Excited Species from a Pulsed Discharge in Helium at One Atmosphere Pressure

Excited Species from a Pulsed Discharge in Helium at One Atmosphere Pressure

WAYNE E. WENTWORTH,* YANG QIN, SOPHIE WIEDEMAN, STANLEY D. STEARNS, and JANARDHAN MADABUSHI Department of Chemistry, University of Houston, Houston, Texas 77204-5641 (W.E.W., Y.Q., S. w.); and Valco Instruments Co. Inc., Houston, Texas 77055 (S.D.S., J.M.)

The relative intensities of atomic emission lines have been analyzed in regard to a Boltzmann distribution of the electronic levels in the pulsed discharge. The analysis confirms a Boltzmann distribution with an excitation temperature of 3200 _+ 220 K, a relatively low temperature compared with that for other excitation sources, such as microwave and radio-frequency discharges. The analysis also suggests that little ionization occurs via direct excitation in the discharge. The emission spectra from excited diatomic helium states have been analyzed and confirm the formation of He2(a3~ +) and the Hopfield emission He2(A']~+ ~ 2He(1 ~S) continuum in the range 72 to 92 nm. Emission intensity-time profiles have been obtained for both atomic and diatomic helium emissions. Analysis of these profiles indicates that excited He2 states are obtained by two reactions: (1) an excited atomic helium reacting with a ground- state helium atom, and (2) recombination of He~ ÷ with electrons. The study concludes that excitation in a discharge through helium at atmospheric pressure yields the following predominant species: He(23S), He~(a3~+), Hopfield emission continuum 72-92 nm, and He~ +.

Index Headings: Emission spectroscopy; Pulsed discharge excitation mechanism.

INTRODUCTION

For the past four years we have been investigating the use of a high-voltage pulsed discharge as a source of ionization/excitation in gas chromatographic detectors. The relatively low power applied to the discharge allows the electrodes to remain cool. This condition is probably one of the principal factors in the stability of the discharge, and stability is critical for a sensitive, reliable detector. Preliminary papers have been published on the use of the discharge as an ionization detector,~ and electron capture detector, 2 and an atomic emission detector for elemental analysis? The source can also be used for ionization in an ion mobility spectrometer and in an atmospheric-pressure ionization/mass spectrometer.

During the course of these studies we carried out spec- troscopic analyses under varying conditions. In the cell, parameters included flow rates and gas composition: for the discharge itself, we varied discharge voltage, pulse frequency, polarity, etc. These analyses included the wavelength-resolved emission spectra as well as the time dependence of the emission, since the discharge is pulsed at a low frequency (~ 2-3 kHz). The duration of the discharge appears to be sufficiently short to allow time resolution of the emission from different species.

The composite of this type of information gives sub- stantive support for various mechanisms for excitation and/or ionization. The purpose of this paper is to present

Received 1 July 1994; accepted 28 April 1995. * Author to whom correspondence should be sent.

this unique information and to analyze the results in regard to various excitation/ionization mechanisms. The results presented apply only to our high-voltage, low- frequency, pulsed discharge in helium. However, there are several other techniques that can be used to obtain atomic emission for elemental analysis. A recent text by Uden 4 summarizes several of the techniques used for this purpose, and to a certain extent the results from the present study apply to those which use a helium carrier gas.

EXPERIMENTAL

This study involves two areas: (1) study of the emission spectra, and (2) measurement of emission intensity-time profiles. This approach requires two separate experimental apparatuses; therefore, the equipment and procedures used for each area are described separately.

Helium Emission Spectra at Atmospheric Pressure. The overall experimental setup is shown in Fig. 1. The helium used is Airco Grade 6 (99.9999% purity, commonly called "6-nines"), which is further purified by passing it through a VICI gas purifier (Valco Instruments Co. Inc.). This purifier contains gettering material that removes all im- purities except the inert gases. The helium flow passes through a valve that controls the flow to the pulsed discharge emission detector (PDED).

The high voltage required for the discharge is supplied by an automotive coil (E-30 Borg Warner). The power to the coil is supplied by a 20-V dc potential from a Heath 2718 Tri-power supply. The frequency of the discharge and the charging time of the coil are controlled by a 4001 Ultravariable Pulse Generator (Global Specialties Co.).

A schematic diagram of the detector is shown in the upper portion of Fig. 2. The discharge electrodes are made from V32-in. stainless steel rods with the ends sharpened to a fine point. We have used different materials and modes of construction for the discharge electrodes, but since no metal emissions are observed, the emission spectra of helium appear to be independent of the type of electrodes used.

The contour of the detector on the right side is designed so that the light emitted through the silica window is directly in front of the entrance slits of the GCA/Mc- Pherson EU700 Monochromator (GCA/McPherson In- struments, Acton, MA). The discharge is aligned parallel to the entrance slit so that the image falls almost within the acceptance angle of the monochromator (F-number = 6.8). The image is thus defined by the entrance slit. This configuration for the detector is awkward, but it offers the best solution when the entrance slits are recessed into the monochromator. Positioning the discharge this

1282 Volume 49, Number 9, 1995 0003-7028/95/4909-128252.00/0 APPLIED SPECTROSCOPY © 1995 Society for Applied Spectroscopy

He 6 grade

electrometer

I

photomultiplier

1

power for PMT

F I G . 1.

t---t

gas purifier

recorder ---t

makeup helium

mono- t ~ . chromator - -

I I

for ___ ignition ___

discharge coil

gas loop - - connection . . . . . . . . instrument

connection

pulse generator

Block diagram of the experimental setup for obtaining helium emission spectra.

close to the entrance slit should give greater sensitivity than the alternative use of an optical train with a con- densing lens. The monochromator contains a 1200-lines/ mm grating with a blaze at 250 nm, with dispersion of 2 nm/mm and maximum resolution of 0.1 nm at 250 nm. The detector is an EMI 9781B UV Glass photomultiplier tube (PMT) (EMI Gencom, Inc.) with a range of 180 to 650 nm. A VICI differential electrometer (Valco Instru- ments Co. Inc.) is used to amplify the PMT signal, and the resulting potential is recorded on an Omnigraphic 100 Recorder (Houston Instrument Co.).

We also have the capability of measuring emission spectra in the vacuum ultraviolet region with the use of an Acton 502 monochromator. Measurements can be made from 60 to 200 nm by purging the monochromator with helium and eliminating the entrance window. The

pulsed discharge source is coupled directly to the monochromator.

E m i s s i o n I n t e n s i t y - T i m e Profiles. The experimental apparatus for measuring the emission intensity-time profiles is similar to that used for making the helium emission spectra, but there are significant differences in the individual components, as well as additional components. A block diagram of the apparatus is shown in Fig. 2. The grade-6 helium is purified in the manner discussed previously, and again the helium flows to the PDED.

A schematic diagram of the detector used in this study appears in the lower portion of Fig. 3. The main body is fabricated from polyetheretherketone (PEEK), and the discharge region is similar to the one in the detector de- scribe above--the platinum-tipped electrodes are opposite one another across the central V~6-in. hole through

He 6 grade

photomultiplier

I I

high speed a/d

converter i [

computer for

data collection

gas purifier

computer to run mono-

chromator

I I

monochromator

power for

discharge

-4 makeup helium

t- E.::T_ I

ignition coil

printer

gas loop connection

Instrumental . . . . . connection

pulse generator 1 :

discharge I

1 I II pulse t_ generator 2:

mode trigger

Fi~. 2. Block diagram of the experimental setup for obtaining emission intensity-time profiles.

APPLIED SPECTROSCOPY 1283

Gas Make Up Outlet I ] Gas Inlet

Chrom~ Columr

IV

Spark Electrode

Stainless Steel m Insulate" (PEEK)

Spark Electrode

Gas Inlet IV

Gas 1 Outlet

Insulator (PEEK) ~ Stainless Steel

FIG. 3. Schematic diagrams of detectors used in this study.

which the emission passes in order to enter the monochromator. The helium gas flow pattern is opposite to that of the first configuration, but for helium emission this factor should not be a concern. The quartz window separating the detector from the monochromator was purchased from Optical Instruments Lab, Houston, TX.

In this setup the monochromator is a DigiKrom 122 double monochromator (CVI Laser Corp). Each monochromator is a ½-m instrument with fixed slits, and the monochromators have two gratings which can be selected by external control. One grating contains 2400 grooves/ mm and is blazed at 240 rim, while the other contains 1200 grooves/mm, blazed at 600 nm. The wavelength

1284 Volume 49, Number 9, 1995

settings, scan speed, and grating selection for the monochromator are controlled by an AT&T PC 6300 computer and software supplied with the monochromator.

The dc power supply and the pulse generator are iden- tical to that in the first part of the study. The nature of the coil and the polarity of the discharge are varied for different experiments and will be specified as the data are presented. The PMT, high-voltage power supply, and electrometer are built into one unit, so that noise arising from the electrical cables commonly used to connect those units will be reduced significantly. This combined unit is sold by Hamamatsu Corp. (Model HC 120-05). The signal from this unit is fed as a voltage to a high-speed

~D

,4

¢'-1 ~

I : 1 : 1 " ~ . 0

O

~ O

m ~ ~ N =11

~ It""- + ~

t"q

eq ~'%

190 FIG. 4. UV-visible emission spectra of helium at atmospheric pressure using the high-voltage pulsed discharge.

590 (nm)

analog-to-digital converter (Model R2000, Rapid Sys- tems Inc., Seattle, WA). The digital data are then pro- cessed with a Lotus 1-2-3 spread sheet in an IBM computer 486SX, and the emission intensity-time profile is displayed on a Hewlett-Packard LaserJet III printer.

RESULTS AND DISCUSSION

H e l i u m A t o m i c E m i s s i o n Spectra. Figure 4 shows the emission spectrum from highly purified helium, obtained with the high-voltage pulsed discharge. The spectrum was recorded with the GCA/McPherson EU700 monochromator with a spectral resolution of 0.26 nm for the helium spectrum and 0.2 nm for the rotational fine structure. In this spectral region the highest energy atomic transition would be at 259.3 nm, the energy change from the ionization limit to the lowest excited state in helium, the 23S state. Indeed, the atomic emission lines observed do have wavelengths in excess of 259.3 nm. The highest energy transition observed with this excitation source is the 73p

23S at 276.4 nm. The atomic spectral lines are relatively narrow, and for the most part are obvious in Fig. 4. However, there are several He2 transitions, and these broader bands also can be seen in Fig. 4. These will be discussed later.

The pressure of the helium used to obtain the spectrum

is probably in excess of one atmosphere, since the outlet & the cell must be restricted so that back diffusion of the atmosphere into the cell is minimized. Note that there is a small N2 + peak at 391.4 nm and a smaller peak at 427.8 nm. These arise from the (0,0) and (0,1) vibrational components of the B2Z + ~ X2~+electronic transition for N2 +. The weakness of the N2 + emissions indicates a low level of back diffusion and relatively pure helium gas. There are some carbon and CO + emissions with wavelengths less than 259.3 nm, probably arising from carbon de- position on the electrodes from some carbon-containing compounds that have been studied previously. We do not think that the carbon at this low level causes any significant alteration of the helium emission.

The relative intensities from helium vs. impurity emissions such as N2 + at 391.4 nm depend upon the conditions of the discharge, such as pulse frequency, discharge voltage, and polarity. The spectrum recorded in Fig. 4 was obtained with a relatively high pulse frequency (9.1 kHz) and a positive discharge potential. These conditions favor the emission of helium, both atomic and molecular, over that from N2 + and C. As will be discussed in more detail later in the paper, a positive discharge potential reduces the N2 + emission, apparently because of enhanced electron-positive ion recombination.


TABLE I. Atomic hel ium transit ions observed from a high-voltage discharge in hel ium at P = 1 atm.

Transition A, I / X (nm) states E~ (eV) I? (108 s ~) g~ (A~,,e E,/*r)

282.9 63p ~ 23S 24.20 1.89 0.0169 9 2.25 294.5 53p ~ 23S 24.03 3.819 0.0293 9 1.22 318.8 43p ~ 23S 23.71 20.588 0.0505 9 1.80 388.9 33p ~ 23S 23.01 296.875 0.0958 9 1.65 396.5 4LP ~ 2tS 23.74 8.52 0.0717 3 0.72 402.6 5 3 D ~ 23p 24.04 16.19 0.117 15 2.31 412.1 53S ~ 23p 23.97 5.835 0.0430 9 1.83 447.1 43D ~ 23p 23.73 136.75 0.251 15 3.65 471.3 43S --~ 23p 23.59 26.567 0.106 3 1.11 492.2 4~D ~ 2~P 23.74 37.50 0.202 5 1.40 501.6 3~P ~ 2~S 23.09 150.75 0.1338 3 1.00 504.8 4IS ~ 2~P 23.67 10.7 0.0655 1 1.00 587.6 33D -* 23p 23.07 3125 0.609 15 4.32 667.8 3~D ~ 2~P 23.07 642.12 0.638 5 1.12

" I, is the relative intensity that has been calibrated for PMT sensitivity, grating efficiency, and silica window transmittance.

The helium atomic emissions that were observed under these conditions are listed in Table I. Note that more intense emissions were observed between triplet states rather than singlet states. The most intense series are n3p --* 23S and n3D ~ 23p. The relative intensities shown for the different transitions have been corrected for the photomultiplier sensitivity and grating efficiency dependence on wavelength. Because of our photomultiplier sensitivity, transitions at wavelengths >600 nm could not be observed with sufficient intensity. For this reason, transitions terminating above the 23p and 2JP energy states could not be observed. A more quantitative evaluation of the relative occupancy of triplet and singlet states will be given below.

Rydberg Character of Excited States. The transitions for atomic helium which we observed are designated by arrows in the energy diagram in Fig. 5. The resonance transition (21p --~ i~S), shown with a wavelength of 58.44 nm, was not observed from our pulsed discharge. Most certainly this behavior is due to reabsorption by the helium at a pressure just above 1 atm, both in the PDED and in the monochromator

As can be seen from Fig. 5, the excited states for helium are at very high energies above the ground state. For this reason one might expect those upper states to be Rydberg- like in character, and the transitions should fit the well- known Rydberg equation.

where R is the Rydberg constant, and nL and nH are quantum numbers of the lower and higher energy states of the transition. We have examined this relationship for the series n3p ~ 23S and n3D ~ 23p. Graphs oft7 vs. [(1/n~) - (1/n~)] for these series are shown in Fig. 6, and from the linearity of these plots it appears that the energy states are Rydberg-like. The data for plotting the n3p ~ 33S and n3D ~ 33p series were taken from the literature?

Boitzmann Distribution and Discharge Excitation Tem- peratures. The mechanism of populating these excited atomic states in the discharge is of fundamental impor- tance to our understanding of the complex excitation pro-

cesses going on in the discharge. The excited states could be populated by direct excitation from the high-energy electrons in the discharge, or by electron-positive ion recombination after the helium is ionized in the discharge. In the former case one would expect a Boltzmann distribution among the atomic energy states according to the well-known Boltzmann equation.

n, = - ~ e-E, j~T (2)

where ni is the number of atoms in the ith energy state Ei; N is the total number of atoms; Z is the partition function; gi is the statistical weight of state i (degeneracy); k is Boltzmann constant; and T is the absolute temperature, commonly called the excitation temperature. The radiant intensity, Ii, for a transition in a series is given by 6

Aihc~ni Ii = 4~" (3)

where A~ is the transition probability; h is Planck's constant; c is the speed of light; ~ is the wavenumber of the transition; and ni is the number of atoms in the ith state. Substitution for ni from Eq. 2 into Eq. 3 and upon re- arrangement yields

1 °bs f h c N E i

In Ad,gi - In 4~rZ k T (4)

wheref i s the fraction of the intensity which is observed, Ii obs.

A graph of ln(I?WAdg~) vs. E~ should be linear with a slope of 1/kT. The data for such a graph are shown in Table I for various transitions, and the graph is shown in Fig. 7. The data fall on a straight line quite well, with the exception of four data points. These four points for the most part have low intensities, and consequently would have a larger error. The data have been fit by least-squares, assuming constant relative error in i?us. From the slope we calculate an excitation temperature of 3200 _+ 220 K.

Even though the graph in Fig. 7 is quite linear, one cannot infer that the distribution is a true Boltzmann type, as would be expected by thermal excitation of the ground state. Rather, Fig. 7 simply shows that the population of excited states fits a Boltzmann-type distribution, and de- scribes the rate of falloff in occupancy of higher energy states. This result is useful in that one can deduce that direct ionization by the pulsed discharge has a very low probability in comparison to excitation to excited atomic states. Also, one can predict from this Boltzmann-type distribution the relative direct excitation to the lower excited states, such as the long-lived He 23S. The excitation temperature is a pseudo-temperature which would be required to give this observed falloffin occupancy of excited states.

Having established the excitation temperature, we can now quantitatively account for the relative occupancy of the triplet and singlet states. In order to evaluate the relative occupancy of the triplet and singlet states we need to account for the transition probability, &; the energy of the transition, 6; and the Boltzmann factor e-e,J~r, where Ei is the energy of the upper state in the transition and T is the excitation temperature. The ratio I /Ad, e -ei/kr


eV

25

24

23

22

21

20

19

18

17

I /

0

1S 1p !

~ \ /~D 501.6 3 S m

:S ,~Z--.

2P._~._L. 58.44

f I

I

I

I

I

I

I

1 D !

5D 438.8 4Dr492.2

667,8

3 S 3p I

4S 471.3 k

3S

7P 276.4 sp~- -

4~ 318.8

388.9

21

2S__L-_

3 D I

7D 370.5

587.6

Fic. 5. Energy level diagram for helium. Wavelength (in nm) for each transition is given at the upper energy level.


~'(crn ~)

40000.0

35000.0

30000.0

25000.0

20000.0

15000.0

10000.0

/

I I I I ~ " I I I 5000.0 ~ 1 ~ n n n I n I ~ 1 ~ ~ I

0 0 . 0 5 0.1 0 .15 0,2 0 .25

(59 nL nH

FIG. 6. Rydberg series graph for a tomic helium: (4) n3P--, 23S series; (X) n3D ~ 2~P; (.) n 3 p ~ 33S; (~) t/3D ~ 33p.

should be a measure of the relative population of the various states, and these are shown in the last column of Table I. There are five transitions for which there are both singlet and triplet states, and these can be compared for the relative occupancy of the triplet to singlet states. The ratio of the I / A d # -E~/~r for the 43p ~ 23S transition to the 4tP ---, 2~S transition is 2.5. Similarly, the ratios for the transitions 3P ----~ 2S, 4D ~ 2P, 3D ----~ 2P, and 4S

obs

In Ii Aivigi

-3

-4

-5

-6

-7

-8

-9 1.84

• +

O

1.86 1.88 1.9 1.92 1.94 1.96

ei (xl0-5cm "1 ) hc

FIG. 7. Graph ofln(I°~VAd,g,) vs. E,/hc: test o f Boltzmann distribution. (~) naP ~ 23S; (x ) 4 'S ~ 2'P; (o) n3D---, 2~P; (~) 4~D ~ 2tP; (+) n3S ----~ 23P; (O) n'P ~ 2~S.

--, 2P are 1.68, 2.60, 3.86, and 1.1 1, respectively. The average of these ratios is 2.34, and obviously the occupancy of the triplet states is considerably greater than the singlet state. The exceptionally low value of 1.1 1 arises from the unusual lower transition probability reported for the singlet state compared to the triplet state for the 4S---, 2P transition. Errors in the transition probabilities can be as high as 50%5

The logical explanation for this preponderance of occupation of triplet states over singlet states is that the threefold degeneracy promotes occupancy of three times

TABLE II. Molecular helium transitions observed from a high-voltage pulsed discharge in helium at P = 1 atm.

Electron configuration Wavelength" (nm) Transition states Intensity b

6p~r- l sa2 2p~ 2s~r 320.7 p31lg --'* a3 ~ Very weak 5prr- 1 set 22pc~2sc~ 335.7 13II~ --~ a 3 ~,,+ Weak 5p~-lsa22p~2so 355.5 k'3~. + ~ a3Z, + Weak 4p~r-lso22p~2scr 367.7 i3II~ --~ a3Z,, + Moderate 6d &~r,a-ls~r22p~2pTr 377.6-378.7 q3A,, 3II,, 3)2,,+ --~ b3II~ Very weak 5d &~r,a-lso'-2pa2prc 397.6-399.7 m3A,,, 3II,, 3~,).---~ b3Hu Weak 6scr-lsa22pcr2p~r 380.4 o3Z~ ~ b3II~ Very weak 4pTr-I sa22pa2so 400.3 P l i e ~ A ' ~,,+ Weak 5sa-lsa22~a2rlr 403.2 k3Z, + --" bqlg Weak 5d 6,7r,a-lsa22pa2pTr 415.8---417.4 MtAu, qI, , tE d ~ B ~ II~, Weak 4d 3,7r,~r-lsa22po2pTr 440.4--445.7 j3A,, 3ii,, 3Z,I ____~ b3iig Moderate 4sa-lsa22pa2p~ - 454.7 h 3 ~ ~ b3Ilg Weak 4d 6,Tr,cr-lsa22po2pTr 462.4.....466.5 J~A,, ql , , t2~,,+ --~ BtII~, Weak 3pTr-lso~2p~r2sa 464.9 e3II~ ~ a3~v,1 Intense 4sa-lso22pcr2p~r 472.5 H~2;~ ~ BLII~, Very weak 3plr-1 so22p~2pr 513.4 Eql~ ---' A 12,,+ Weak 3d &~,o-lsa22pa2p~ 573.5-595.9 f3A,, 311,,, 3E,~ ~ b3Hg Moderate 3d ~5,Tr,a-lsa22p~2p~r 611.2-631.4 F'A,, 3II,, ~Z,, + -- ' Bqlg Weak 3sa-lsa22pa2prr 640.1 d3]~ ---' b3llg Weak 3sa-lsaZ2pa2prr 659.6 DlE;~ --~ BtII, Weak

" All the wavelengths (nm) are based on (0,0) transition between two electronic states. b Intensities are not correct for PMT sensitivity or grating efficiency and wavelengths greater than 600 nm are beyond the PMT range.


eV

25

24

23

22

21

20

19

18

17

I J

0

1 + 3 + Eu li] u llqg 1A u Eu 3Iqu 3Hg 3A u I I 1 1 1 I I

M 417.4 M 416.7, J 466.5\ j 465.1\

: 634172~4~'5 ~ F 624.7 ~

335.7 397.6 M415~ m 399,7 m 398.9 I m, 440~.0 //~r~~ '4405 400.3 d 462.4 403.2 i~45~.7 | !F ~-==

454.7 o f

B .~_.V.._

a

Fio. 8. Energy level diagram for diatomic helium. Wavelength (in nm) for each transition is given at the upper energy level.


Q(3) Q(1)

I ~ Q(5)

R(3) R(5)

R(7), ,

R(1)

Q(7)

P(3) Q(9)

P(5) P(7)

P(9)

P ( l l )

A P(13)

462.0

Fro. 9. H% rotational fine structure in the electronic spectrum for the (0, 0) transition of e3IIx ~ a~E2.

472.0 (nm)

that of singlet states. The average ratio of 2.34 is considerably lower than the expected 3.0; however, there is considerable scatter in the five ratios, as mentioned previously.

Diatomie Helium Emission. The emission spectrum for helium shown in Fig. 4 also contains several emission bands arising from excited states of diatomic helium. Some of the more intense bands which were produced from the pulsed discharge source appear at 440.5 and 464.9 nm. Other less intense bands are also noted in Fig. 4. These become obvious when the spectrum is run with a higher photomultiplier sensitivity.

The observed emission bands from He2 are given in Table II, along with the states that have been identified with each transitionY An energy level diagram for diatomic helium is shown in Fig. 8, so that the energy levels can be compared with those of atomic helium. As described previously, we also have a helium-purged monochromator with a wavelength range 60 to 550 nm, which allows us to observe the Hez(A~: + --* X~Z +) continuum in the range 72 to 92 nm. The X~: + dissociates to 2He(1 ~S).

Some of the He2 emission bands contain sufficient fine structure resolution to allow a rotational analysis. There are points at which this analysis cannot be precise because intense atomic emissions overlap, such as the emission at 440.5 nm overlapped by the 447.1-nm line. However, there are three transitions, at 464.9,367.7, and 513.4 nm, that are sufficiently intense and free of atomic helium emission to ensure that a precise rotational analysis can be performed. The rotational fine structure spectra for these transitions are given in Figs. 9, 10, and 11. Note the somewhat different shapes in the P, Q, and R branch-

es. This difference stems primarily from the difference in the internuclear distance in the two electronic states. In all of these transitions the internuclear separation in the upper electronic state exceeds that of the lower state, resulting in fine structures similar in appearance.

The spectra shown in Figs. 9, 10, and 11 appear to be well resolved, but this is true because only the transitions of odd J are allowed. 9-~ The assignments for the various rotational transitions, A J -~ J J, within the P ( A J = - 1), Q(zXJ = 0), and R(2xJ = + 1) branches are shown in Figs. 9, 10, and 11. The J value shown for each transition is the rotational quantum number associated with the lower electronic state in the transition. Equations for the R and P branches can be combined in the following way:

gR(J) -- gp(J)= B'~(2 + 4J) (5)

~R(J - 1) - gp (J + 1)= B~"(2 + 4J). (6)

For each combination of R and P branch data a B'~ or Bf can be calculated, and from this the internuclear separation, re. The average of each of these re is given in Table III, along with the standard deviation. Also shown for the re for each electronic state is a value from the literature. Generally there is good agreement between the values calculated in this study and the literature values. With this agreement in the re, there is little doubt that these are H% transitions.

Of even greater significance is the use of the intensities for the rotational fine structure to estimate the rotational excitation temperature, assuming a Boltzmann distribution over the rotational states. For the P branch we have the relationship between the transition intensity Ie(j+~)


Q(3)

R(7) I ~ ( 5 )

] ' I Q(7)

! ~R(3) ~ IQ(9). w, P(9)

R(1) Q(ll) Q(13)I P(ll)

P(13) P(15)

i i

365.0 FIG. 10. He2 rotational fine structure in the electronic spectrum for the (0, 0) transition of i3II~ ~ a3Z, +.

374.6(nm)

and the energy expressed in terms of the rotational quantum number j:12,t3

In Ie(]+ 1) 17_ 4 = constant hcB~J(J + 1)

kT (7)

and for the R branch

In IR(J- 0 - - 4 = constant j u hcB'vJ(J + 1)

kT (8)

where Ie(j+ 1) is the intensity of P branch J + 1 transition; is the wavenumber of the transition; h is Planck's con-

stant; c is the speed of light; k is Boltzmann constant; and By is the rotational constant.

Since the transitions in the P branch are better resolved than those in the R branch, the intensities of the transitions in the P branch were used in Eq. 7 to obtain the "rotational temperature" of the molecules. A graph of the left-hand side of Eq. 7 vs. J(J + 1) should be linear, and the rotational temperature can be calculated from its slope. Graphs for the three transitions (Fig. 12) reflect reason- able adherence to a straight line. The calculated rotational temperatures for the three transitions are given in Table IV. Note that the rotational temperatures calculated for

Q(3) Q(1) Q(7)

3 ~ Q ( 5 ) P(3)Q(9) P(~(11) p 7

R(iR~7)~5)R~) R ( 1 ) I ~ / ~ A , ~ ( ) P(9)

I I 510.0

FIG. 1 1. He2 rotational fine structure in the electronic spectrum for the (0, O) transition ofELIIg ~ A~2. +.

APPLIED SPECTROSCOPY

520.0(nm)

1291

In-! Pa+ L)9 -4 2.6 J+l

-39 •

F -40 ~ o

-41 b o ~< .o r ~ × ~ i x

-42 r-

r

-43 f \ '--. -44 ~ . . . . I _ . . . . . . . . _ ~ L ~ I . . . .

0 50 100 150 200 250 300 J(J+l)

FIG. 12. Graph of ln([l~j+~/(J + 1)]~7 4) vs. J(J + 1): evaluation of rotational temperature. (x) e3Ilg ---, a3Z~ (0, 0) 464.9 nm; (A) i3Ilg a3~,, + (0, 0) 367.7 nm; (.) EII~.----.A"2,, + (0, 0) 513.4 nm.

2 . 4

2.2

2

1.8 0

8 1.6

1.4

1.2

N 1 ¢~

0.8

0.6

0 . 4

0 . 2

0

1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 1 5

Delay Time (gs)

FiG. ] 3. Individual emission-t ime profiles for the He(33p --~ 23S) transition at 388.9 rim.

these three transitions are consistent with an average rotational temperature of ~808 + 23 K. The rotational excitation temperature is considerably lower than the discharge excitation temperature of 3200 K described previously.

Emission-Time Profiles for Atomic Helium Transi- tions. In order to further our understanding of excitation mechanisms occurring in the pulsed discharge, we have undertaken a study of the time dependence of the emission. The initial study involved the atomic helium transitions, the 33p ---* 23S transition at 388.9 nm in particular. This is one of the most intense emissions, so the time profile can be established most accurately. Figure 13 shows the profiles for six successive pulses in order to show the uncertainty in the measurements. There is some variation in the time for the profile maximum, but the major uncertainty is in the intensity. We found that taking an average of 15 profiles gives reproducible results. That average profile for 33p ---* 23S is shown in Fig. 14. The lifetime of this transition is 0.098 #s, much shorter than the decay shown in Fig. 14 of ~0.85 us.

The halfwidth of the pulse was determined from the profile for the current and was found to be 0.18 #s. Ob- viously the spectral emission is significantly delayed from the discharge. The other atomic helium transitions show a similar intensity-time profile, as shown in Figs. 15 and

TABLE Ill. Determination of internuclear distance in excited states of HE2.

Fe Transition (0,0) X (nm) r,, calculated (A) literaturC (,~)

i3He---*a3E# 367.7 1.0909 ± 0.0091 1.0754 1.0516 ± 0.0091 1.0457

e3H~---,a3~ 464.9 1.0884 ± 0.0051 1.078 1.0516 ± 0.0427 1.0457

E~H~-->A'E;~ 513.4 1.1019 ± 0.0114 1.0764 1.0624 ± 0.0146 1.0406

16. The 33p ---* 23S transition is included for comparison. Note that all profiles have similar shapes, with maxima at ~0.8 us, with the exception of the 43D--* 23p transition at 447.1 rim, which shows a shoulder at ~2 us. As will be shown later, this observation is due to a spectral in- terference from diatomic helium. The first maximum in the profile for this transition is at ~0.8 us, similar to that for the other atomic helium transitions. At present we do not have an explanation for the delay in the He atomic emissions from the pulse discharge.

Emission Intensity-Time Profiles for Diatomic Helium. The emission intensity-time profiles for five diatomic helium emissions are shown in Fig. 17. All profiles show two obvious maxima: one at ~0.8 us, similar to that for the helium atomic emissions, and another at a later time of ~2.5 us. This result suggests that there are two mechanisms for the formation of the excited states of He> Since the relative intensities of the two peaks vary for each transition, the population of the different excited He2 states by each of the mechanisms is different. An explanation for the relative intensities will be given later that is based upon the proposed mechanism. Figure 18 shows three emission intensity-time profiles at wavelengths of closely spaced maxima. The relative intensities of the two maxima are similar for the emissions at 440.5 and 443.0 nm. These wavelengths could correspond to two of the rotational branches of the same transition. The intensity-time profile for the emission at 441.4 nm ap-

TABLE IV. Excitation temperature for rotational motion in He2 excited electronic states.

X (nm) Transition B,.' Slope T ± AT(K)

367.7 i3H--*a3~d 7.131 0.012792 803 ± 28 464.7 e3H--->a~Z~ 7.173 0.012531 820 ± 43 513.4 EIH---*AIE~ 7.163 0.013402 770 + 210


2.2

2

1.8

1.6

1.4

1.2

-~

0.8

0 . 6

0 .4

0.2

Delay Time (Its)

FiG. 14. Average emission-time profile for Hc(3~P ----~ 2-'S) transition at 388.9 n m .

pears to be different, and probably represents a different electronic transition.

Effect of Polarity of Discharge. All the previous results were based upon the use of a negative discharge. Changing to a positive discharge had a rather dramatic effect on some emission spectra. For the atomic helium emission lines, there is no noticeable effect on the emission intensity-time distribution; however, the intensity of the emission is increased by ~ 80%. As we will see shortly, other

12

0

}1

~0

9

B

7

6

5 i

4

Q/ (2)

(3)

3 5 7 9 II 13 15

Delay Time (Its)

FiG. 16. Emission-time profiles for various Hc atomic transaction. (1)) 587.6 n m (33D --~ 23p); (2) 388 .9 n m (33P ---~ 23S); (3) 501.6 n m (3'P----~ 2 'S) ; (4) 706.5 n m (33S ~ 23P). Emission intensities for 501.6 and 706.5 n m have been multiplied by factors of 3 and 2, respectively.

emissions also increase when the positive discharge is used, but the percent increase is not so dramatic.

The emission intensity-time profiles for the He2 transitions undergo the most dramatic change. Surprisingly, the double peaks observed in Figs. 17 and 18 merge into a single peak, with a maximum intermediate between the

o

>

(t)

(2)

(3)

(4)

I 2 I 4 F 6 I 8 10 I 12 [ 14 I 3 5 7 9 11 ~3 15

Delay Time (Its)

Fio. 15. Emission-time profiles for various Hc atomic transitions. (1) 388 .9 n m (33p ~ 23S); (2) 447.1 n m (43D ~ 23P); (3) 318.8 n m (43p

23S); (4)402 .6 n m (53D --~ 23p). Emission intensities for 447.1, 318.8, and 402 .6 n m have been multiplied by factors of 3 and 2, respectively.

0.7

0.6 [

I A ~,)

0.5

0 g~ 0.4

~u 0.3 >

0.2

(2)

(3)

(5)

0 r ~ [ - 4 6 8 10 12 - 14 1 3 5 7 9 11 13 15

Delay Time (its)

Fic . 17. Emission-time profiles for various He2 molecular transitions. 3 + . (eqI e a - ~ , , ) , ( 3 ) 4 0 0 . 3 n m (1) 367.7 n m (PII,, ----~ a ~,, ), (2) 464 .8 n m

(I'IlI~,---~ A~Z;J); (4) 377.7 n m (q3(,5,,, I1,,, Z,~) ----~ b3II,); (5) 454 .7 n m ( h 3 ~ ~ b3I lg) .


0.45

0 . 4 '

0.35

0.3 0

o 0.25

;~ 0.2

N 0.15

0.1

0.05

-0.05

t

(1)

(2)

(3)

3 5 7 9 l1 13

, f----rE

1'4

15

Delay Time (p_s)

Fro. 18. Emission-time profiles for various He2 molecular transitions. (1) 440 .5 n m (j3A. + b3IIe); (2) 441 .4 n m ; (3) 443 .0 n m (j311. --~ b3Ilg).

two previous maxima. The profiles shown in Fig. 19 show the obvious transition from the double maxima (negative discharge) to a single maximum (positive discharge at 1.05 as. Also note the increase in intensity with the positive discharge, similar to that observed for the atomic helium emission (33p ~ 23S).

(1)

3.5 (2)

~ 3 I (t)

2.5 +

,g 2 o

1.5

>

Y 0.5

I ~ I ~ I ~ [ 1BI t'211'4[ 1'61 &I2B 12~ Id4 I 5 7 9 11 13 15 17 19 21 23 25

0.6

0.5

0.4

o.3 ~ o

o.2 4

0.1 ~

Delay t ime (p.s)

1 ...--). 3 + n FIG. 19. Emission-time profiles for He2 (e-Ilg a Z,, ) molecular tra - s i t ion at 464.85 nm as generated by (1) a negative and (2) a positive discharge.

1.4

1.3

,.,"2I//(,, 1

0.9 O

©

0.7

0.6 (2)

",~ 0.5

~ 0.4

0 .3

0.2

0.1

-0.1

1 3 5 7 9 I1 13 15 17 19 21 23 25

Delay Time 0as)

FIO. 20. Comparison of emission-time profiles of different He2 molecular transitions. (1) 573.5 n m ( f 3 A , --> b3IIg); (2) 440 .5 n m ( j3A, , ---, b3II~). Emission intensity of 440.5 nm has been multiplied by a factor of 2.

Since the discharge depends upon the difference in voltage between the electrodes, the polarity should not affect the discharge itself. However, since ionic species are produced, one should consider the effect of the potential on the charged species. Since the electrons are the most mo- bile species, they would feel the greatest effect of potential. An obvious rationale for the polarity effect is that the negative potential would repel the electrons away from the discharge region, whereas the positive potential would attract them. This behavior would then affect the positive-ion-electron recombination rate. If the concentra- tion of electrons is lower with a negative discharge, the recombination rate will be slower.

Boltzmann Distribution of He2*(n > 2) States. As shown in Fig. 17 and discussed above, the emission intensity- time profiles for various Hea*(n > 2) --* He / (n = 2) + hv transitions suggest two mechanisms for the production of He / (n > 2) when a negative discharge is being used. The first occurs around 0.8 as and the second around 2.0 as after the discharge. As was noted in the previous section, the second peak was affected by the polarity of the discharge, whereas the first peak seemed to vary only slightly, except for the intensity. In this section we examine the intensities of these two maxima in light of the excitation energy of the upper state of the transition. In other words, we wish to examine the molecular transitions to see if the occupation of these excited states sat- isfies a Boltzmann distribution similar to that observed for atomic helium.

Again we examine the diatomic helium transitions for the different series, shown in Table II. The first series starts at excited 3A, states and terminates at the b3IIg state. The two most intense emissions in this series occur at


0.4 . . . . . . . . .

0.35 (1)

0.3

0.25

(2)

['~ 0,2

'~ 0,15 ~D

0.1

0.05

° --A-TTV-i 1 3 5 7 9 I1 13 15 17 19 21 23 25

Delay Time (~ts)

Fro. 21. Compar i son o f e m i s s i o n - t i m e proliles o f different He2 molecular transi t ions. (1) 464.85 n m (eqI~ ----, a3Z,+); (2) 367.7 n m (?H~

4.5

4

3.5

i

3 (])

g~ • 2.5

.~ 2 (2)

1.5

0.5

0 I '21 ' , [ ; 1 1(01,'21,'41,'61,' 12'012 -r 1 3 5 7 9 11 13 15 17 19 21 23 25

Delay Time (gs)

FIG. 22. Compar i son o f e m i s s i o n - t i m e profiles of different Hez molecular t ransi t ions. (1) 640.1 n m (d3Z,, + ~ b31I~); (2) 454.7 n m (h3Z, + ~ b311~). Emiss ion intensi ty o f 454.7 n m has been mult ipl ied by a factor o f 10.

573.5 and 440.5 nm. The higher energy transitions at 397.6 and 377.6 nm have very low intensity, so that the emission intensity-time profiles could not be recorded accurately. The emission intensity-time profiles for the 573.5- and 440.5-nm transitions are shown in Fig. 20. Note that the intensity of the first maximum is much greater for the lower energy transition at 573.5 nm than for the 440.5-nm transition. However, the intensities for the second peak are comparable for the two transitions. Since these two transitions terminate at the same state, the higher energy transition (440.5 nm) originates at a higher energy state (j3z~,) than the lower energy transition (573.5 nm) from thef3dx,). If the transition probabilities for the transitions are similar, the population of the higher energy state (j3dx,) is much less than that for the (f3A,,) state.

A similar situation occurs with the e3II~ --* a3y + and ---* a 2;, transitions in the last series of Table II. The i 3 i t g 3 +

emission intensity-time profiles for these transitions are given in Fig. 21. Again, note the decrease in the intensity of the first maxima in going from the lower energy transition (464.8 nm) to the higher energy transition (367.7 nm). As before, if the transition probabilities are the same for these transitions, we conclude that the higher energy state (i3IIg) is less occupied than the lower energy state e3lIg). Conversely, the intensities of the second peak are similar, suggesting a different mechanism for the excitation for these transitions.

Finally, in Fig. 22 we show the emission intensity-time profiles for the transitions d3z + ---* b3IIg (640.1 nm) and h3F_, + ---* b3Hg (454.7 nm). In this case we see a similar trend, in that the intensity of the first maximum decreases as we go from the lower energy transition at 640.1 nm to

the higher energy transition at 454.7 nm. Again, the second maximum shows less of a decrease than the first maximum. However, note that the emission intensity- time profile for the 454.7-nm transition is lower by a factor of ten than that shown in Fig. 22. In other words, the intensities of both transitions decrease considerably in going from the lower energy transition to the higher energy transition. This is quite different from the results observed in Figs. 20 and 21.

Previously we examined the population of atomic helium states for a given spectral series to see ifa Boltzmann distribution was obeyed. In principle we could do the same thing for the He/transitions in the different spectral series. However, the He2 emissions are broader because of the rotational fine structure, and consequently the intensities are much lower. Also one might note in Table II that some transitions overlap (namely, the q, m3II, ---* b3IIe and the q, m3Z + ---* b3II~ series), and it is difficult to ascribe the appropriate intensity to each transition. The transitions whose emission intensity-time profiles are shown in Figs. 20-22 have sufficient intensity to be analyzed in regard to a possible Boltzmann distribution.

The previously derived Eq. 2 also can be used for the analysis of the He2 emissions. However, we are much more limited in our analysis of He2 populations, since we have only two transitions in each series and we do not know the transition probabilities. Since each of the pairs of transitions is reasonably close in energy with a common lower energy state, we will assume that they have the same transition probability. The statistical weights should be the same since they belong to the same series. With those assumptions, the following equation can then be


T A B L E V. Excitation temperature of He= and He electronic states.

He2 transi t ion (0,0) t~.. (~s) )~ (nm) T _+ A T (K)

f3A,, ~ b3Hx 0.75-0 .80 573.5 j3&,--~ b31I Ie 0 .80-0 .85 440.5 4,510 ± 390 d 3 ~ ~ b3Ile 0.75-0 .80 640.1 h3~, + ~ b3IIx 0 .75-0 .85 454.7 2,770 _+ 260 e3IIg ~ a3~, + 0 .75-0 .80 464.85 i3He ~ a3E, + 0 .70-0 .80 367.7 4,590 ± 440 f3A,, --* b3llg 1.75-2.05 573.5 j3A,--~.b3IIx 2.15-2.45 440.5 19,400 _+ 7180 d3E~ ~ b3IIe 2 .2-2.35 640.1 h3E, + ~ b3H~ 1.95-2.35 454.7 3,230 _+ 170 e3IIg ~ a3~,, + 1.85-2.25 464.85 i3Hg '--*" a 3 ~ 2.0-2.4 367.7 11,500 _+ 2770

Atomic He t ransi t ion T = 3,200 _+ 220 K

derived with the use of only two transitions of frequencies, ~a and ~,:

In la ( g A ) ~ l , Ea - Eb Ib ( g A ) j a - k T (9)

where/ , g, and A have been previously defined for Eqs. 2 and 3. The excitation temperature can be calculated for each of the three pairs of transitions by solving for T. Since the He2 emission intensity-time profiles consist of two maxima when a negative discharge is being used, we can calculate excitation temperatures for each maximum. The results of these calculations are given in Table V. The times of the maxima are given for both the first peak and the second peak, and the corresponding excitation temperatures are given for each of these maxima. The measurements for each of these maxima were repeated at least five times, and the average of these along with the standard deviation is given in Table V.

As expected, the calculated excitation temperatures with the use of the intensities of the first peak are less than those for the second peak. This observation is consistent with our general observation from Figs. 20-22 that the intensities of the higher energy transitions are all lower than the intensities of the lower energy transitions. The last row of Table V contains the excitation temperature which was calculated from the atomic helium transitions, as presented above. Since the transition probabilities are known for the atomic transitions and there are several lines that can be used from each series, the calculated excitation temperatures should be more precise. The calculated excitation temperatures with the use of the intensities of the first maximum agree reasonably well with that calculated from the He atomic transitions. The more reliable calculations of T with the use of thefj3A, ---* b3IIg and e, i3IIg ---* a3E + series both give excitation temperatures which are high by 30%. The discrepancy is greater than expected, but not so great that we can rule out the possibility of a Boltzmann distribution among the He2 states. Obviously the calculated excitation temperatures with the use of the intensities of the second peak give values which are much larger than those calculated from the He atomic transitions, and we can definitely conclude that the population of these states does not follow a Boltz- mann distribution. The only exception to this conclusion comes from the d, h3Z + ~ b3IIg series, which gives very

low excitation temperatures with the use of data from both maxima.

SUMMARY OF RESULTS AND PROPOSED MECHANISM

The reactions that can occur as a result of excitation in helium are numerous, and they depend upon the mode of excitation and the pressure of the helium. In our system the high-voltage pulsed discharge causes excitation over a short period of time (-0.18 us), in pure helium at -1 atm pressure. Under these conditions only certain reactions are predominant, and in this study we attempted to make sufficient measurements to delineate the reaction steps appropriate for these conditions. The following is a summary of our results and the conclusions which we have reached.

(1) We observed helium atomic emissions in the UV- visible region which arise from transitions in the upper electronic states (n > 2) down to the n = 2 level, resulting in helium 21S, 2'P, 23S, and 23p states. There were 19 transitions in the triplet manifold and seven transitions in the singlet manifold. Analysis of these data allowed us to conclude that the high-voltage pulsed discharge excites the atomic helium in electronic states according to a Boltzmann distribution. The "discharge temperature" was calculated to be 3200 _+ 220 K, which is rather low in comparison to that of other excitation sources. As a con- sequence, occupation of the high-energy states is quite low, and the possibility of electron excitation to give direct ionization of atomic helium is even more remote. Ionization in helium can be carried out by other processes, as will be discussed later. Since the atomic helium excitation follows a Boltzmann distribution, the excitation of electronic states by electron-He + recombination must not occur.

(2) The atomic emission-time profiles were measured, and all were found to occur with a maximum intensity at ~0.8 us. That time period is somewhat behind the maximum in the current profile which is passing through the discharge maximum at -0 .12 us, but the -0.68-~s delay could be due to the amplifier used on the photomultiplier tube output. This He atomic emission must be the primary initial excitation of He in the discharge, with little or no direct ionization.

(3) We observed 15 triplet and 13 singlet He2* emissions, as shown in Fig. 8. A rotational analysis was carried out on five transitions, and the calculated internuclear separations of the electronic states were in agreement with those reported in the literature, confirming that these are He2 emissions.

(4) The "rotational temperature" was determined from the Boltzmann distribution over the rotational energy levels, and found to be 808 _+ 23 K.

(5) The He2* emission-time profiles showed two maxima when a negative discharge was used, suggesting that two processes are present in the formation of He2*. (a) The first process occurs at about the same time as the

He*. The He2* emission from this first process for He2* formation showed a Boltzmann distribution over the He2* electronic states. Since the transition probabilities were not known, the "excitation temperature" could not be determined with absolute accuracy; how-


TABLE VI. Proposed mechanism for the pulsed discharge excitation/ionization in helium at 1 atm.

Time (us) Comments

<0.30 1 e (discharge) + He --~ He* (n -> 2) + e Boltzmann distribution <0.59 2 He* (n > 2)---, He* (n = 2) + hu UV-visible spectra observed

3 He* (2'P, 23P) --, He* (2'S, 23S) + hv t (2'P) = 0.00055 us r (23p) = 0.098 us

0.5-1.5

1 . 5 - 3 . 5

10 11 12 13 14 15

He* (23S) + He* (23S)~He + + e + He He*(2'S) + e ~ He* (233) + e He*(n >_3)+He~He~ +e He*(n>2)+ 2He~He2*(n> 2)+He He2* (n > 2)--~ H%* (n = 2) + hv He2* (2qlg, 23I I~ . ) - -* H%* (2rE,, +)

He* (23S) + 2He --, H%* (23~u +) + He 2He2* (23~i~) ~ (He ~ + He or He2 ~ ) + e He2* (23~,?) + He* (23S)--~ (He + + He or He2 ~ ) + e H%* (2'~,?) ~ 2He + hv (13.5-17.7eV) He ~ + 2He--H%' + He He~ ~ + e --~H%*(n >2)

Fast reaction

Boltzmann distribution of H%* states

UV-visible spectra observed, n = 2 refers to the states ALE,,, BIH~, a3Z;~,, b~g~,

Hopfield emission observed

ever, a value o f ~ 4 0 0 0 K is somewhat in agreement with the "exci ta t ion tempera ture" determined from the He* emission (3200 + 220 K).

(b) The second process for the formation o f He2* occurs later, at 1.5-3.5 us, and the intensities o f this emission do not show a Boltzmann distribution over the electronic energy levels. This second process for He2* for- mat ion depends upon the polarity o f the discharge. We therefore assigned this process to electron recombination, presumably with He2*, which is consistent with the lack of a Bol tzmann distribution and the dependency on the polarity o f the discharge.

(6) The Hopfield emission arising from the He2* (2~E +) transition to the dissociative ground state was observed, confirming that we do obtain populat ion of the He2* singlet manifold o f energy states.

On the basis o f these results and additional informat ion f rom the literature, we propose the mechanism shown in Table VI. React ion step 1 represents excitation by the electrical discharge. As revealed by measurement of the current on a digital oscilloscope, the discharge has a halfwidth o f 0.18 us, and is completely decayed at 0.30 us. The discharge causes a Bol tzmann distribution o f excited a tomic hel ium states which undergo transitions to the n -- 2 level with the emission o f radiation. Noting the much greater intensities f rom the lower energy states, we concluded that the lower energy states are more highly occupied and that direct ionization o f the atomic hel ium by the discharge is negligible. React ion step 3 is the expected 2P ~ 2S transition. However , we were un- able to observe this step, since it occurs at much longer wavelengths, outside the range of our PMT. The lifetimes o f the 21P state and for the 23p states are 0.55 ns and 98 ns, respectively, both of which are short in comparison to the rates o f collision processes. This sequence of steps 1-3 p r o d u c e s a high c o n c e n t r a t i o n o f 2~S and 23S He.

React ion step 4, first proposed by Biondi '4 to account for ionization after a potential is removed, is probably the principal source of ionization in our system. Reaction step 5 is very fast, with a cross section o f 100 A 2, according to Phelps. Is The net result o f reaction steps 4 and 5 de-

pletes the populat ion of the 2~S state. Reaction step 6 is an associative ionization first proposed by Hornbeck and Molnar. ~6 Ionization can only occur when n >_ 3. The ionization cross sections for energy levels have been measured by Wellenstein and Robertson~7 and Collins et al.~8

Reaction step 7 is in compet i t ion with reaction step 6, and leads to excited He2* states. The subsequent emission is shown by reaction step 8. We observed 15 triplet and 13 singlet He2* transitions in the UV-visible region. These are depicted in Fig. 8. F rom the emiss ion- t ime profiles, we concluded that there are two processes for the for- mat ion o f He2* excited states. The process which occurs in the region 0.5 to 1.5 us would appear to be formed via reaction step 7, since it occurs along with or just following the He* atomic emission. Fur thermore, the He2* transition intensities suggest a Bol tzmann distribution similar to the observed Bol tzmann distribution over the He* a tomic states. Reaction step 9 is the expected II ~ E transition, which appears at wavelengths that are longer than our P M T sensitivity region. Reaction step 10 is analogous to reaction step 7, except that it is restricted to the He*(23S) state. Myers and Cunningham '9 give a rate constant for this reaction o f 0.18 Tor r 2 s-~. At 1 atm pressure, this gives a lifetime for He*(23S) o f less than 10 us.

* 3 + The He2 (2 ~ , ) state produced in reaction steps 8-10 is used in reaction steps 11 and 12 to give additional ionization in the form of He + or H% +. Myers and Cunningham L9 have studied these reactions and show a long lifetime for the He2*(23E~) state. Deloche et al. 2° give a rate constant for reaction step 12 in the range (0.8-2.5) x 10 9 cm 3 s- l .

Reaction step 13 is the well-known Hopfield emission, 2~ which arises from the transition o f the

* 1 + H% (2 ~, ) state to the dissociative ground atom He(a)S) states. We observe this emission in the region 72 to 92 rim, but we have not measured its intensity t ime profile. He + is produced via reaction steps 4, 1 1, and 12.

In reaction step 14 we include the reaction o f He + with He to give He2 +. According to McDaniel, 22 at helium pressures below 0.7 Tor r nearly all the ions are He +. Between about 0.2 and 2-3 Tor t there is a mixture o f He + and He2 + , the latter of which is formed by reaction


step 14. Above 3 Torr, and certainly at atmospheric pressure, the dominant ion is He2 +.

Reaction step 15 gives the charge neutralization reaction which leads to excited He2* states. This is the second process we observed for He2* formation at ~ 1.5-3.5 us. These He2* emission intensities do not suggest a Boltz- mann distribution in agreement with an e- + He2 + ion recombination step. Also according to McDaniel, 22 molecular emission is observed at 15 Torr pressure and higher, and the light intensity is proportional to the square of the average electron density. This result was attributed to e + He2 + recombination, and a lower limit of 3 × 10 '° cm 3 s ~ could be placed on the recombination coef- ficient at 15 Torr. The He2* (n >- 2) could then react via reaction steps 8, 9, 11, and 13.

In order to see if reaction step 7 can compete with reaction step 2, we calculated the collision frequency of an He* with helium atoms where the helium is at 1 atm pressure. We took the diameter of a helium atom as 0.2 nm. Of course, the atomic radius is not definitive, and there is likely an error of +25% in this value. We calculated the collision frequency for a single He* from the formula 23 Z = "x/2~CN, where ~ is the cross-sectional area = 1.3 × 10 ~ g m 2 ; c i s t h e a v e r a g e s p e e d = 1300m/s; and N is the atom density = 2.5 × 1025 atoms/m 3. The collision frequency is estimated to be 6 × 109 collisions/ s, or a time per collision of 1.7 × 10 -~° s or 0.17 ns. This is shorter than the expected transition time for an allowed atomic transition; consequently, the He encounters numerous collisions before undergoing photon emission via reaction step 2. From Table I we see that the lifetimes (Ti) for the atomic helium transitions range from 15 to 591 ns (ri = 1/Ai). The numbers of collisions occurring during these lifetimes range from 88 to 3500, and it would appear that the formation of the He2* from He* is quite competitive with the radiative process.

The proposed mechanism for helium excitation, reaction steps 1-15, appears to be self-consistent. However, it should be pointed out that reaction steps 1-3, 7-10, and 13-15 are supported by experimental evidence given in this publication. We observe ionization in helium at 1 arm, but it is at a very low level when the helium is pure. Since we observe the Boltzmann-type distribution of excited atomic helium states, the direct ionization by the discharge is highly unlikely. Therefore, we have as- sumed reaction steps 4, 6, 11, and 12, as taken from the literature, as possible ionization processes. We have no direct support for these steps and very likely some may be more important than others. A time profile for ionization would assist is assessing the relative contributions of ionization from excited atomic vs. diatomic helium species. Reaction step 5 is reported to be fast, and we presume that it would affect the singlet vs. triplet population of excited atomic helium, but again, we have no direct evidence for this reaction step.

From this proposed mechanism the most reactive species will be He*(23S) and He + at very short times (< 1 us) and He2+(23~, +) and He2 + at longer times (> 1 us). Studies are underway which will measure the intensity-time profiles of excited atoms and diatomic species in order to determine the mode of excitation in the pulsed discharge in atmospheric helium. This is most important for our work, since we intend to use this technique for qualitative analysis of effluents from a gas chromatographic column.

ACKNOWLEDGMENTS

The authors would like to thank the Robert A. Welch Foundation and Krug Life Sciences for financial assistance on this project. We would also like to express our appreciation to Valco Instruments Co. Inc. for providing the facilities to carry out the emission intensity-time profiles. Finally, we would like to thank David Salge, technical writer at Valco Instruments, for his assistance in manuscript editing and preparation.

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3. S. V. Vasnin, W. E. Wentworth, S. D. Stearns, and C. J. Meyer, Chromatographia 34, 226 (1992).

4. Element-Specific Chromatographic Detection by Atomic Emission Spectroscopy, P. C. Uden, Ed., ACS Symposium Series (American Chemical Society, Washington, D.C., 1992), Vol. 479.

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Excited Species from a Pulsed Discharge in Helium at One Atmosphere Pressure

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