negatively biased grid of the electron gun pro- duces a pulsed electron beam of 0.5-2-psec pulse width, 200-kHz rewetition rate. and 55-nsec cutoff time. The electron beam passes through a field-free collision region positioned along and adjacent to the monochromator entrance slit, is collected in a deep Faraday cup, and is monitored by a sampling oscilloscope. Standard delayed-coincidence photon-counting techniques are used to detect the resulting collisional radiation.

Intensity-versus-wavelength scans of the 0 I spectrum, produced by dissociative excitation of the 0 2 target gas, are used to identify the observed spectral lines. For example, Fig. 1 displays the wavelength scan used to identify the 4p =P +

3s 5S0 transition occurring at X = 3947 A. The scan, obtained a t a spectral resolution AX = f 3 A, represents the worst-case situation with respect to overlap of unwanted spectral lines observed in the present work. The X = 3947-A Line experi- ences line blending to shorter wavelengths because of the presence of the X = 3945-A line of the 0 11 spectrum. How- ever, the fraction of its integrated intensity that is observed within the X = 3947-A bandpass is only about 6% of the total integrated intensity of the X = 3947-A line under observation. In addition, although the lifetimes of the two transitions are well separated ( ~ 4 6 versus 191 nsec), a short-lived decay mode corresponding to the X = 3945-A transition is not observed in the present data. Therefore the X = 3947-A line is spectrally resolved for the purposes of the present experiment.

Time-resolvedin'ea~ureme~ts of the radiative decay, after the cessation 'of,the electrbg beam, are obtained with the center of the monochromator band$s%AX 5 3 A, set at the wavelength c o r r e s p ~ n d i g t c ~ e a k l ~ n e iptemity. The relative

0030-3941/81/070851-05$00.50 O 1981 Optical Society of America

Journal of Applied Physics, v 5 1, n 8, p 4088-93, 1980

oes obtained from a recent multichnrmel qurn,hm defect ted by 20-W bursts of 1 . 0 6 ~ radiation f m an active the or^ anrly.1~ of the optical 01 the even par- Q-swi tched Nd:Yag laser. The laser. a1 though capable it, J=Z l w e l s of m. ms rort h.. b- supported by Of delivering 50 pps. i s operated in a ngle- hot the A i r Force Office of Scientific Research under mde to produce pwer densities near lOf1 ~ d i ~ i -

~ m t m c t ~ 4 9 6 a o - ~ 9 - ~ 0 z i a . dent upon solid aluninum targets. Thennalized plasma plumes, generated by the laser irradiation, are al- lowed to d r i f t t o a 3-kV gridded extractor gap where the ions are extracted. accelerated and subsequently

The Slew Rate Dependence of the Electric Field e lec t ros ta t ica l ly focussed. Ion pulses of < 2bS

Ionization of Highly-Excited Sodium. T H r duration ( N H H ) are collected in a deep Farzday cup. The temporal width of the ion pulse i s increased to

OUST * F G * > 5 b s (MJ in a magnetic bottle before i t reaches miiZ ;ice i. , &ston, TX. Highly-excited sodiunatolm created tn zero €he extractor. The f eas ib i l i t y of micmwave heating

e lec t r ic f i e ld and then subjected t o a continuously of the plasma as a means producing high charge states increasing e lec t r ic f i e ld may exhibit two major i s also being investigated. Both procedures are

ionization thresholds (1). These different thresholds ~ ~ ~ ~ ~ ~ l ~ c ~ ~ ~ ~ ~ ~ l ~ t ~ ~ ~ ~ :i ~ r ~ ~ ~ c ~ ~ ~ ~ y s $ i ~ ~ are the resul t of predainantly-adiabatic and predominantly-diabatic passage of the excited s t a t e for conventional accelerator-based experiments. to f ie ld ionization. Data demonstrating the e lec t r ic *supported by NSF Grant PHY 75815988. f i e ld slew-rate dependence of the diabatic threshold will be presented. The predominantly-adiabatic threshold is typtcally ccrmposed of many closely-spaced p",.$uction of Ba+ and Ba++ Plasmas by Resonant thresholds. A simple mdel of adiabatic ionization will be presented along with data on the slew-rate Laser-Driven Ionization. T. 8. Lucatorto and dependence of th i s threshold. 7. J. Hcllrath*. %--Resonant laser-driven

ionization has been applied to Ba vapor a t 10" an -' densit ies to produce a fu l ly ionized ~ a + or Bau plasma. A detailed model for the ionization f rm'Ba+ to Bau has been developed. The effect of a l tght buffer gas on the efficiency of the tonizatfon

H65 Oeternination of a f r o m the Stark Effect. h a ~ b e e n stuqied. The success in obtaining 'Y. B. PHILLIPS. National Bureau of Standards*, M. G. Ba from Ba ( I . P. - 1OeV) using 2.5eV LInM4N. Princeton University.--Ue pmpose a new method photons (493510 i s an important verification fo r measuring the fine structure constant. a. using of the electron-superelasti coll ision model. atanic Stark spectruscopy. The f i r s t order Stark Absorption spectra from 1 0 d to 600A were frequency s h i f t in hydrogen my be written as v = obtained for Ba. 0a+ and Baf+. (3/8r)n(n n2) (e/h)E(a/L) where n, nl and n are the principle and parabolic quantum numbers. 'Since #Is0 a t the Untversity of Haryland. College Park. KJ the Rydberg and 2elh in HzINBS volt, a re known to parts in 1 4 o r better, a may be determined from v i f E is measured in NBS voltslmeter. An accuracy of 1 ppn and possibly 0.1 ppn should be attained. H 69 Laser-induced Col l i s i o m l Energy hansfer*, 1 p p w u l d be competitive with the best atomic GRR;ORY A. PAR-, Universit of .OPIaKoma and John C-

spectroscopy measurements of a and 0.1 p p with the Light. Lhiversit 0-1 decou~lfng . best QED independent value of a. approximation isYdevelopedgfor the interaction of

s-state atom with a E-state diatomic mlecule in the presence of an intense laser f ie ld . Results are presented for the f ie ld dependent coll isfons of tfe with HF and 'IF. The resul ts indicate that an i n t m e f i e ld strength i s necessary to induce a significant m u n t of translational-to-vibrational energy transfer- tdoUWtr with an intense laser f i e ld significant trms- lational-to-vi6rational transit ion probabilities are

Electric M m l e Traugi t iae for the Cblorfne Iwelec-. obtained fur fnqumcies which are nearly 200 m-? Off A

t r o n t t S m c e . n ' ~ . V . S ~ ~ . and^. resonance fima the nearest dlpolr at1m.d . t ~ ~ ~ f i ~ * ~ ~ h y ~ i c ~ ~epartment, Johns ~ o ~ ~ i n ~ hi.,. 0: the isolated diatom.

E r c i e a t h a-1- and osci l la tor strengths of s e v e r a l . . Work m p p o ~ e d by the Department of Energy and per- fd a t the University of Chicago.

Ion beams from laser-generated plasmas R. H. Hughes, R. J. Anderson, C. K. Manka, "' M. R. Carruth, b1 L. G. Gray, a n d J. P. Rosenfeld Physics Department, University of Arkansas, Fayetreuille, Arkansas, 72701

(Received 26 December 1979; accepted for publication 14 April 1980)

Space-charge-limited heavy ion beams have been produced by utilizing the plasma blowoffs generated by 20-MW bursts of 1.06-p radiation from an active Q-switched Nd:Yag laser. Laser power densities near a modest 10" W/cm2 on solid targets generate thermalized plasma plumes which drift to a 15-kV gridded extraction gap where the ions are extracted, accelerated, and subsequently electrostatically focused. The spatially defined ion beams are then magnetically analyzed to determine the charge-state content in the beams. Results are presented for the more significant amounts of charge states Z < 5 contained in the beams formed from carbon, aluminum, copper, and lead targets. The extraction and acceleration technique preserves time-of-flight (TOF) information in the plasma drift region, which allows plasma ion temperatures and mass flow velocities to be determined from the Maxwellian ion curve T O F shapes for the individual charge species.

PACS numbers: 52.40.Mj, 29.25.C~

I. INTRODUCTION

The concept of using directed laser-generated plasma plumes as a source of heavy ions for accelerator-based phys- ics is an intriguing one.'.2 Among the potentially useful properties of these plasma plumes are (1) a copious supply of ions per laser pulse, (2) high stages of ionization, (3) short plasma generation times, (4) highly directional plasma plumes which can be directed along the accelerator axis when certain targets are used (an invaluable characteristic for a low-emittance ion source), (5) versatility in producing not only a multiplicity of charge states, but also a variety of nuclear species, since all solids can be used for plasma sources, (6) the "freeze" in the ion charge distribution dur- ing the plasma expansion with little recombination effects, (7) simplicity in generation since, in principle, only a solid target is required a t high potential which is optically con- nected to the laser a t ground potential, (8) generation ofplas- mas from solid targets does not require differential pumping and gas transport through an anode aperture.

Ions have already been extracted from laser-generated plasmas and have been put to various use^,^.^ but generally without appreciable consideration as a possible ion source for an accelerator. However, without modification the pulsed, explosive nature of the laser blowoffs is best suited for pulsed, high-current accelerators. Conventional pulsed machines, such as cylcotrons, require a higher duty cycle than can be currently attained with a laser ion source. Al- though both high power and moderately high repetition rates are currently available in both Nd:Yag and CO, lasers, our choice was a Q-switched 20-MW Nd:Yag laser, (Holo- beam model 5050Q), capable of generating 15-nsec-0.3-J pulses at a wavelength of 1 . 0 6 ~ and a maximum repetition rate of 50 pps.

"'Permanent address: Physics Department, Sam Houston State University, Huntsville, Tex.

"'Permanent address: Jet Propulsion Laboratory, I'asedena. Calif.

II. APPARATUS

Figure 1 is a schematic diagram of the experimental apparatus. The laser beam from the laser H, is bent by a right-angle prism P and sent through a window W in the high potential terminal. The laser beam is reflected through an angle of 45" by a mirror M and directed through a 4-cm focal length lens L a n d a thin glass cover slip C placed irnme- diately in front of the lens. The focused laser beam impinges on a 2 I-in.-diam target disk T, which can be rotated by an electrically isolated st,epping motor SM. The entire target assembly, including M, L, C, and T , is mounted on a trans- latable bench contained within a vacuum chamber. The laser beam produces a divergence limited spot of about 100 ,u in diameter, corresponding to a power density near 10' ' W/cmZ at full laser power. (Note: The glass cover slip C plays an important role in protecting the laser vacuum op- tics, since coating of the focusing lens with laser-sputtered target materials such as aluminum eventually creates a se- vere laser backscattering problem that can destroy both the lens arid mirror. The protective cover glass is replaced as routine maintenance even though n o deleterious effect of the sputtered material on the cover glass has been noted.)

The resulting plasma plume PL expands freely from the target to the extraction gap G, where positive ion extraction takes place across a a-in. gap. Both the extractor electrode and the anode have t-in-diam apertures with 100-mesh 1- mil-diam tungsten grids placed across them. After high-volt- age extraction the ions then immediately pass through two grounded 16 mesh, 6-mil-diam tungsten screens S , which ease the space-charge repulsion effect (i.e., beam blowup). The ions are focused onto a pair of collimating slits CS by a double-gridded decel-accel short-focal-length Einzel lens El operating at the extraction voltage. The collimated ion beam then passes through the gap between the 4-in.-diam pole faces of the analyzing magnet A. The ions are magnetically separated into the various charge states present in the beam

14088-06gOl . l o 8 1980 American Institute of Phys~cs 4088

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Electron excitation of the singlet9 states of Hi' R. L. Day and R. J. Anderson Department of Physics University of Arkansas, Fayeneuille Arkanurc 72701

F. A. Sharpton Northwest Naurrene College, Nampa Idaho 83651 (Received 22 June 1979; accepted 23 July 1979)

Tl~e optical method and time-resolved spectroscopy have been used to study the electron impact excitation of the G 'Z:(V' = 0,1.2,3) and I1n,(u' = 0) Rydberg etates and the K 'Z:(u' = 0,1,2), M 'Z:(v' = O), and N lZ:(u' = 1,2) doubly excited states of the H, molecule. Emission cnws section measurements in the range 0.2 to 3 . 3 ~ lo-" cma and radiative lifetime mcesurcments in the ranp -21 to 70 ns are obtained at 50 eV incident electron energy and -30 mTorr H, gas pressure. In addition, HI-HI* quenching rate data arc obtained for several rovibronic levels of the singletg stata over the pressure range - 10 to 400 mTorr.

1. INTRODUCTION

Extensive investigations of the spectrum of Ha and i ts isotopes by Richardson,' Dieke, a and EIerzbergs form the foundation of the spectroscopic literature. Their works include identification of rotational-vibrational t e rm values, evaluation of spectroscopic constants and identification of perturbing interactions. F o r the case of the X'Z; ground electronic state, the rovibronic level s t ructure has been extensively studied and excellent agreement exists between theory and experiment. ' How- ever, in the case of the excited singlet-g states, the situation i s more complicated. Early experimental ob- servations of the Hz molecular spectrum attributed some of the excited '2; levels to the Rydberg configurations (lso)(nso) and (lso)(nda) while others , because of a sig- nificantly smaller rotational constant, were identified with the doubly excited configurations (2poY and (2po)(npo). However, theoretical calculations on the f i r s t excited '2; state carr ied out by avids son^@^ in 1960-61 indicated that the potential energy curve of the doubly-excited (2po)' configuration crosses the (lso)(2so) Rydberg potential in a manner which results in a single adiabatic electronic-energy curve characterized by a double minimum. Subsequent calculations by Kolos and ~ o l n i e w i c z , ' based on Davidson's work, exhibited satis- factory quantitative agreement with ~ i e k e ' s ' experi- mental measurements of the rovibronic levels. There- fore, since ~ e r z b e r g ' had previously identified the inner and outer minima a s the (lso)(2so)E and (2po)'F states, the double minimum potential curve was labeled the EF' 2; electronic state.

In 1977, Wolniewicz and Dresslere expanded the theo- retical studies on the second excited ' ~ f state initiated by ~ e h l ' and by Glover and Weinhold." In accordance with ~ e r z b e r ~ ' s ' notation of (lso)(Sdo)G for the inner minimum and (2po)(3po)~ for the outer minimum and in analogy with the E F state, they designated the second double-minimum singlet-g s ta te as GK'Z;. Their work included calculations of Born-Oppenheimer potential energy curves for the EF and GK states of Ha, D, and

* ~ e s e a r c h supported by the Atmospheric Sciences section of the National Science Foundation.

HD and correlated the existing experimental data with the calculated vibronic levels up to 6 = 30 for the E F state and v' = 7 for the GK state. In addition, Dressler et al. " have recently computed the mutual vibronic coupling between the E F and GK states in an adiabatic basis of electronic-vibrational states. However, the latter calculations have resulted in a revision of their previous assignments of several of the experimental levels to either the EF,, o r GK,. vibrational levels. Therefore, since the assignments of these strongly mixed levels a r e s t i l l somewhat arbitrary, the singlet-g levels investigated in the present work will be designated using the Herzberg notation and the wavelength of the observed rovibronic transition. Table I l is ts the vari- ous electronic designations currently used by experi- menters to identify the dnglet-g s tates of Hz.'

Previous experimental and theoretical investigations of the G i g (v'= 0) state have yielded values of the ex- cited-state lifetime,'2-'5 the, c r o s s section1' and the

(v'= 0)- BIG, (v'= 0) emission c r o s s ~ e c t i o n . ~ ~ ~ ~ " ~ ore recently, lifetime and diealignment c ross sections have been carr ied out by Chien, Dalby, and Van der ~ i n d e " (CDV) for several rovibronic levels of the GIG,

IiIl; and I In; Rydberg s ta tes and for the K'G doubly ex- cited state. The CDV measurements, obtained using the Hanle effect, indicate a dependence of the radiative life- t ime upon rotational quantum number for several rovi- bronic levels of the G'C, state. This variation of radi- ative lifetime with rotational quantum number is as- cribed by CDV to perturbing interactions between the G 'C, and I 'I, states, (L uncoupling). The present work expands on these ea r l i e r investigations and includes measurements of the radiative lifetime, emission c ross sections and H~-H: collisional quenching rates for sev- e r a l rovibronic levels of the singlet-g s tates of Hz. In addition, the rotational dependence of the radiative life- time, observed by CDV, is not observed in the present work.

I I . EXPERIMENT

A. Technique

Emission c ross section measurements a r e obtained using the optical method, while radiative lifetime and

J. Chem. Phyr. 71(9), 1 Nov. 1979 OM 1 -9606/79/213683-06$01.06 O 1979 American Institute of Physics 3683

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Radiative decay constants of the H, Fulcher bandsa) Ray L. Day and Richard J. Anderson

Deparrmenr of Physics, University of Arkansas, Fayetreville, Arkansas 72701

Francis A. Sharpton

Department of Physics, Northwest Nozarene College. Nampa. Idaho 83657 (Received 26 June 1978)

The radiative decay constants corresponding to the Fulcher bands of the H, molecular spectrum are obtained using pulsed-electron beam techniques. Data in the range - 1.8 to 2.8X 10' s-' are obtained at 25 eV incident electron energy for the d ' m u - a 3H: (v', v")Q(K) and k 3 ~ , _a 38: (vt,v")Q(K) rotational lines with K = 1,2,3 and v' =0,1,2,3,-v' =0,1,2,3. Additional measurements of the d 'n,-+a 'Z8+(0,0)~ I rotational line intensity indicate the presence of a two-mode decay over the energy range 50-300 eV. The long-lived decay component, identified as radiative cascade from upper levels, contributes - 15% to the total d3.rr,(v' =0) population.

1, INTRODUCTION grid structure of a 6EM5 radio tube a r e used a s an elec-

The Fulcher bands of the molecular spectrum of hydro- gen a r e atomiclike in appearance because of the open ro- tational s t ructure of the spectrum. Prominent rotational lines occurring in the red-green portion of the spectrum a r e the result of transitions from the ( l s u ) ( 3 p ~ ) 'nu and ( 1 s ~ ) ( 4 ~ n ) ~ n , s ta tes to the ( ~ s u ) ( ~ s I J ) ~ z ~ state. The s tates a r e designated d3n ., k3n ,, and a 'c; and give r ise to the Fulcher a and p bands respectively. Although these a r e triplet s ta tes , the interaction between elec- tronic spin and orbital motion i s usually considered to

tron gun. A positive pulse applied to its negatively biased control grid at a repetition rate of 200 KHz pro- duces a pulsed electron beam of 5 5 m ~ / c m ~ peak cur- rent density, 500 ns pulsewidth, and < 2 nsfall t ime. The

pulsed electron beam enters the collision region through a grounded collimating aperture ( L = 1 .0 cm, W = O . 3 cm) and passes adjacent to and along a vertical monochroma- tor entrance slit . The resulting collisional radiation en- t e r s the slit without the aid of external optics and i s de- tected by means of time-resolved counting techniques.

be negligible ( ~ u n d ' s case b) and K = A , A + 1, . . . , where Several control experiments, including measurements K represents the total orbital angular momentum ex- of radiative lifetime and dc light intensity versus HZ gas cluding spin. ' Therefore, the Q-branch rotational lines pressure and electron beam current, indicate that, at observed in the present work will be identified a s 25 eV electron-impact energy, secondary excitation (TI', u") Q(K). a r e absent over the ranges 3 to - 5 0 mtorr

Previous experimental investigations of the Fulcher a band system have included spectral line identifica- tion, relative intensity measurements of rotational lines, emission c r o s s section measurements, '" and polarization m e a s u r e m e n t s . ~ h e o r e t i c a l calculations of Franck-Condon and of the d 3 s u direct ex- citation c r o s s have also been carr ied out. The present work used pulsed-electron beam techniques to obtain measurements of the radiative decay constants fo r both the CY and p band systems. The radiative cas- cade contribution to the population of the d 3nu (7)' = 0) lev- el i s also estimated from the decay measurements.

II. EXPERIMENTAL APPARATUS

A block diagram of the experimental apparatus used to ca r ry out the radiative decay measurements is shown in Fig. 1. The apparatus includes a vacuum system capable of producing residual gas pressures of 5 5 x lo-' to r r ; research-grade purity Hz gas samples monitored by a capacitance manometer vacuum gage; a monokinetic electron beam with AES 0 .8 eV over the energy range 10 to 300 eV; a fieldfree collision region; a Faraday cup to collect the electron current; and a grating monochro- m a t y , operated at spectral resolutions of Ah =i 0.8 to - 4 A to isolate the rotational lines. The cathode and

* ~ e s e a r c h supported by the Atmospheric Sciences section of the National Science Foundation under grant ATM 74-12763.

and 50 pA to - 2 mA, respectively. In addition, mea- surements of Light intensity versus wavelen~th a r e c a r - ried out over the range A = 3 9 0 0 to A = 6900 A. The ex- tremely open rotational structure of the H, spectrum a l - lows identification of the rotational lines under investi- gation and prompt verification of the absence of ~mpurity gas spectra.

Ill. RESULTS AND DISCUSSION

Radiative lifetime measurements of the dsn, and k3nu vibrational levels u' =O,1,2,3 a r e carr ied out a t

Colllslom

Faraday Amp.

Dlscr. Sampling

[*1""1 Plotter -

FIG. 1 . Block diagram of the experimental apparatus used to obtain the radiative decay measurements. Time calibration of the MCA spectrum i s obtained using a BNC model 7030 pro- grammable digital delay generator a s a standard delay line.

5518 J. Chem. Phys. 69(12). 15 Dec. 1978 0021-9606/78/6912-5518$01 .M) 63 1978 American Institute of Physics

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Electron impact excitation of the G ' ~ f , state of H,' Richard J. Anderson and J. Watson, Jr.*

Departn~ent of Physics, University of Arkansas, Fayetteville, Arkansas 72701

Francis A. Sharpton Department of Physics, Northwest Nazarens College, Nampa. Idaho 83651

(Received 7 July 1977)

The optical excitation function for the G 'B: -+B 'B,+ transition of the H2 spectrum has been determined for the electron impact energies in the range 0-500 eV and at H2 gas pressures 5 30 mtorr. Measurements of the absolute emission cross section for the X = 4634 A [G -+B(O,O)Pl,R2,R 31 vibration-rotational line blend have been carried out at 200 eV incident electron energy. Measurements of the G ' Z l ( v ' = 0) state radiative lifetime have also been carried out and yield a single-mode decay, with a lifetime of - 30 11s. Both dc intensity and time-solved lifetime measurements are observed to be independent of H, gas pressure and electron beam current over the regions - 2 to 50 mtorr and 5 pA to 1 mA, respectively.

1. INTRODUCTION tense adjacent spectral lines corresponding to i r r e l e - vant transitions.

The optical-method' is used in the present investiga- tion to examine the electron-impact excitation of neutral A. Absolute emission cross sections H,. In particular, excitation of the G'C; s ta te i s studied through the process Intensity measurements of the A = 4634 A rotational

X ' ~ i ( l s o ) ~ 2 G lC;(l so) (3do) 2 B lC:(l so) (2po).

The te rm assignments of ~ i e k e ' a r e used to identify the rotational lines associated with G- B vibrational transi- tions3 and to allow comparison with previous spectro- scopic studies of the Hz spectrum. However, i t should be noted, that ( i ) Dieke's assignments of the vibrational levels v = 0 - 3 a r e somewhat uncertain due to the strong perturbations that exist between the G ' ~ ; ( l s o ) (3du) and the H 'C;(lso) (3so) s tates , and (ii) recent calculations2 indicate that the G s ta te i s actually character ized by a double-minimum Born-Oppenheimer potential function and has the t e r m symbol G + K 'C',(lso3do+ 2pn2).

Absolute m e a y r e m e n t s of the emission c r o s s section f o r the A = 4634 A [G-B(O, 0) P I , R2, ~ 3 ] spectral lines a r e obtained a t 200 eV incident electron energy. In ad- dition, time-resolved techniques6 a r e used to obtain a measurement of the G 'xi(; = 0 ) s ta te lifetime and to show that population of the G s ta te by radiative cascade transitions is negligible.

Il. EXPERIMENT

Emission-cross-sect ion measurements have been c a r r i e d out using experimental techniques and apparatus s imi la r to that described in previous papers7*' and therefore will not be discussed in detail. A graph of the light intensity versus wavelength obtained with this apparatus is shown in Fig. 1 for the spectral region about !he A=4634 A line a t a spec t ra l resolution of Ah = 0.9 A, a n acceleration voltage of 100 eV, and a H, gas p r e s s u r e of - 3 5 mtor r , The P I , R2, and R3 rotational lines appear ,as a blend a t this resolution. Recent studies of the A=4634 A line by FreundS also indicate an inability to completely resolve the R3 and P1 rota- tional lines even a t a spec t ra l resolution of AX=O. 13 A . The other rotational lines shown in Fig. 1 also repre - sent blends of various unresolved rotational transitions. However, the A=4628 A l ine corresponding to the G - B(0,O) RO transition is spectral ly isolated from in-

line blend a r e ca r r ied out a t AAS 4 A spectralresolut ion, 200 eV electron energy, and 5 30 m t o r r Hz gas pressure. Graphs of light intensity versus p r e s s u r e and light in-

WAVELENGTH [ANGSTROMS]

FIG. 1. Relative photodetector signal-vs-wavelength scan of the spec t ra l region about the h= 4628 and h= 4634 A spectral l ines corresponding to the G - B(0, 0)RO and G - B(0, O)Pl, R2, R3 t ransi t ions at AA= 0.9 A effective spectral s l i t width, - 35 m t o r r H2 gas pressure and 100 eV incident electron energy. The t race i l lustrates the open rotational s t ruc ture of the H2 spectrum and the dominance of the h= 4634 A rotational Line blend.

1641 1. Opt. Soc. Am., Vol. 67, No. 12, December 1977 Copyright 1978 by the Optical Society of America 1641

Bulletin of the American Physical Society, v 22, Issue 1 1, p 1330-1 330, 1977

. .~ - ~ ~ -

process in medimZ,Li-l ike multicharged ions.

*ark supported ir. par t by the Division of Magnetic Fusion Energy and i n par t by the Division of Physical Research of US ERDA.

*staff Member, Quantum Physics Division, h'ational Bureau of Standards.

%RAU summer student a t Oak Ridge National Laboratory. $operated by Union Carbide Corporation for US ERDA.

C l 3 Electron Imoact Excitat ion of T r i o l e t States of p* R:-L. ~ a y and'R. J. Anderson, Universit o f rkansas. F. 1. Sharpton. Northwest h e e.--

m t r o n impact exc i ta t ion o f the H7 t r i p l e t sgtates -(lso)(npn) (n=3.4;5) i s studied through observations of the r a d i a t ~ v e transit ions (lso)(non) %Inll + (lso)(2po) . . . . 3ri. Rotational l ines ~ 0 r r e s p o n d i n ~ ' t o the (vi=O + v"=O) 91. ( v t = l + v"=l)Ql . (v =2 + vn=2)Q1 and (v'=3 + vN=3)Q1 transit fons are isolated a t a spectral resolution o f AI < 314 FWW and are studied using the optical method and delayed-coincidence techniques. Optical excitat ion functions are obtained f o r the e lectmn energy range 0-500 eV a t H2 target gas pressures < 50 mtorr. Intensity measurements of the rotat ional lines-at 25 eV are characterized by a l i n e a r dependence upon H2 gas pressure and are placed on an absolute scale by d i rect comparison with the I& and H s ect ra l l ines produced by dissociative exc i ta t ion o f H ~ . * R:diative l i f e t i m e measurements f o r the rota t iona l transit ions y i e l d values i n the range approximately 35 to 50 nanoseconds.

'Research supported by the Atmospheric Sciences Section o f the National Science Foundation.

G14 Bydrogen ( n = L+2) result in^ f m m a e c t r o n h c t Dissociation of H,. G. J. FISANICIC-EIGLUl'. D. E. LYJEOBUE*, and R. S. EELmD. Bell Laboratories, 600 Mountain Ave., m a y Bill, R.J. 07974-When Hz is dissociated by electron impact, the resul t ing Balmer-0 radia t ion shovs a c@ex Doppler broadened Lineshape. There i s a cen t r a l peak resul t ing f rom dissociation of Rydberg s t a t e s and b r d vlngs r e milt- f m m dissociation of dnubly excited a ta tes . Polarization of the e n t i r e l i n e has been measured a s a function of excit ing electron energy f r o m thres- hold t o 500 eV as a function of pressure. Etsrimum poLarlz6tion for the entLre l i n e occurs a t 40 eV exci ta t ion energJr v l t h a value of 165, extrapolated t o zem pressure. A t high op t i ca l resolution, t he polarization is found t o vary across t he lineshape, and t h f s var ia t ion is strongly dependent on t h e exci ta t ion energy. *Present address. B e l l Laboratories. Holmdcl. H. J. 07733

GI 5 P o l a r i z a t i o n o f V U V R a d i a t i o n f o l l o w i n q E l e c t r o n Impac t o n M o l e c u l a r Hydrogen.* I.C. W U ) L M , H.W. DASSM and J .W. McCONKEY, UNV. o f Windsor. - A t r i p l e r e f l e c t i o n p o l a r i z i n g d e v i c e h a s b e e n u s e d i n c o n j u n c t i o n w i t h a con- v e n t i o n a l crossed-beam e l e c t r o n i m p a c t e x c i t a - t i o n a p p a r a t u s t o carry o u t p o l a r i z a t i o n s t u d i e s in the vacuum u l t r a v i o l e t down t o 50 nm. Wavelength s e l e c t i o n was a c h i e v e d u s i n g a S e v a -N&oka monochromator whose ~ o l a r i z a t i o n c h a r a c t e r i s t i c s w e r e checked using- polarized r a d i a t i o n . Measurements will be p r e s e n t e d f o r b o t h a t o m i c and m o l e c u l a r r a d i a t i o n f rom Hz, t h u s p r o b i n g b o t h t h e d i s s o c i a t i v e a n d direct e x c i t a t i o n p r o c e s s e s .

+ R e s e a r c h s u p p o r t e d b y t h e B a t i P n a l Resea rch C o u n c i l o f Canada.

G I 6 M e t a s t a b l e Fr tat ion of N 0 u n d e r E l e c t r o n Impact? C. G C * and J.W.'MCCONKFI,

, UNV. o f Windsor.- A t i m e o f f l i g h t a p p a r a t u s h a s been wed to make a d e t a i l e d study o f t h e break-up of 820 into m e t a s t a b l e f r m t s m d e r clect=on impac t . Bo th a t d c a n d molecular f r -enta have been ronitored and p & i d u

13

a t t e n t i o n h a s been p a i d t o f r a g m e n t s in high- Rydberg s t a t e s where i d e n t i f i c a t i o n was p o s s i b l e u s i n g mass a n a l y s i s f o l l o w i n g f i e l d i o n i z a t i o n .

* Resea rch suppor ' ted b y t h e N a t i o n a l Research Counc i l o f Canada and Environment Canada.

GI7 On S t a t i s t i c a l Equilibrium Among the Lower Rydberg Levels [~( 'P ," 'D,F) , n = 4-71 in Helium. W. R. PENDLETON. JR. and TA-YUNG WOAG. Utah Sta te U.--The approach to s t a t i s t i c a l e q u i l i b r i c r f the lover Rydberg iebei populations i n helium has been investigated in the pressure range 0.2 - 400 mTorr for the case of primarg excitation of the energy-level manifold by =loo-eV electron impact. Level population r a t i o s were inferred from the r e l a t i ve i n t ens i t i e s of appropriate near-IR He I nu l t i p l e t s terminating on levels of the ls3( configuration. The r e su l t s indicate that nlD n " ' ~ ~ s t a t i s t i c a l equilibrium occurs a t much lover helium densit ies than does equilibrium v l t h the complete nP fine-structure manifold. S t a t i s t i c a l equilibrium of n ' ~ and nP is closely approached a t the highest pressure used i n the investigation. The in tens i ty r a t i o I(4F * ~ ) D ) / I ( ~ F -+ 3 ' ~ ) was obsened to exceed 1.5 a t low pressures and to approach 3 a t the highest pressure. in conf l ic t with conclusions drawn from e a r l i e r indirect studies. A possible explanation of the low-pressure resul ts i s afforded by recent r e su l t s r e l a t i ng t o the high Rydberg s t a t e s i n helium.

~ 1 8 N e g a t i v e I o n Resonances a s s o c i a t e d w i t h I n n e r Shell E x c i t e d S t a t e s . " J.W. McCONKEY, G.C. K D i G a n d F.H. READ, M a n c h e s t e r Univ. - A c r o s s e d g a s - e l e c t r o n beam s y s t e m h a s b e e n u e d t o s e a r c h f o r r e s o n a n c e s a s s o c i a t e d w i t h i n n e r s h e l l e x c i t e d s t a t e s o f a toms and m o l e c u l e s . D e t e c t i o n is v i a t h e i o n wh ich r e s u l t s a s t h e e x c i t e d c o n f i g u r a t i o n decays . R e s u l t s which w i l l b e p r e s e n t e d i n c l u d e d a t a o n r e s o n a n c e s a s s o c i a t e d w i t h K-she l l e x c i t a t i o n o f C, N and 0 in N2, CO and C02.

* Resea rch s u p p o r t e d by U.K. S c i e n c e Resea rch Counc i l .

GI9 Asymmetric Decomposition of Organic Wolecdes & Longitudinally Polarized Electrons L.A. nodge, A.R. Hdr- rison, R.H. White, F.B. Llunning, G.J. Schroepfer, and G.K. Walters, Rice U.-In biological systems a l l p m i n s a r e composed exclus iwly of amino acids of the t t p chirality, whereas laboratory syntheses generally resul t in equal mixtures of D- and types. 1t has been sugges- ted that in terac t ion with longf tudhal ly polarized elec- trons from &decay may have &en r e s p n s i b l e f o r the origin of t h i s aspmetry through preferent ia l destruction of the Ecwfiqura t ion . Experiments attempting to verify this hypothesis employing natura l &emitters have ~ i e l d e d d l i c t i n g and inconclusive resul ts . Recently, hawe-, Banner. ~ . l reported asymmetric degradation of .the amino ac id DLleucine upan i r radia t ion with 120 keV e l e c trans having polarization of 10-239. 'Ihe improved pol- ired elec tmn source developed a t a c e , which p m i d e s beam currents of 1~ a t a popolarizatim of 2459 is P* a t l y being used to investigate t h i s effect . The of i r radia t ion and analys is of DGleucine samples by te* n-S s h d l ~ to those of Bornerr 5 &. will bed is cuss^ laanner, W.A., Van cart, LA., ~earian, nR, ma- 258, 4l9 (1975).

G2O E l c c t r o a Momentum D i s t r i b u t i o n s i n a - E l e c t r o n Molecu le s by E l e c t r o n Impac t I o n i z a t i o n . A . L . MICDALL, H.A. COPLAN. J - A . TOSSE?, and J . B . M O R E . U n t v e r s i t Y O f Maryland. --The momentum d i s t r i b u t i o a s o f t h e v a l e n c e e l e c t r o n s o f a e e t v l e n e and e t h y l e n e have been d e t e r m i n e d f rom r e a b u r e m e a t a o f t h e t r i p l y dLf f e r e n t f o l c r o s s - s e c t i o a f o r e l e c t r o n impact i o a f z a t i o n i n t h e r e g i o n where t h e p l a n e wave impu l se f p p r o x i m a t i o n is v a l f d ( ( e ,Ze ) 8PeCtroscopy) . There aomentum d f e t r i b ~ t f o ~ ~

Excitation of the E,F1zC, states of H, by electron impact*

James Watson, Jr.t and Richard J. Anderson

Department of Physics. University of Arkansas, Fayetreville, Arkansas 72701 (Received 25 February 1976)

Optical excitation functions produced by low-energy electron impact excitation are reported for several rotational lines originating from the E , F 'z: and the H '2: electronic states of neutral H,. The E ' 2 1 ; ~ ~ '1:. F '1:-B '1:. and H '2: +B 'I: band systems are observed at an effective spectral slit width of 5 2 A. over an electron energy range 0-300 eV. The observed transitions are identified as originating from v ' = 2,3.4 vibrational levels of the E state, the v ' = 5 level of the F state, and the v ' = 2 level of the H state. Absolute emission cross section measurements arc obtained at 2W eV electron energy and 30 mtorr H, gas pressure through direct comparison with the A =4686 A (n = 4 ~ 3 ) line of the He11 spectrum. Analysis of the E-B(2,l)RO rotational line intensity yields (1 .0*0.3)~ lo-" cm' as a lower- limit estimate of the E ' P i direct cross section at 200 eV.

I. INTRODUCTION duce the buildup of impurity gases, while the target gas

The present work employs the optical methodL to ex- pressure is continuously monitored by a capacitance

amine the electron impact excitation processes manometer pressure gauge attached adjacent to the col- lision region.

E, P ' ~ ; ( l s u ) (2sw) + ( 2 ~ 0 ) ' The electron gun employs a planar grid structure to produce a - 2 mm diameter monokinetic electron beam

x 1 1 s u 2 [ O r ] iB iE:lsu)(@o) with currents in the range 5 pA to -1 mA at accelerat- H 'E;(lso) (3.~0) ing voltages of 10 to 300 eV, respectively. Application

in o rder to obtain optical excitation functions, absolute emission c ross section values, and an estimate of the E state direct excitation c ross section. These ex- cited s tates a r e of special interest because of their prominent emission bands in the H, visible spectrum and their double-minimum potential structure.= The lat ter characteristic has stimulated several experi- mental and theoretical studies, especially in the case of the E, F 'c; state. Experimental observations of spectral emissions from this f i r s t excited ' ~ f s ta te to the B 'c: state were initially carr ied out by Dieke, 'u' who assigned the observed lines to w o separate band systems, E - B and F-B. However, the theoretical studies of avids son' and of Kolos and ~ o l n i e w i c z ' have firmly established the double-minimum structure of the E, F potential energy curve (Fig. 1). Their work has led to attempts by other investigators to apply simplified techniques such a s the "frozen-core approxi- mation"' and the "expansion m e t h o d 8 to describe the E, F-B transition. The existence of a s imilar double- minimum potential energy curve representing the H s ta te i s not as well documentede a s that of the E state and consequently is shown a s a broken line in Fig. 1.

II. EXPERIMENTAL APPARATUS

A block diagram of the experimental apparatus is shown in Fig. 2. The vacuum system is constructed of stainless s tee l components, and employs a liquid- air-trapped oi l diffusion pump to produce residual gas p ressures of -5x torr. Research-grade purity H, gas passes through a cryogenic t rap and flows contin- uously into the collision chamber through a calibrated leak valve. The collision chamber and the cathode r e - gion of the electron gun a r e differentially pumped to re -

Internuclear Distance ( A) FIG. 1. Potential energy cu rves and consecutlve vibrational levels f o r the B 'I;, E , F 'Z:, and H '2; ejtcited s t a t e s of the hydrogen molecule. F o r the E, F s t a t e , solid l ines indicate a localized vibrational wavefunction6: broken l ines indicate vibrational Levels f r o m whlch t ransi t ions have not been exper i - mentally observed. 3*4

The Journal of Chemical Physics, Vol. 66. No. 9, 1 May 1877 Copyright O 1977 American Institute of Physics 4025

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Excitation of the C3n, state of N, by electron impact* S. T. Chen and Richard J. Anderson

Deportment of Physics, University of Arkansas, Fayettevillc, Arkansas 72701 (Received 1 April 1975)

The h = 3371 A spectral band of the C311 ,~B311 , (v' = 0, v" = 0) second positive system is investigated using time-resolved spectroscopy with electron beam pulse widths of 100 nsec, 260 nsec, and 5 psec. Lifetime data for the v' = 0 vibrational level of the C 'nu electronic state are obtained for incident electron energies and N, gas pressures in the range 25-200 eV and 2-380 mtorr, respectively. In the case of the 260 nsec electron pulse, a single radiative decay component is observed which exhibits a dependence upon electron impact energy. However, in the case of a 5 psec excitation pulse the resulting radiative decay is observed to consist of two decay components. The short-lived component is identified as the primary C 'rl, state excitation, while the long-lived component is identified as the radiative cascade contribution from higher electronic states, including possibly the metastable E 'El state. When the long-lived cascade contribution is subtracted from the total C 3 n , population. the C state natural radiative lifetime becomes independent of energy. In addition, the lifetime is observed to be independent of N, gas pressure over the pressure range - 2-380 mtorr.

I. INTRODUCTION A gated electron beam acts a s the primary excitation

The second positive band system of the N, spectrum i s one of the most important spectral systems in atmo- spheric, plasma, and lase r physics. Therefore, a number of experimenters have used various techniques to examine the C 3 ~ , - B3n, radiative decay and to ob- tain measurements of the radiative transition probabili- t ies associated with the C 3n, vibrational levels. '-a (A partial energy level diagram of N, is shown in Fig. I). In the present experiment, time-resolved spectroscopy has been used to study the electron-impact excitation of the C 'nu (v'= 0) state and to obtain measurements of i t s natural lifetime. Previous experimenters have re - ported a significant dependence of the C state lifetime upon the electron impact energy, and have attributed this dependence to radiative cascade contributions to the C state population f r o m higher long-lived stateslq2 o r possibly the spectral mixture of the second positive and f i r s t negative band systems.' In addition. an unusual dependence of the C s ta te lifetime upon t i e N, gas pres- sure was also reported by some i n v e ~ t i ~ a t o r s . ~ * ~ In the present work the lifetime of the C 'n, ( v ' = 0) state is measured a s a function of incident electron energy, N2 gas pressure, and excitation pulse width. The previ- ously observed pressure and energy dependencies a r e re-examined under experimental conditions which allow the contribution of radiative cascade to be quantitatively determined.

mehcanism. Electron pulses of variable energy from 25 to 200 eV, peak amplitudes of 0.01 A, and durations of 100 nsec, 260 nsec, and 5 usec a r e produced a t a repetition rate of - 20 kHz and with cutoff times of - 5 nsec. The energy distribution of the electron beam is determined to b e 5 5 eV a t 50 eV incident electron energy by means of the retarding-potential method. The pulsed electron beam passes into a field-free excitation region through a 0.3 cm wide by 1.8 cm long slit and is col- lected and monitored in a deep cylindrical Faraday cup (6 cm longx4.3 cm deep). The excitation region is sur - rounded on three sides by electrically grounded walls 2 cm long and 3 cm high. The width of the chamber is variable and can be changed from 1 to 4.5 cm. The modulated light signal passes out of the collision cham- ber at a right angle to the electron beam through a 1-in. diameter quartz window placed immediately adjacent to the excitation region.

The collision induced radiation is focused by a lens, with unit magnification, parallel to the entrance slit of a 0.5-m plane-grating monochromator (AX = 8 A), and is detected by a photomultiplier tube. The PMT output signal is then analyzed by a sampling oscilloscope. The signal-to-noise rat io of the sampling system i s improved by a factor of - 5 by use of a double modulation detection scheme in which the 20 kHz pulsed radiation is modu-

11. EXPERIMENTAL APPARATUS

The experimental apparatus shown in Fig. 2 utilizes a version of time-resolved spectroscopy developed a t the University of Arkansas.' Spectroscopically pure nitrogen g a s passes into the excitation chamber through - a precision gas-metering valve. Target gas p ressures 5

Att. of 50 mtor r and above a r e obtained by operating the sys- tem in a static state, while lower pressures a r e achieved by continually flowing the gas through the exci- tation chamber a t a constant rate. Pressure measure- ments a r e ca r r ied out by means of a well-trapped McLeod gauge, and a calibrated ion gauge. The residual gas p ressure is held below - lom5 tor r throughout the ex- o perirnent. FIG. 1. Partial energy level diagram of N2

1250 The Journal of Chemical Physics, Vol. 63, No. 3, 1 August 1975 Copyright O 1975 American Institute of Physics

Dbwnloaded 11 Mar 2008 to i30.164.237.6. Redistribution subject to AIP license or copyright; see http://jcp.aip.orglj~~~lco~~yrigtlt.js~i

P H Y S I C A L R E V I E W A V O L U M E 1 2 , N U M B E R 2 A U G US'I' 1 9 7 5

Excitation of the B 31t, states of N2 by electron impact*

S. T. Chen and R. J. Anderson Department of Physics, Univ'ersity of Arkansas. Fayetteville, Arkansas 72701 (Received 24 December 1974; revised manuscript received 18 March 1975)

Radiative lifetime measurements of 13 vibrational levels of the B 3n, state of N, have been obtained at 50 eV incident electron energy and 30 mTorr N, gas pressure using a 50-psec electron excitation pulse. Two radiative decay components with lifetimes of approximately 5 and 25 psec are observed in each case. The short-lived com- ponent is identified as the contribution from direct excitation of the B 3 n , state. The long-lived cascade contri. bution is observed to decrease from 37% of the total B'n, + A 'H: (u',vn) radiation for the u ' = 0 vibrational level to 15% for the v' = 12 level and is consistent with theoretical predictions by Cartwright et a l .

I . INTRODUCTION

Electron-N, collision p roces ses have par t icular significance in t e r r e s t r i a l a tmospher ic sc ience because of the abundance of molecular nitrogen in Ear th ' s a tmosphere . A knowledge of absolute transit ion probabili t ies and l ifetimes for the elec- t ronic t rans i t ions of N, i s a l so important in many problems in as t rophysics , p lasma physics, chem- ica l kinetics, and l a se r s tudies . In the present experiment, t ime-resolved spectroscopy has been used to study electron-impact excitation of the B3n , s t a t e s of N,. Lifetime measurements of 13 vibrational levels of the B311gelectronic s t a t e a r e obtained by observing the t empora l decay of the first-posit ive band s y s t e m corresponding to B3n, -A3C: ( u ' , v") transit ions. (A par t ia l energy-level d i ag ram of N, and N; is shown in Fig. 1.)

Severa l investigators1-3 have previously mea- su red the l ifetimes of vibrational levels of the B%, s t a t e of N,. In addition, Chung and in^ have calculated the electron-impact excitation c r o s s sec t ions of the B3n , s t a t e and found that the calculated c r o s s sec t ions were about one- half that of the experimental apparent c r o s s s ec - t ionsSe6 correc ted f o r cascade f r o m the C311, s t a t e . Meanwhile, theoretical calculations by Cartwright et ~ 1 . ' ' ~ indicate that radiative cascade f rom high- e r vibrational levels of the W3a, and A3Et s t a t e s could play a significant ro l e in populating the low- e r vibrational levels of the B311g s t a t e and hence change the cha rac t e r of i ts apparent excitation ci-oss sections. In the present work, lifetime measurements of 13 vibrational levels (ul=O - 12) of the ~ ~ 1 1 , s t a t e a r e obtained a t 50-eV electron impact energy, 50-gsec e lec t ron excitation pulse width, and 30 -mTor r N, g a s p r e s s u r e . A signifi- cant long-lived cascade contribution is observed for each vibrational level and i s consistent with theoretical predictions by Car twright el al.'"

11. EXPERlMENT

A. Apparatus

The experimental apparatus shown in Fig. 2 util izes a version of t ime-resolved spectroscopy developed a t the University of Arkansas.' Spec- troscopically pure nitrogen gas pas ses into the ex- citation chamber through a VACOA precision gas - metering valve. The gas flow i s held a t a constant

4 i N2

2 1 J 0 x 15;

FIG. 1. Partial energy-level diagram of N, and N;

PHYSICAL REVIEW A VOLUME 8 , NUMBER 2 AUGUST 1 9 7 3

Excitation of the 4 'S and 3 'P Levels of Helium by Electron Impact*

R J. Anderson, R. H. Hughes, J. H. Tung, and S. T. Chen Department of Physics University of A r k a m , Fayetteville, A r k a m 72701

(Received 26 February 1973)

The exchange excitation of the 4 ' s and 3 'P levels of helium by elactron impact has been s t u d y by means of time-resolved spectroscopy of the 4 'S + 2 'P (A = 4713-A) and 3 'P - 2 'S (A = 3889-A) transition$. Men.?uranents were carried out for incident electron energies in the range 50-400 eV at helium-gas pressurea of 8 and 4 mtorr. Radiative cascade transitions from tbe n3p levels and from the n 3 s and n ' ~ levels contributed 12 and 10% to the total 4 % and 3 3 ~ populations, respectively, at 50eV. Thesc fractions increased to about 30 and 40%, respectively, at 400-eV electron-impact energy. The 4 'S and 3 'P directexcitation moss sections were obscsved to decrcase with increasing electron impact according to the relation (energy)-'. This result is in agreement with the theoretical predictions of Ochkur and Bratsev.

I. INTRODUCTION

Many experimenters have used the optical meth- od to examine the phenomenon of electron-exchange excitation in electron-atom collisions. Particular emphasis has been given to the study of the triplet levels of the He1 spectrum since they can be direct- ly excited only through the exchange mechanism. Absolute measurements of the direct-excitation c ross section and i t s dependence upon incident- electron energy have been obtained for several of the He1 triplet levels. However, discrepancies presently exist between many of these measure- ments and the corresponding theoretical predic- tions. One example is the apparent discrepancy between theory and experiment in predicting the relative energy dependence exhibited by the direct- excitation cross-section curve (excitation function).

In the case of helium the Born-Oppenheimer the- ory predicts that triplet-state excitation functions will have a peak just above the threshold energy and then experience a rapid monotonic decrease with increasing energy of the incident electron. This rate of decrease of the direct cross section with electron energy E is calculated to be E-=, E-3, and Em4 for the %, 'P, and 3~ levels, re- spectively.' Calculations of the direct-excitation c ross section carr ied out using the expansion technique of Ochkur and Bratsev predict a E-' de- pendence f o r all of the triplet-level cross s e c t i ~ n s . ~ Previous experimental studies of this energy de- pendence have produced contradictory result^.^ The theoretical calculations become more valid a t the higher energies (above 100 eV). However, this is a difficult region to interpret experimental re- sults because secondary excitation mechanisms, such as radiative cascade, generally make signifi- cant contributions to the low-level optical measure- ments. The 4 3S, 3 3P, and 3 3D apparent-cross-

section energy dependence determined by Kay and Showalter' indicates a single E-' dependence for the 4 'S cross section for the higher energies. However, they found that the 3 'P and 33D levels have excitation functions which exhibit a far more complicated energy dependence at these higher en- ergies. A log-log plot of their apparent cross sec- tion versus incident-electron energy shows that the 3 P- and 3 %-level excitations each exhibit a E -' dependence for energies above approximately 100 eV. Subtracting this energy dependence from the original data points leaves a second component that varies a s E-"' and E-3.7 for the 3 9 and 3 'D, respectively. The slow components a r e attributed by the authors of Ref. 4 to the effects of secondary excitation on the levels in question.

The present work uses time-resolved spectros- copy to determine the energy dependence of the direct-excitation cross section. The effects of secondary excitation mechanisms which have tem- poral decay rates different from the radiative rate of the upper level of the transition under study a re subtracted off from the total light intensity. This technique provides direct information about the excited-atom density produced by direct electron- impact excitation. The time-resolved technique is a simple and useful method to isolate, identify, and quantitively measure the effects of secondary excitation mechanisms and therefore can be used effectively in conjunction with absolute-intensity measurements to determine direct-excitation c ross sections.

11. EXPERIMENT

The time-resolving apparatus shown in Fig. 1 was similar to that used in our previous experi- m e n t ~ . ~ Helium gas passed through a cooled trap and into the excitation chamber through a small orifice. The rate of helium-gas flow was held

1194 M A C D O N A L D , F E R G U S O N , C H I A O , E L L S W O R T H , AND SAVOY - 5

bration.

CONCLUSIONS

Electron-capture and-loss cross sections have been measured for fluorine ions in nitrogen gas in the energy range from 5 to 54 MeV. The velocity dependence of all the capture cross sections i s de- scribed by a power law with the power ranging from - 4 to - 8 , being larger for lower incident charge states and for more than one electron cap- tured in a single collision. Good agreement with

the theoretical predictions of Nikolaev has been obtained for the capture cross sections by the fully stripped fluorine nucleus. The ratio of double- to single-electron-capture cross section has been discussed in terms of a geometry-independent component of the capture process. Broad maxima have been observed in this ratio for the higher in- cident charge states. Maxima a re observed in the velocity dependence of the single- and multiple- loss cross sections and these a re consistent with the predictions of the model of Bohr and ~ indhard ."

*Work partially supported by the U. S. Atomic Energy Commission under Contract No. AT(l1-1)-2130.

IS. K. Allison, Rev. Mod. Phys. 30, 1137 (1958). 's. K. Allison and M. Garcia-Munoz, in Atomic and Mo-

lecular P r o c e s s , edited by D. R. Bates (Academic, New York, 19621, p. 721.

3 ~ . C. Northcliffe, Ann. Rev. m c l . Sci. 2, 67 (1963).

9. S. Nikolaev, Usp. Fiz. Nauk 85, 679 (1965) [Sov. Phys. Usp. 8, 629 (1965)l.

5 ~ . S. Nikolaev, Zh. Eksperim. i Teor. Fiz. 2 1263 (1966) [Sov. Phys. JETP 2.1, 847 (1967)l.

%. ItT. Martin, Phys. Rev. 140, A75 (1965). I. S. Dmitriev, V. S. Nikolaev, L. N. Fateeva, and

Ya. A. Teplova, Zh. Eksperim. i Teor. Fiz. 3 16 (1962) [Sov. Phys. JETP l5, 11 (1962)).

'J. R. Macdonald and F. W. Martin, Phys. Rev. A 4, 1965 (1971).

9 ~ . M. Ferguson, T. Chiao, L. D. Ellsworth, J. R. Macdonald, and S. A. Savoy, Bull. Am. Phys. Soc. & 74 (1971).

"s . Dalz, H. 0. Lutz, L. B. Bridwell, C. D. Moak, H. D. Betz, and L. D. Ellsworth, Phys. Rev. A l, 430

(1970). "H. D. Betz, G. Ryding, and A. Wittkower, Phys.

Rev. A 3, 197 (1971). "G. Ryding, A. Wittkower, G. Nussbaum, A. Saxman,

and P. H. Rose, Phys. Rev. A 3 1382 (1970). I3s. M. Ferguson and J. R. Macdonald (unpublished). he Hewlett-Packard Calculator, mode1 9100B, Pro-

g r a m Library. "H. Schiff, Can. J. Phys. 32, 93 (1954). i 6 ~ . R, Macdonald, S. M. Ferguson, L. D. Ellsworth,

T. Chiao, S. A. Savoy, and W. W. Eidson, in Seventh International Conference on the Physics of Electronic and Atomic Collisions, Amsterdam, 1971, edited by L. M. Branscomb et al . (North-Holland, Amsterdam, 1971). p. 516.

'IF. W. Martin and J. R. Macdonald, Phys. Rev. A 4, 1974 (1971).

"N. Bohr and J. Lindhard, Kgl. Danslce Videnskab. Selskab, Mat.-Fys. Medd. 28, No. 7 (1954).

"w. L. Wiese, M. W. Smith, and B. M. Glennon, Atomic Transition Probabilities (U. S. GPO, Washington, D. C . , 1966), Vol. 1 , p. 113.

P H Y S I C A L R E V I E W A V O L U M E 5 , N U M B E R 3 M A R C H 1 9 7 2

Excitation of the n = 4 Level of He' by Electron Impact on He ? R. J. Anderson and R. H. Hughes

Department of Physics,, University of Arkansas, Fayetteville, Arkansas 72101 (Received 14 September 1911)

The simultaneous ionization and excitation of helium to the n = 4 level of ~ e ' by 200-eV electron impact on helium has been studied by time-resolved spectroscopy of the h=4686-A (n -4-3) line of ~ e ' . The study revealed that the excitation process is complex. It appears that only about 70°h of the excitation of the h=4686-A line can be accounted for by direct simul- taneous ionization and excitation of the helium atom to the n = 4 level of He' under single elec- tron-atom collision conditions.

Absolute c ross sections for the production of the A = 4686-A (4 - 3) line of He' by the simultaneous ionization and excitation of He under electron im- pact has been determined by two different groups with results that a re in very close agreement.'" However, these experiments simply measured the total light output and did not resolve the fine-struc-

ture components in the line. Some means of reso- lution must be used in order to determine the cross section for exciting the individual fine-structure levels.

There a r e at least two ways that this line com- plex, which consists of the 4 s - 3 p , 4 p - 3 s , 4 p - 3d, 4 d - 3 p , and 4 f - 3d transitions, might be resolved. The frequencies of many of these components a r e sufficiently different that high-resolution interfer-

JOURNAL OF THE OPTICAL SOCIETY OF AMERICA VOLUME 60. NUMBER 4

Detection of Weak Light Signals* RICHARD J . ANDERSON

Departntent of Physics, UniversiCy of Arkansas, Fayetteville, Arkansas 72701

AND

J . CLEARY Department of Physics, University of Newca.de, N . S. W . 2308, Australia

(Received 23 October 1969)

The experimental signal-to-noise ratios (S/N) of four photomultiplier light-detection systems are obtained for two values of the dc anode signal and dc anode dark current. The photodetection systems investigated are dc amplifier, lock-in, noise power, and combined lock-in plus noise power. Although there is no significant difference between the experimental S/N ratios of the noise power and lock-in-plus-noise-power system, they both produce significantly greater S/N ratios than the other methods. Theoretical S/N ratios are calculated assuming that shot noise only is present, and are compared with the corresponding experimental values. INDEX HEADINGS: Detection.

In an earlier note Clearyl suggested that the noise- by l/fl from the dc case, when I,>>Id, and by the power detection method of Pao, Zitter, and Griffiths2 factor + when I,<<Id. On the basis of this, the addition (hereafter referred to as the PZG method) be used in of a lock-in detection system to the PZG system might conjunction with standard lock-in detection techniques either increase or decrease S/N compared to that of in order to enhance further its signal-to-noise (S/N) the PZG system alone. ratio. The earlier note overlooked the decrease of S/N The superior S!N ratio of the PZG method over con- which is always expected when a signal is chopped, ventional lock-in detection has been demonstrated by owing to the reduction of the average or effective Pao and Griffiths3 and by Alfano and Ockman4 in two amplitude of the signal. This effect is demonstrated by different applications of the PZG method. They ob- examining the theoretical S/N voltage ratios (based tained experimental values for the S/N ratios of photo- on shot noise alone) for dc and chopped detection multiplier detection systems, including lock-in systems schemes. and PZG systems. Unfortunately, Alfano and Ockman

Let I, and I, be dc cathode and anode currents of a did not measure the dc values of the photomultiplier photomultiplier, g the photomultiplier gain, B the signal and dark currents. Hence, their results do not effective noise bandwidth, e the electronic charge, in permit comparison between theoretical S/N ratios cal- the rms anode shot-noise current. Then for a dc detec- culated from the shot-noise expression and their experi- tion system, neglecting the contribution of all later mentally measured values. Alfano and Ockman assumed stages than the first to the shot noise of the photo- that their experimental S/N ratios were exactly those multiplier, to be expected from shot-noise alone and used this as-

sumption to calculate the dc signal and dark currents. in =g[2eI,B]i = [2egIaB]k, However, the results obtained in the present work show

since that the experimental S/N ratios may be many times la = gl,. less than the theoretical values. This is to be expected,

Since I , = I , + I d the S/N ratio is owing to the effect of l/j noise, whose contribution was shown to be significant by Pao and Griffith~.~ Pao

(S /N)dc=18/ [2eg(18+Id)Bl t , and Grfiths also showed theoretically that the PZG method, in addition to reducing lljnoise, discriminates

where '8 and I d are the ~hotomult i~l ier anode signal .gainst dark-current noise when the signal is less than and dark currents, respectively. the dark current, if the dark current is amplified less

For a PZG 'ystern, and Griffiths have shown than is the signal. This effect may be considered as that, to a first the S/N based on shot being comparable to pulse-height discrimination used noise is the same as for a dc ~ y s t e m . ~ in single-photon-counting schemes, where only signal

In a 'Ystern, the ~ h o t o m u ~ ~ ~ ~ ~ ~ ~ ~ anode cur- pulses above a specified height are counted. Dark- rent swings from Id to I d + I a . If the is current pulses generally receive less amplification than tangu1ar, with the rms current signal pulses, and therefore are smaller and are dis- is The mean dc current that causes the criminated against, resulting in an improved S/N ratio. noise is (Id+I,/2), and The aim of the present work was principally to

(S/N)l0~k-i~=I~/2[2eg(I,/2+Id)B]~. determine whether the system proposed by Cleary has a signal-to-noise advantage, as originally suggested,

Thus for a photomultiplier, chopping reduces the and to determine what other advantages or disadvan- theoretical S/N voltage ratio (based on shot noise alone) tages it has. We did this by comparing experimental

531

DF itronium ?ine stmcture Density sh i f t s in the captuze md the t i p br*sstr

Ga ::. H.H. YAM, p.0. =AN, W.E. ERIEZE, and V.W. ed togethess because at at idse Univ,- The fine s t ~ c t u r e interval bv i n shapes a? energies close t tho y~ state of Ps formed in the noble gases has similar and vary s10vl.y vl barn ~ ~ o a m r d u ing the microwave induced Zeeman reson- ionization. As a resul t .nc. teehAquelf for gas densities D ranging from 1 to 3 t i t i e s , the qlLantUJll defec a u . Udng $he musured value ~ v ( ~ & ) - ~ . ) 8 4 9 ( 1 2 ) OH2 b e n ~ i t y ~ need t o be c d c

the linear fractional denslt shif ts riel. As an example, r : : ($324o) /U) were r u t o be, a ( H e j - ( t l . ~ . 3 ) w n s tiy Mo ions rill &I-5, ( ~ e ) - ( - l . ~ . k ) r l @ 5 , a(~r)-(d.~&.j)x10-5 the dependence on inc r ( ~ r ) - ( - 6 . 6 4 . 5 ) d ~ 5 , a(~e)-(-7.3*1.7)~10'5(atm-1-0~~). Corpparin~ tlw Pa density shifts with those of HI we note:

' in part 1) the Pa shif ts are an order of magnitude larger, 2) In Under m74 !'I@ the Pa s h i f t 1s negative but the H shif t i s positive, C'M. Lee R'H. 1) In tJm heavy gases ( ~ r , K r , ~ e ) the Ps shif ts are m appmriautrly sgud but the corresponding H shif ts at differ appreciablyr Theoretical calculations of Ps dm11 ty rhif t1 are needed. DF 5 On t h e Connection B e t ~ t n ~ t o m i c Photoeffe~t and de

Internal Conversion* SUIPG DAHM'OH and R. H. PRATI'. *Rerearch rup~orted i n part by HSP under Grant hiP43&+'& of Pitt8burah.--It i s sho

I)rutach & S.C. B-, Phys. Rev, 3, 1047 (1952); range K. L. and

2 ~ - ~ . @MI U-d* lrieze, V.U. Hughes and U.H. Yam, sections are proportional to bo prblishal i n Phys. Lstt. A. efflcients from the same

dipole approxiaation the Pendent of shell and subs th i s ra t io holds t o 1 MeV

D F 2 ~lrubc~CrossSecttonsforExcItatIonofthe31 a t l o v e n e r g i e s , r e a c N n g oqen Atan by Electron Impact. F.T. C-nd threshold. lversit of Arkansas.-- s ng the Integral 'I niques if Thcinas mdce:.jkyl. the Glauber

rcdtterlnp smplItudes for the excitation of the 31 Supported i n part by the ?lati Science ~oundation (1 Oe),2) levels of atanlc hydrogen by electron Impact under Grant HPS?k-03531A01 h v e been obtaImd I n closed form. I t Is shown that the Gl rd t r mplltudes can be written In terms of four 9anW4tIng functtons: Two of these (for 3s. 3p) have rlrtrdy been dcrlved by Thomas and Gerjuoy. the detailed DF 6 raductlon of the other two (for 3d) t s gtven In thts PPlr. Csmpdrlson Is made with other theoretfcal models and with recent measurements.

' B ~ K . Thanas and E. Gcrjuoy, J. Math. Phys. 11, 1576 (1971)

Dr 3 Cxcltatlon of the C1rt Level of H by Electron . , . an

r a w . Uninnit of z d BGCI

~iscmsm!-The optical excitation fmctLm o t - '1. t r m s i t i m of the H, spec-

h a t A n ; C f ; d i n j for electron impact energies In t h ran 0-300 eV. M c a s u m t s of absolute optical CroJJ SeCtrmS have been carried out a t 200 eV electron ewrw md 30 mtorr, 112 gas pressure. Radiativc l i fe - t l m neasurrnrnt~ have 8130 ken carried out for the I

C'C' state and yield a single nude decay with a decay cdtmt of 38 nwc. Thc experimental measurements are I

mod almg with theoretical Franck-Condon factors t o cstlla3to the G ' Z ~ direct u c i t a t i m cross sect im.

*ksorrch Slprported by the Atmospheric Sciences Secticn, kit imnl Sciencc Fomdjtim.

Dr 4 R.d$.tirc capture or t e V elcctrans by ions.. c. u. a h - - hblat ivc c m t m o r el.ctmns & atamic ions is an i m - portmt p & ~ , in plasmas. ~ u e h an radiatirc tmv-bod J

recorbimtion my be clarmifid ar 1 ) indirect capture through ~utoionl t i ry rtater and 2) direct capture into th edtwrg r a i r r of the "electmn + ionC ~ s t a a . We h.w 8tudl.d dimct mdlatlru capture or electrons wlth kia.Cic mrrgies ~ r t w thn 1 KrV. The captund el=- t r m r l a dereribed u m i w in a Rut r teS1a tm central po te t i r l . A p p l y l u quaat= defect theoxy. the radiative

Dr 7 Collislonal uenching of Metastable X-Ray Emlk t i n States i n a F a 4 Beam of - ke.Fluorine.* R. J. -b Lab.--High resolution x-ray spectral me- deter

Dr 7 Collislonal uenching of Metastable X-Ray Emlk t i n States i n a F a 4 Beam of - ke.Fluorine.* R. J. -b Lab.--High resolution x-ray spectral meZG$%iE

Bulletin of the American Physical Society, v

KTe = 10 eV. A laser propagating in a direction opposite t o the direct im of propagation of the Nby laser is used to s r h i b t e the Thomwn scatter- ing. Q n calculaticns *te a single pass gain of aban 10-3. Although this gain is low it may be possible to masum it using p o l a r i z a t h enhancement as intro- k e d by Wieman and tHmich.1 Fossible future appli- c a t i m of stirmlated Thanson scattering w i l l be dis- assed .

C. Wi- and I. W. mmch, Pfr)rs. Rev. Letters 36, . 1170 (1976).

H34 Gaip. f o r Stinulated hmn S u t t e r i n g and . c ~ u i s i o ~ ~ n d u c e d n c ~ r c s ~ g l u : n. C. B~YXEB gnd J. L. URLS'EN. JIWL. UV. of Calo. 6 NBS.-We have masured the wins, both fonmrd and backuard, for s t i m l a t e d R&n scat ter ing (SRS) and stimulated col l i s iopinduced fluorescence (SCF), vhi+ vere ob- served upon Lrradiating a thallium-argon vapor v i t h a pulsed dye laser. We have used r a t e equations fo r dressed levels t o model the ti-dependent s t i m l a t e d sca t ter ing , thus providing an explanation for an ea r l i e r discrepancy betvera our e x p e r h n t and steady-state the0ry.l We found that, while the SRS gain is unaf- fected, t he SCP gain is suppreascd by the transient nature of the l a s e r pulse. In addition, w e obselsed tha t the SES gain coeff ic ient is much larger i n the forward than the backward direction, the r a t i o being equal to the r a t i o of the Lsser bandvidth t o the atomic l i n e vldth. a s predicted.'

+work supparted by the Office of E l a d Research.

1. W. C. R a p r and J. L. Carlsten. Phys.. Bev. Lett. 39. 1326 (1977). -

2. S. A. Akhanov, Yu. E. D'yakov and L. I. Pavlov. h. Phys. JETP 39, 249 (1974). -

H35 Infrared Radiation Facil i ty f o r Upper Amos- pher ic Gases.. R.J. C i r P i w . Department of Physics. University of Wisconsin and A.B. Fairbairn and S.J. -, ~ar r~cam~be t t s .++ -A f a c i l i t y f o r 8tudyi* upper a m s p h e r i c infrared radiation hrr bem c o ~ a u c t e d . A p a t h l w c h of 4 f t . is wed fo r the exci ta t ion of a lw density gau(3-?&I). Lov power electron, 6-8 LV. a c i t a t i o n is wed to pro- duce the infrared radiation. The i d r a r e d r a d i a t i o ~ is detected by e i the r a nichelson in ter fenmeter or a contimtously variable f i l t e r spectrmneter. Typical mpectra of atiwrpheric gasem w i l l be presented. % u b n i t t d by R.D. S h a m *%upported by AF Projec t 2310C405 a d DNA - Subtask llSBAM632

H36 ~i-~ejolved lnvest i~at ion of the G,K'X: states of &.* R. L. Oay and R. J. Andetson. University M Arkansas. -The radiative lifetimes of the f i r s t seven vibrational . . : levels of the second excited l q s ta te are obtained using pulsed-electron beam techniques., The levels. identified a s 6Kva(v'=0-6). correspond to consecutive vi b tional levels f" thin a double dnirmm potential function Rotatiom1 nes corresponding to the transition &s1e + BIZ+ are

ncd a t a spectral resolution of dl = 1 . k M kiden! trcn energy of 50 eV and H g& press- O W the e 5-400 mtorr, kasurenenfs of the - 8 radiative . '

times, extrapolated to zero pressure. are within 9 e l 25 m to s 90 ns while va ues of H 2. m h i n g s sections r a n fm r 90 d to 1 6 x :.,.

23, Issue 9, p 1104-1 106, 1978

~arfolmeci und& the auspices of the U.S. D e w - ment of Energy. 1- P- M. Dehmer an 1. Chem. m5. a

Calculation f Proton E-ection of Electmns fna Foils N. .Y IWN. BPtte le-~o$hwest, and H. -pAmWE' m L t f:r Strahlm:hut:.-- Wntc U r l o is de- "ribed for u l c u l a t i g e n e r g deposition and f0nfza t fa a b u t f a s t ion t racbh T k s e calculations i c o ~ r ! ~ " t e n ~ f ~ e use of expe i-tal ionfzation cross sect'onr and recent improveme & in electron transport bas4 O" the cri t ical evalua; n of data on electron interactio** Absolute double d i f f tO ntfal yields for elecVOn m'ss'on

thin foi ls are oRbined and compared dth d f W t m s u r a n t s for 1 neb protons pss ing through thin b n foils. The frac a n of the m t a l stopping w"' ap ing the foil in thf fom of energetic sc~ondaO electrons i s evaluate: for various foil thicknesses-

'S~ppo- in part by U.S. DOE contract ~~-76 - t -o~ -"~ ' -

m- Bulletin of the American Physical Society, v 2 1, Issue 2, p 157- 15 7 1976 I h 1

BE-4 Influence of the electron density on the modes Of shape ; the equivalent point source of the l a se r nvr a pulsed submillimetre laser*, A. MLMONIC AND M.C. SEXTUN, University Colleqe, Cork, Ireland -- The l ine determined and the characterist ics f the discharge

emission a t 3 3 7 ~ from a pulsed HCN l a se r (= 1 t o a , b

uas related to the l a se r performanc Skv, ZSA f o r 25usec) was monitored with vvroelectric

rd rut ;

C-

detectors. Simultanecus electron densiG-measurements during the lasing action were made with a B mm inter- f e r m t u . Both the asymmetry of the l i ne shape and cyclic variations i n output power with pulse frequency strongly indicated the presence of excited HCN molecules up to 20 m s into the afterglow. Moreover, calculations taking r ad ia l electron distribution^ into account supported quantftatively the presence of mde mmpres- sion near the c r i t i c a l density.

*Work supported i n part by the USAF (EQARD) and in part by culham Laboratory (KAEA).

BB-5 A Very Low Divergence N, W Laser. BRUNO Ubaratoires de Marcoussis, France -- A very low divergence (0.2 X 0.3 mad) E2 @V l a s e r is

described ; an in tens i ty of 5 Wd/mrad2 was obtained.

The average brightness was 2 .10 '~ ~/strd X cm2.

These r e su l t s were reached by using a re la t ively low

voltage transmission l i n e with an adequate electrode

ion . or

B e 6 E n e m Pathways FoUowine ~mdm !&citation of k-N2-SF6 Gas C. H. CHEN~J. P. JUDISH,G. S. HURST, and M. G. PAYNE,Oak Ridee Nz/tfl. Lab. --Time- resolved vw and visible specka due t b pulsed proton exci-

TUESDAY AFTERNOON, 21 OCTOBER 1975 BALLROOM EAST. UNIVERSITY CENTER AT 1:30 P.M. A Phalp, presiding

Reseasch sponsored by the Energy search ard Development k i n i s t r a t i o n der conkact with?. lhlim carbide carp. t~oetdoctora l research Kentucky supported by

C-3 E las t i c Scattering of Electrons i n Iblecular ?!&dragen. B. VAN YINGERDEN, F.J. DE BEER. E. UEIaLD , and K . J . NYGAARD'. FOKInstitute f o r Atomic and Molecular Physics, Amsterdam. The ~e the r1ands . -~ la s t i c scat ter ing Of electrons i n 82 has been studied i n the energy range from 100 t o 3000 eV and with laboratory scattering angles from 5 to SO0. Ihe r e su l t s are normalized to the absolute cmss sections of Jamen e t a1.l i n nitrogen and ampared with various theoret ica l Z u i d es aud several p r e 6 0 ~ relative experimental measurements.

*permanent addresses: Plinders university. ~ e d f o r d Pa** South Australia and (Jniv.of M.-Rolla, USA, ~ e s p e c t i v e l ~ .

R.8.J. Jansen, P.J. de Beer, B.J. Luyken, B. Uingerden, a d H.J. Blaaw, J . P ~ ~ s . B ( i n press).

C-5 E l e c t r o n I m p a c t Cross S e c t i o n b!easure- m e n t o f H e l i u m 2 J S + 3 J P . . . M L LAKt. u n i v e r s a Energ: y s emsSLDayton. 0 . and A. GARSClDDEN, AFiPL; YPAFB. 0.--A d u a l e l e c t r o n beam d e v i c e i s used t o measure the e x c i t a t i o n c r o s s s e c t i o n f o r t h e He 2's + 33P l e v e l s . Me tas tab1 es a r e c r e a t e d by i n t e r - S F c t i n g a n e u t r a l a t o m i c beam w i t h a f i r s t e l e c t r o n beam o p e r a t e d a t 23 eV. The meta- S t a b l e s a r e t h e n e x c i t e d t o t h e 3'P s t a t e

c o l 1 i s i o n w i t h l o w ene rgy e l e c t r o n s f rom a second e l e c t r o n beam, w h i c h i s moduTated. The Subsequent m o d u l a t e d 388.9nm emf s s f on from t h e second e l e c t r o n beam r e g i o n f s m o n i t o r e d . A b s o l u t e v a l u e s a r e o b t a i n e d b y comparison WI t h the d i r e c t 3 ' ~ exf cf ta t fo ! - D a t a i n d i c a t e t h a t Q(Z3S + 3") % 1 x 10 "cm'

SESSION C ELECTRON COLLISIONS I: ELECTRON TTERING, EXCITATION. AND IONIZATION k a t 15 eV, as compared d i c t i o n s o f 8.2 x 10 ,x 10-17cm2(Eikona l ) . 1

-. - . . . - - ham beenLoG'med f o r severdl s w d t a l l i n e s co rm- - -... -- .- - ~- ~~

spending t o the ZA+ ZB and 3 ~ - 6 4lectranic t rans i t ions of the neut ra l H spectrm. The t ansi t ions we ob- I C M ~ a t an e f d c t i v e spectral s l f t width o f I2 over an electrcm energy range-of

f o r the 2A state. tain an est inate o f the t o t a l appa nt cross section f *Research S q p x t e d by the Atno r i c Sciences Secfim. National Science Foundation.

1R. J. Spiadler Jr.. J. w t . A r c . M a t . Transfer. 9. 1041 (l569).

I

C-7 Measurements of the ~ l e c k r o n -act Ionization

m s s ~ect icms of Bbf IOIIS.* m. Y. E. suLE -uia -e of =edIno1---- be absolnte cross s e c t i m fo r t h t single ionhation of Bb+ ions by e l ec tmn b t i w o f incident eLecm

--- Bulleti~l of the American Physical Society, v 20, Issue 2, p 240- 40, 1975 i - I

WEDNESDAY MORNING. 23 OCTOBER 1974 CRYSTAL BALLROOM AT 9:00 AM. M.F.A Harrison, presiding

SESSION E ELECTRON EXCITATION OF MOL 4 CULES I

E-1 V i b r a t i o m l Exc i t a t i on of N 7 . CO. and N,O by Law E-4 V i b r a t i o n a l E x c i a t i o n a n d T r a n s m i s s i o n Energy ~ l e c t r o n s . " S.F. UONG and G.J. SCHULZ, Yale U -- ~ p e , - ~ r o s c o ~ t , in dr0 entsal ides.* J. p. z ~ E ~ ~ ~ A crossed-beam appa ra tus w i th e l e c t r o n bean r e s o l u t i o n 1. N-ER and c- :. sLULZ, Y a l e U. - -v ibra t iona l -. 22-35 meV i s used t o s tudy the abso lu t e d i f f e r e n t i a l v i b r a t i o n a l c ros s s e c t i o n s t o V-1 and v-2 of t h e ground near threshO1 has been studied in

s t a t e of N I B i n t h e e n e m range 1-30 eV. h o neu broad HCh (V = l a 2 ) and D C h ( v 2 , 3, with a trapped- i . resonant p;aks a r e observed nea r 8 eV and 13.7 eV respec- e l e c t r o n a p p a r a t u s . r he v i b r a t i o n a l c r o s s 1 t i v e l y , and they a r e i n t e r p r e t e d a s shape resonances.-In CO, which is i s e l e c t r o n i c v i t h N2 , an a d d i t i o ~ l mecha- nism was found:' Sharp s t r u c t u r e s have been observed i n t he e l a s t i c and v i b r a t i o n a l c ros s s e c t i o n s a t en r g i e s 5 co inc iden t w i th the PO, 1 and 2 l e v e l s of t he a n - s t a t e _ of CO. The l a r e e l e c t r i c d i p o l e moment (1.38 D) associ- a t ed wi th the aSn s t a t e appears s u f f i c i e n t t o bind tempc- r a r i l y t he incoming e l e c t r o n s a t o r near t he top of t he p o t e n t i a l w 11.-In N20. t he v i b r a t i o n a l e x c i t a t i o n v i a t he 2.3 eV ' 1 resonance is found t o c o n s i s t of fou r s e r i e s of modes (nOO.nlO,nOl and 002). The present ex- periment t oge the r w i th the calcuLat ion of Dub6 and Herzenberg sh t h a t t he impulse-limit theory is appl i - :able t o t h e 4; s t a t e of N20-. Work supported by NSF and by DNA through AROD

~ s . F . Yong and C.J. Schulz, Phys. Rev. L e t t e r s z, 134 (19 74)

E-2 v i b r a t i o n a l and R o t a t i o n a l E x c i t a t i o n o f CO b y Low-Enerqv E l e c t r o n s * - - ~ . ~ o r r i s o n t - 2 a n d N.F.Lane, p h y s i c s Depar tment . Rice U. I n e l e c t r o n - C 0 2 c o l l i s i o n s , r e l a t i v e l y l a r g e A j = i1 r o t a t i o n a l - e x c i t a t i o n a n d d e - e x c i t a t i o n c r o s s s e c t i o n s a r i s e f rom t h e ( t r a n s i e n t ) d i - p o l e moment w h i c h a c c o m p a n i e s t r a n s i t i o n s be- tween t h e g r o u n d state (000) a n d t h e b e n d i n g mode (010) . A model1 i n w h i c h t h e e l e c t r o n i s s c a t t e r e d f r o m a " f r o z e n d i p o l e m is shown t o y i e l d a t o t a l s c a t t e r i n g c r o s s s e c t i o n u n d e r r e s t r i c t e d c i r c u m s t a n c e s . C r o s s s e c t i o n s f o r t r a n s i t i o n s i n v o l v i n g t h e b e n d i n g mode 010 w i l l b e p r e s e n t e d a n d d i s c u s s e d .

s e c t i o n s r ise s t e e p l y n e a r t h r e s h o l d a n d l e v e l o f f a b o u t 6 0 meV a b o v e t h r e s h o l d . The magni- t u d e s o f t h e s e c r o s s s e c t i o n s h a v e b e e n measured ( a b o u t c m 2 f o r HCL v = 1 ) a n d a r e i n t e r - p r e t e d i n terms o f r e s o n a n t c o n t r i b u t i o n s . These l a r g e v i b r a t i o n a l c r o s s s e c t i o n s p r o d u c e s t r u c t u r e i n t h e t r a n s m i s s i o n s p e c t r a o f HCL, HBr and t h e i r i s o t o p e s . A d d i t i o n a l f e a t u r e s a r e a t t r i b u t e d t o Wigner c u s p s .

*Work s u p p o r t e d b y ONR + P r e s e n t a d d r e s s : C o l l i s i n s E l e c t r o n i q u e s . ~ n i v e r s i t ; P a r i s - s u d , 914 5 O r s a y ( F r a n c e )

f o r i nc iden t e l e c t r o n e n e r q i e s fn ;he ranqe 0-200eV. The

1 i nes were observed a t a spect a l 11 i t wid th of 811. and were a s s iqned t o correspondinq v i b r a t i o n a l and e l e c t r m l c t r a n s i t i o n s usinq t h e wavelenqnh t a b l e s of ~ i e k e ' . They include t h e s i n q l e t t r a n s i t i o n s nA + 2B, mD + 28 and mE +2B, and t h e t r i p l e t t r a n s i ions mc + 2a and 3d -t 2c where n=2,3 and ~ 3 . 4 . In a d d i t i o n t h e 2K + 28. 2U + 2B

'Suppor ted in p a r t by U.S. ~ t ~ ~ i ~ Energy * Research Supported by t h e Atm s p h e r i c Sciences Sect ion.

a n d R o b e r t A. Welch Foundat ion . National Science Foundation. * P ' F a n n i e a n d J o h n H e r t z F o u n d a t i o n Fe l low.

'R. T i c e a n d D. K i v e l r o n , J. C h m . phys . &j,

4748 (1967) .

t ion

24 0

coherence corresponding t o a few t imes the diffraction excimer deact ivat ion by Penning i o n i t a t i o n is evident

limit, and a sharp o s c i l l a t i o n threshold. Energy trans- a t high excimer dens i t i e s . The resu ts a r e compared f e r processes and ape ra t ion of t h e xenon syst0ll have been with recent experimental measurement of excimer decay explored in argon/xenon mixtures. following pulsed exc i t a t ion under co d i t i o n s of both

high and low e x c i t a t i o n density. Th app l i ca t ion of - S t a t e s Atanic Energy h i s s i o n .

1 This mrkw.s performed mder the auspices of the United the r e s u l t s t o the development of uv l a s e r s based on

the r a r e gas excimer bands w i l l be d scussed. I 6 3 Bxcimer Formation and Decay Processes i n Elec- t r o n Excited High Pressure Rare Cases. D. C. IAWml'S Stanford Research I n s t i t u t e . -- A t h e o r e t i c a l model has been developed t o descr ibe exci ted dimer formation and decay i n r a r e gases a t pressures above 1 atm. The model is based on a v a i l a b l e knowledge of the e l ec t ron ic energy deposi t ion processes, and c o l l i s i o n a l associa- t ion, recombination and re l axa t ion react ions occurring i n exci ted r a r e gases. Excimer formation and decay r a t e s i n Ar and Xe a r e ca l cu la t ed a s a function o f atom and e x c i t a t i o n densi ty . The importance of excimer-

G-4 S ntaneous and Superradian e Fmnission a t 1730 i i n E n o n G a s a t La* TenperatJe. Franck C o l l i e r and Chris t ian Co l l e t , Laboratoires d+ krcwssis-- Spectra l and time d e ~ e n d e n t s t u d i e s bf vacuwn u l t r a -

WEDNESDAY AFTERNOON, 17 OCTOBER 1973 SESSION HA WISCONSIN CENTER LAKE SHORE ROOM AT 3:45 P.M. Electron-Molecule and Charge Trontfer C W.L. Bont, presiding

. - copy has been ! exc i t a t ion of the C3il.. s t a t e o f N- bv ~ ~

U ~~

la*-enerqy eiectron Impact. k a s u r e m n t s were carrfed ' out on fhe A.3371 8 spec t ra l l i n e , correspondinq to the CQ,O) C il;~3il t r a n s i t i o n , a t 50eV incldent e lectron Seam enerqy an1 p u l n widths of 0.3 and 5 usec and for gas pressures in the range 1.4 t o 300 mtorr. The l i f e - t l m of the c3nU ( ~ 0 ) level i s deterrnlned t o be 39*4 nanoseconds whlch i s in aqreemnt with o the r expe r imnta l r e su l t s . In the s s e o f the 5 usec exc l t a t lon pulse width the ,413371 1 radtat ion . was observed t o contain a lonq-lived component ( r z l ~ s e c ) which comprises l e s s than 1.15% of the t o t a l radiat ion and which exh ib i t s a dependence upon the - t a rqe t qas pressure . Secondarv exc i t a t ion o f the c3nU s t a t e by co l l i s iona l transf;r from metastable s t a t e s i s beinq invest iqated as a func- tion of the qas pressure , e l ec t ron beam cur ren t , exci ta- t i on pulse width, and c o l l i s i o n chamber s i ze .

his work supported in pa r t by the National Science Foundat i m and the Research Corporation.

W- 2 Dissociat ive Exc i t a t ion o f Nr by Electron Impact: Transla t ional S ~ e c t r o s c o u y of Lom-Lived Riah-Rydberq Fragment Atoms. Kermit C . SMYTH, James A. SCHIAVONE, and Robert S. FREUM), Bell Laborator ies . Murray H i l l , Rev Jersey 07974.

I Nitrogen atoms i n long-lived high-Rydberg s t a t e s have been produced i n t h e d i s soc ia t ive exc i t a t ion of B2 by e l ec t ron impact. Four p r inc ipa l f ea tu res were found i n t h e the -o f - f l igh t d i s t r i b u t i o n s o f the d i s soc ia t ion fragments and i n t h e corresponding t r a n s l a t i o n a l (kinet- i c ) e n e r m d i s t r i b u t i o n s . Appearance po ten t i a l s and

i e x c i t a t i o n hrnct ione were measured f o r the high-Rydberg

'! a tons ; f o r the slowest fregments t h e exc i t a t ion h c - t i o n exh ib i t s sharp, resonance-like s t r u c t u r e near threshold. The core-ion model of high-Rydberg dissocia- I t i on . vhich t r e a t s t h e Rydberg e l e c t r o n e s s e n t i a l l y a s I a spec ta to r i n the d i s soc ia t ion process , i s used t o i n t e r p r e t t he da ta i n terms of t h e d i s soc ia t ive core ion

HA-3 Dissociative Ionization J. A. D. STOCKDALE, LILIANA COMPTON. Oak ~ i d e Na

elec t run energies from threshold

110".

k . w h sponsored by h e U. S. Atornil h e r g y Commission d e r con t rac t with Union Carbide Corppration.

HA-4 Vibrat ional Exci ta t ion of 0, b y ~ l e c t r o n Impact.* S. F. WONG, M. J. W. BONES and &. J. SCHULZ, Yale U.-- Vibrational exc i t a t ion of 0, bv eleedron imoact i n the

- . - - - ~ ~ - energy range 4 - 1 5 ev has g e e i s tud ied with a crossed- beam double e l e c t r o s t a t i c analyzer. The s c a t t e r i n g angle i s f ixed a t 25' and the r e so lu t ion is 60 mev. The d i f f e r e n t i a l v ib ra t iona l cross sec t ions f o r the v - 1 - 4 l eve l s of the ground x3z - s t a t e a r e found t o have s imi l a r bell-shaped energygdependences, v i t h a broad peak near 9.5 ev. The maximum d i f f e r e n t i a l v i b r a t i o n a l cross sec t ion f o r v - 1 is approximaoely 4.6 x 10-18 cm2/sr and the peak crosa sec t ions t o consecutive higher v ib ra t iona l s t a t e s diminish by a f a c t o r of about two. The d i f f e r e n t i a l cross sec t ions a t 25' f o r the exci ta- t ion t o the alhg and bll+ metastable s t a t e s a r e i n su s t a n t i a l agreement v i h the r e s u l t s of T r a j m r e t P a 1 . and s h w p a d s near 8.0 ev and nkar 9.0 ev respec- t i ve ly . The v ib ra t iona l and electrondc c ross sec t ions i n the range 4 - 15 ev a r e in t e rp re t ed i n terms of the resonant contr ibut ions of exci ted 02 s t a t e s .

*Work supported by NSF end by DNA t h r 1s. Trsjmar, D. C. Car tvr ight , and W. Rev. A 4.1482 (1971).

I s t a t e s of N2+ and t h e appropria te d i s soc ia t ion l i m i t s . In addition, the high-Rydberg kinetic dietrib,+ M-6 Light Emission from He+/Rare Gas Collisions. B . M . t i o n s u e found t o be i n reasonable q u a l i t a t i v e agree- '. " and^^^^^^ ment v i t h t h e k i n e t i c energy d i s t r i b u t i o n s of N+ Research Laboratory, Aerospace Research ~aljoratories.-- measured by d i s soc ia t ive ion iza t ion experiments. A new apparatus developed in our labolatories has been

I

Bulletin of the American Physical Society, v 18, Issue 5, p 814-814, 1973

FRIDAY MORNIlUG, 20 OCTOBER 1972 ROOM 200 T 8 : 30 A.M.

Sess ion OC

Elec t ron Impac t Excitation

Chairman: Paul Marmet, Universite Laval, Quebec. Qiebec

. 0c1 Excitation of fhe 3 3 ~ Level of He1 ium by Elec- ,tr?n Impact*; R.J. Anderson. R . H . Hughes and J.H. Tutig, Imversi t v o Arkansas.--The exchanoe exci tat ion of the

~ J P i e v e r o f helium by electron impact has been s tu led g' bv means of time-resoTved snectroscoov of the 3 3 ~ + 2 5 1;38891) t r ans i t ion . The study was ib r r i ed out fo r electron eneroies within the raise 40-400eV. a t a helium 0.825* and 0.725. 1

~

gas pressure i f -4mTorr. Under i h se conditions the total l igh t intensi ty of the A38998 lirle was observed to vary l inearly with both electron beam current and t a rge t gas pressure. Radiative cascade t rans i t ions from upper n 3 ~ and n 3 ~ s t a t e s wer observed to contr i - bute about 10% of the total A38891 l igh t ou.tput a t 40eV. This f rac t ion increased t o about 50% a t 400-eV electron impact energy. M i thin xperimental e r r o r , the 5 energy dependence of the 3 P d i r e c t exci tat ion cross sect ion was observed to decrease with increasing energy according t o the relati,on ( h e r g y ) - ) . This resu l t i s in agreement with he theoret ical calculat ions of Ochkur and Bratsevi which predict a s imilar dependence above -50eV.

1 . V.I. Ochkur and V.F. Bratsev, Opt. S'pectry. (USSR) 19, 274 (1965)

* Work supported by the National Science Foundation

OC2 Electron Excitation of H and Hz.' R . A . MICKISH and R . M . ST.JOHN, u. of Oklahoma.--Collision processes of electrons impacting upon H and Hq have been studied in a crossed beam experiment. A Wood discharge was used fo r dissociat ion of the molecules t o produce an atom en- riched beam. Optical cross-sect ion data was obtained by observation a t 90 t o the beams by a monochromator and photo~nult ipl ier system w i t h an absolute scale se t with a tungsten standard lamp. A s t a t i c system was used for observation of e-H co l l i s ions as it permitted d i rec t determination of t i e molecular number densi ty. Excita- t ion functions measured were f o r : (a) e-HZ co l l i s ions i n the flow system yielding molecular l ines , 5 l ines u i th the Wood discharge off and on; (b) e-H2 co l l i s ions i n the s t a t i c system and i n the flow system u i t h the Wood discharge off yielding atomic Balmer l i n e s through dissociat ion and exc i t a t ion , H, and HB; (c) e-(H+H2) co l l i s ions in the flow system with the Wood discharge on yielding Balmer l i n e s , H and t ig . Subtraction of (b) type data from (c) type 8ata yielded e-H co l l i s ion data. Relative exci tat ion functions were ohtained in each case from onset t o 190 eV and absolute cross sect ions were obtained f o r e-HZ co l l i s ions leading t o H, and HB Balmer l ines .

*Work supported by AFOSR, Grant AF-AFOSR 71-2051.

Oc3 Measurements of Electron Exci tat ion Cross @c- ions o t e i v l

t a t e s o tassium." ""N- a high-resoluti&&

o s i t y scanning Fabry-Perot interference swctrometer . a l l s i x Ze- components of the 4 2 ~ j / 2 + 4 2 ~ 1 / 2 trans: i t i o n of potassium produced by electron-beam exc i ta - t ion have been resolved under a magnetic f i e l d of 1.8 kG. At incident energies of 10.5, 16.8, and 23 eV, the r e l a t i v e i n t e n s i t i e s of the MJ = 3/2 + 1/2 and

* Work supported by the U.S. Air Force Office of

Sc ien t i f i c Research and by the Air Force Cam r idge Research Laboratories, off i ce of Aerospace 4 search.

f a s t orimary electroni ( ~ 1 keV). ~ e s u i t i n q band emir-

OC6 Excitation of 8and Emissions in Secondary and Primary Electrons. WALTER L . p s University

OC7 S c a t t e r i n g o f Alk.a.l.&_H_alides B y Low=r.- g y E l e c t r o n s , * M. G. F i c k e s , R. C. l a t e r , a n d R. C. S t e r n , C-ol-un,i-a. g.-- L a b o r t o r y d i f - f e r e n t i a l c r o s s s e c t i o n s - f o r t h e s c a t e r i n g o f t h e r m a l beams o f CsC1, K I , a n d T1F b 0 . 5 t o 1 5 ev e l e c t r o n s h a v e b e e n m e a s u r e d u i n g t h e m o l e c u l a r beam r e c o i l t e c h n i q u e . A 1 o v e l k i -

Nitrogen by BORST and a t

t h e o r y .

Carbondale.--5econdarv electrons were oroduckd in H9 by

* S u p p o r t e d b y t h e N a t i o n a l S c i e n c e F IR. C. S l a t e r , M. G . F i c k e s , a n d R. Phys. Rev. L e t t e r s 2, 333 ( 1 9 7 2 ) .

FRIDAY MORNING

O D 1 Measurement H +

nation coeff icients determined by mean: trometer apparatus. decnys i n helium-h) the decay of mass-i the microwave cavit sures of hydrogen i to dominate the ior electron densi ty dt controlled electror U(H +) (2.9 f 0.1 10-1 cmjls, a r e OW

respectively. A t t where ~ t j + i s the dc f 1.0) x 10-6 ,311 tions of these rest the atmospheres of ~ t e l l a r medium a r e

*mis research has (NGR 39-011-137).

OD2 Col l i s io Plasmas Produced i Veatch+ and H.J. 0 ninneapolis . The time de endenc 4, a, net8 and glow period of p la 0.01, 0.02 and 0.1 varying from 1 t o ionization of H2 b cm eec-1. The s t u the f i r s t t h e , of l i s ions between m e radiat ive l i f e t i m be 2.20.2 msec. l i s i o n processes u H$ i n helium was d sec1-1. Work mupported by the Air Force Comb *resent address: General E lec t r i c C

OD3 Mass Helium-Cesium A L.M. CHANIN, !JJ t i m e d e p e n d e n c e ~e:, a n d CS' we c o n t a i n i n g s m a l s p e c t r o s c o p i c t i o n s i n the r a n h e l i u m , and u p o f t h e CS+ d e c a

Bulletin of the American Physical Soc 7

:iety, v 24, Issue 4, p 663-663,1979

device using a short pulse Nd: glass laser i s proposed. A l i g h t u l s e propagates in a plasma with group velocity c(l - '/&I% and the pondermotive force of the photons leaves gehind a t r a i n of plasma waves as a wake with phase veloci ty same as thc photon group velocity. Such plasma waves t rap electrons and are eff icient accelera- to rs of electrons t a high energy. Either by preaccelera- t ion or by densi ty gradient it might also be possible t o accelerate ions. The maximum energy electrons can gain i s W = 2mc2u2/c {W = (w/w )kfc2[1 + (2u/u )4(m/~)%] f o r ions) . Wave brgaking s e t s l i m i t on the ?lectrostat ic f i e l d a t E = mcw / e . With a laser light focused to 1018 w/crn2 and plasmaPof densi ty l ~ ~ ~ c m - ~ , i t wrll take 1 cm t o accelerate electrons t o 1 GeV with electrostat ic f i e l d of 10' V/cm through t h i s mechanism. Computer simulations on the 1-2/2 D r e l a t i v i s t i c electromagnetic code have demonstrated t h i s conce t and confirm the scaling law fo r W a t least up t o (w/w 1' = 40. Applications to pulsars and cosmic rays a r e a l s o suggested.

*Work supported by NSF.

HI 3 Transport Coefficients in Haloqen-Nitroqen and Halogen-Rare Gas Mixtures*. K.J. NYGAARD, S.R. HUNTER+, H.L. BROOKS, S.R. FOLTYN, and R.A. SIERRA, University of Missouri-Rolla. --Usi ng the method developed by Grunbergl, we have measured the attachment coefficient and electron d r i f t veloci t ies for several halogen- containing gas mixtures. Specifically, resul ts have been in 12-N2, NF3-Ar, F2-Ar and CC14-Ar, f o r the range

of E/N from 1 to 40 Td (1 Td = 10-l7 1 cm2). Halogen concentrations were varied from 0.1% to 1 .OX, with t o t a l gas pressures of 10 Torr to 100 Torr. Cal- culations of the r a t e coefficient have been made from our data and compared t o the resul ts of other researchers.

'Supported i n p a r t by ARPA/ONR and Los Alamos Scient if ic Laboratory

t ~ r e s e n t address: Dept. of ~ l e c t r i ' c a l Engineering, Untv. of Liverpool, Liverpool, England.

'Grunberg, R., 2. ~aturfo;sch. z a , 1039 (1969).

HI 4 Electron Transport Coefficients in CO,&

mixtures have been measured a t total pressures ranging from 10 t o r r to 200 t o r r and over an / N range of ro r d to 220 Td (i Td = 10-17 vol t - c i ) . ~t low E/ri where Ionization in pure CO i s very small the attach- ment coeff icient (q/N) has Been measured using an integrated charge method. A t higher E / N the spatial current growth has been monitored t o obtain both attachement and ioni zation (a/N) coeff icient<. In addition, temporal analysts of the electron transient wave form has yielded d r i f t velocities In these gases. The resul ts are compared w i t h theoretical calculations

S and with ex i s t ing experimental data. I

*Supported in par t by Los Alamos Sc len t i f l c Labpratory.

HI 5 Time-of -Fli~ht Analvses of M a a n e t i c a l l v - S e o a r a t e d C h a r ~ e S t a t e s of I o n s from L a s e r B l o w o f f s . C . K . MANKA,Sam Houston S t a t e U . a n d M . R . CARRUTHJ: ,L.G. GRAY ,R. H. HUGHES ,R. J . ANDERSON,

b. a n d 0.H.ZINKE ,Uof Arkansas*"- -Plasma produced f rom blowoff by 25-MW ,Q-switched YAG l a s e r b u r s t s o n C , A l ,Cu , a n d Pb was a l l o w e d t o d r i f t , and t h e ions w e r e t h e n e x t r a c t e d and f o c u s s e d by a n E i n z e l l e n s o n t o a m a g n e t i c a n a l y z e r which p r o d u c e d c h a r g e - s t a t e s e p a r a t i o n . Each c h a r g e s ta t was a n a l y z e d b y t h e t i m e - o f - f l i g h c t e c h n i q u e 1 > $ . E x c e l l e n t f i t s t o Maxwell- ~ o l t z r n a n n distributions were observed and t h e

= plasma t e m p e r a t u r e and f l o w v e l o c i t i e s were c a l c u l a t e d . Non-common t e m p e r a t u r e s f o r i o n s of d i f f e r e n t c h a r g e s t a t e s from s i n g l e plasmas are i n d i c a t e d .

la ted Compton mode i s derived witho density of t h e electron beam employ the exact r e s u l t in Powers af the 1

* Work supported by AFOSR contract 44620-75-C-0055. 1

1x1 reproduces the vacuum gain fotmtlal , the leading plasma correction causes ment (not reductlonl) of the vacuuri also generalizes past work by incluc

ated payload.

and shows t h a t a s l i g h t enhance- gain. The theory

ing a s t a t i c guide

HI 8 Intense Re la t iv i s t i c ~ l e k t r o n Beam Expanding in to a Field-Free Vacuum*. P. MU J. SMITH, and W. 0. DOGGETT, Nort Ralei h N. C.--An intense r e l a t i v s t i c electron beam produbced i n NCSU's 7 ohm diode fO.1 MeV, 70 W) is f i red

magnetic f i e l d and an a rb i t ra ry d i s momenta; t h e principal assumption ava i lab le length (1.e.. a recycled beams ( I x ( > l ) , i t i s shown tha t s t i scatterfng p e r s i s t s In a f in i te - len

*Supported by AFOSR Contract No. 49620-76-C-0007 **On eabbatical leave from U. of cranton. Scranton, PA 1E. J. Henley and D . Richman, Ana :Chem. 2, 1580 (1956). i

the Compton gain can e a s l l y r iva l Ramn gain which i s derived f o r possibility of an x-ray lase r operated regime (plasma-modtfied Compton

t ? e f ini te- length com>arison. The

in t h i s new e f f e c t ) i s discussed.