Polarization enhancing optical system for a high-resolutionVUV spectroscopic facility at a synchrotron light source

Charles M. Brown and Marshall L. Ginter

In this paper we describe a reflecting optical system designed as part of a new high-resolution VUV spec-trometer facility at the SURF II light source at the National Bureau of Standards. The system employsthree cylindrical mirrors to focus an image of the storage ring source on the entrance slit of a 6.65-m vacuumspectrometer located 10 m from the source point. Image properties and focusing techniques are demon-strated with ray-trace diagrams and visible light images. Reflectance values in the VUV are large and themirrors are oriented so that the electric vector of horizontally polarized synchrotron radiation is always per-pendicular to the plane of incidence. Depending on the wavelength and coating material chosen, large dif-ferences in the transmittance for the s and p polarization components of the light beam are possible, withpolarization purities >95% expected throughout the 500-2000-A range and values as high as 99% possiblefor A > oo A.

1. Introduction

A unique optical system has been designed for use ina new high-resolution spectroscopic facility being in-stalled at the SURF II synchrotron light source locatedin Gaithersburg, Md. The project is a joint effort by theNaval Research Laboratory (NRL), National Bureauof Standards (NBS),.the University of Maryland, andthe National Science Foundation aimed at placing ahigh-resolution vacuum spectrometer on a synchrotronbeam line for experimental studies in atomic and mo-lecular physics. A variety of optical arrangements wereconsidered, and a system has been constructed whichmeets a diverse set of experimental design require-ments. This paper discusses the design of and the ini-tial tests on the optical system which couples the lightsource to the spectrometer. Details of the beam lineconstruction, components for differential pumping, andthe spectrometer and its scanning mechanism will bedescribed in a separate paper.

The facility is designed to provide a national capa-bility for research involving high-resolution (0.004 A orbetter) photoabsorption, photoionization, and photo-electron measurements in the VUV region. The highpolarization purity of the beam will greatly improve thequality of data obtained from experiments involvingexternal fields, the angular distribution of photofrag-ments, etc., and will simplify their interpretation be-cause the experimental apparatus can be in a knownorientation relative to the electric vector of the radia-tion.

Charles Brown is with U.S. Naval Research Laboratory, E. 0.Hulburt Center for Space Research, Washington, D.C. 20375; M. L.Ginter is with University of Maryland, Institute for Physical Science& Technology, College Park, Maryland 20742.

Received 7 July 1984.

11. The Light Source and SpectrometerThe SURF II light source, a 280-MeV electron storage

ring dedicated for use in research which employs VUVradiation, is described in detail elsewhere.1 The at-tractive features of this source for high-resolution VUVspectroscopy are the following: (1) the intensity of thesource can be calibrated from first principle calculationsbecause the electron's orbit is very nearly circular andquite stable; (2) the source has high brightnessthroughout the wavelength range of interest because thecross section of the electron beam is small; (3) highlypolarized (nearly 80%) light is emitted most strongly ina horizontal plane parallel to the floor, and (4) a portproviding line-of-sight access to 60 mrad horizontaland 6 mrad vertical apertures is available for thespectrometer facility.

The spectrometer is a modification of a 6.65-m focallength concave grating instrument originally manu-factured for NRL by the Jarrell-Ash Co. This spec-trograph, which has an off-plane Eagle mounting 2 3 witha horizontal entrance slit and a vertical plane of dis-persion, has been modified for oil free pumping, focalplane scanning, and photoelectric detection. It also canbe used as a monochromator for experiments operatedbehind its exit slit. The horizontal slit arrangementsare appropriate for optimal utilization of light from thestorage ring source, while the rulings of the grating alignwith the major direction of polarization of the syn-chrotron radiation.

111. The Optical System

A critical element in the construction of this high-resolution spectroscopic facility is the design of an op-tical system which maximizes the throughput of shortwavelength radiation and which operates inside theultrahigh vacuum (UHV) lines connecting the storagering and the spectrometer. While there are a numberof side conditions constraining the design, we will dis-cuss only the few which we found to be of greatest im-portance.

4034 APPLIED OPTICS / Vol. 23, No.22 / 15 November 1984

M2

Ml M3

.1 I mM2

MI ~ - 20.33' 2.7 ~ M3 S

e 1.70m 3.00 m 2.50 m 2.70 m

Fig. 1. The upper schematic drawing of the optical system shows

the orientation of the three mirrors and the beam characteristicsbetween optical elements. Concave wave fronts indicate converging

light, convex wave fronts diverging light, and straight wave frontsindicate parallel light. The lower drawing is a schematic represen-

tation of the positions of the system's principal elements and thesurrounding vacuum system.

The first consideration is the matching of the aper-tures of the light source to those of the spectrometer.Since the divergences we wish to collect from the SURFII source are -60 mrad horizontally and 6 mrad verti-cally and the spectrometer accepts -20 mrad in bothdirections, an optical system between the electron beamand the spectrometer slit that magnifies -3X horizon-tally and demagnifies -3X vertically is required. Aperfect optical system of this type would convert a 1- X0.1-mm source line to an image 3 X 0.03 mm at thespectrometer slit. In addition, the requirement for highthroughput at short wavelengths limits the optical el-ements to front surface mirrors with large angles of in-cidence.

Major design constraints are imposed by (1) theminimum allowable distance between the source elec-tron beam and the first optical.element ('1.5 m) and theminimum distance between the last optical element andthe slit of the spectrometer (at least 1.5 in for insertingexperiments), (2) the clearance required between thenew beam line and existing and future apparatussharing the laboratory floor space, (3) a requirement toshare the light with another experiment which is alsoon the line of sight to the synchrotron port, and (4) themaximum overall dimension of the line of sight availablein front of the slit (-17 m) at the SURF II facility.Additional important considerations are (5) to havelight as highly polarized as possible (to accommodateseveral major VUV scientific programs), (6) to havetransfer beam cross sections as small as practical (toreduce vacuum pumping demands and the sizes of op-tical elements), and (7) to have segments of the transferbeam where the light is held nearly parallel (to allowinsertion of capillary arrays to inhibit molecular flowwhen conducting high gas load experiments).

We employed a ray-tracing procedure4 5 to evaluatethe optical properties of the many systems possiblewithin the constraints outlined above. Most of the

optical systems studied were combinations of simplecylinders, although systems using complex mirrorshapes also were considered. By examining ray-tracespot diagrams we were able to view the aberrationsproduced by trial optical systems and to reject thepoorest configurations. The decision to use separatecylindrical mirrors with orthogonal axes enabled us toarrive at a system with both the horizontal and verticalmagnifications desired and with simple and relativelyinexpensive optical elements. The final result of theseconsiderations is shown schematically in Fig. 1.

As can be seen from the lower diagram in Fig. 1, thelight source (S1) is located 1.7 m from the first mirrorMl. Ml is cylindrical mirror which sends a collimated(in the direction corresponding to the deflected hori-zontal plane from the source) beam upward at a 200angle to the plane of the electron ring. This beam iscollected by a second cylindrical mirror (M2) located4.7 i from the source and 1.1 m above the plane. M2reflects the beam downward in a direction that inter-sects the original plane at a 240 angle at a point 7.2 infrom the source. M2 also focuses the source's horizontalcomponent (as a vertical line image) at the spectrometerentrance slit (S2). Mirror M3 collects the beam fromM2, where it intersects the horizontal plane and focusesit vertically (a horizontal line) on the spectrometer slit.Si, S2, and all three mirrors lie in the same verticalplane. In addition, all but M2 lie in the plane of thestorage ring. Fig. 1 also includes a schematic outlineof the vacuum lines and chambers containing the threemirror system.

The upper diagram of Fig. 1 is a schematic of theoptical system showing the orientations of the threemirrors. The wave fronts on the figure indicate theconvergence/divergence of the beam as it passes throughthe system. Diverging light is indicated by convex wavefronts and converging light by concave. Of particularinterest in this design are the divergences obtainedbetween Ml and M2. The curvature of Ml was chosento focus at infinity so that the light passing to M2 is anearly parallel beam, with -6-mrad divergence in thevertical direction only. This arrangement is importantto the differential pumping design (see below) and tothe ultimate magnification of the beam along the lengthof slit S2.

The radii of curvature of the mirrors are calculatedusing the conventional formulas:

(1)R=cosa(-+T)r r')

R = cos r+ )

where the object and image distances are r and r', anda is the angle of incidence. Equation (1) is used for Mland M2 and Eq. (2) for M3. For Ml, r' is taken to beinfinity as is r for M2. The S1-Ml optical path is r forMl, and the M2-S2 optical path isr' for M2. Togetherthese two mirrors form the horizontal focusing compo-nent of the system. M3 is the vertical focusing com-ponent, with r being the S1-M3 optical distance and r'being the M3-S2 distance. The final mirror radii, and

15 November 1984 / Vol. 23, No. 22 / APPLIED OPTICS 4035

(2)

Table 1. Elements In the Optical System

Element positiona (cm) Angle of Radius of OrientationElement Type X Y Z incidence curvature of axisb

S1 Source 0.0 0.0 0.0 - - YMl Cylinder . 0.0 0.0 -170.0 79.8360 60.0 cm XM2 Cylinder 111.1 0.0 -470.0 67.8520 410.0 cm XM3 Cylinder 0.0 0.0 -720.0 78.0160 19.25 m YS2 Slit 0.0 0.0 -990.0 0.00 - Y

a The laboratory coordinate system has its origin at the source tangent point, and the synchrotron radiation propagates in the -Z direction.X is vertical, and Y is horizontal.

b The coordinate system of each optical element is body centered with Z along the surface normal and Y parallel to the laboratory Y axis.The axis of a cylindrical surface is that direction along which the surface is not curved. The length of the spectrometer slit is along the laboratoryY axis as is the largest dimension of the light source.

optical parameters, and element locations (X, Y,Z) forthe system are summarized in Table I.

The sizes of the mirrors are determined mainly by thebeam cross section at each mirror. Ml and M2 werefabricated 100 mm square, although Ml need be only60 mm in the X direction. M3 must be 200 X 50 mm tocollect the full beam and was fabricated as two piecescut from a single square mirror and placed end to end.All mirrors were produced by conventional grindingtechniques6 for cylindrical optics.

Each mirror is mounted in an adjustable three-pointkinematic holder. In addition, screws are provided ineach mirror mount to permit alignment of the mirror'saxis with the beam axis. The holder for M3 has anadditional set of adjusting screws for co-aligning the twoparts of the split mirror. Figures 2 and 3 are photo-graphs of a typical mirror holder and of the holder in itskinematic adjustment mount. Finally, the holder forMl is provided with a mechanism which raises themirror above the S1-S2 line to allow alternate use of thebeam by other experiments.

Prior to coating and installation in the vacuum sys-tem, the mirrors were tested in the laboratory. Thisprocedure was practical because the angles of incidencein the optical system (see Table I) are large enough towork easily without reflective coatings using visiblelight. The three mirrors were mounted to the opticalspecifications detailed in Table I and illuminated bylight diverging from a 1-mm diam pinhole at S1. Amesh of 0.030-mm diam wires was placed over the pin-hole to facilitate finding the foci. Figure 4 is a photo-graph of the aligned and focused image of the pinholeat the S2 position, while Fig. 5 is a ray-trace spot di-agram simulating the image photographed in Fig. 4 (lessthe wire mesh). In Fig. 4, the 0.030-mm wires are wellresolved in the horizontal focus (vertical wires) but arepoorly imaged in the vertical focus (horizontal wires).This observation is consistent with the ray-trace resultsin Fig. 6 on the image at S2 of a 0.030- X 1-mm object atS1, first oriented horizontally and then vertically. Thefinal magnification factors for the system in Table I are3.19X in the horizontal direction and 0.354X in thevertical direction. Thus for an aberration free system,a 0.030- X 1-mm wire would be imaged 0.096 mm wideand 0.354 mm long if oriented vertically and would beimaged 0.010 mm wide and 3.19 mm long if orientedhorizontally. Aberrations degrade these results in the

Fig. 2. Front view of cylindrical mirror M3 in its holder. Holder ismounted on its kinematic mount which is suspended from above as

it will be in its mirror box.

Fig. 3. Side view of mirror M3 on its adjustable kinematic mountwith the left vertical suspension arm removed.

actual system, most severely in the vertical focus. Theray traces in Fig. 6 show that the horizontal wire has animage with slightly more than 56% of the rays falling ina line 0.25 mm wide; thus we could not resolve the im-ages of the horizontal wires, which is consistent with thephotographed image in Fig. 4. The magnitude of theaberrations in the horizontal focus is about the same

4036 APPLIED OPTICS / Vol. 23, No.22 / 15 November 1984

12

Z ()

10

Fig. 4. Image (at S2) of a 1-mm diam source (at S1) covered by amesh of 0.030-mm diam wires on 0.30-mm centers. The two halves

of M3 have been adjusted so that their images are superimposed. Thescale has 1-mm divisions.

8

0.9 .O I I 1.2

h (m)

10.0

5.0

E 0.0

-5.0

-10.0_ I( -5.0 0.0 5.0 10.0

y (mm)

Fig. 5. Spot diagram at S2 from ray-trace calculation for a 1- X 1-mmsource at S1.

2.5

EX

0.0

O.C

-9':.

I I I I I I I I

I-5.0

I I I __ I I-2.5

. . . .. . . . . . . . . . . . .I .~I

0.0 2.5 5.0

y (mm)

Fig. 6. Spot diagrams for a 0.030- X 1-mm line source at S1 with theline oriented vertically (lower diagram) and with the line orientedhorizontally (upper diagram). The images shown indicate (see text)that the vertical wires of Fig. 4 should be well resolved with the hori-

zontal wires remaining unresolved due to the aberrations.

Fig. 7. Computed focal distances Z for the horizontal and verticalfoci as a function of the height of M2 above the line of sight betweenS1 and S2. The horizontal focus (a vertical line) is indicated by bythe dashed curve. The solid curve is for the vertical focus (a hori-zontal line). The intersection of the horizontal and vertical focal

curves gives the best focus for the system.

with more than 53% of the rays falling in a 0.25-mm line,but the magnification places the images of the verticalwires 0.95 mm apart so that they are easily resolved inFig. 4.

The focusing procedure for this system involves ad-justing the height of M2 above the line joining S1 andS2 and the collateral fine adjustment of the angles of themirrors. This procedure is employed since the positionsof Ml, M2, and M3 along the line joining S1 and S2 cannot be adjusted easily because of the surrounding UHVvacuum chambers. Figure 7 is a plot of the computedhorizontal and vertical foci of the system as a functionof the height of M2 above the S1-S2 line. The idealfocus is at the point of intersection of the two curves.Most critical is the vertical focus (from M3) since it fallson the narrow dimension of the spectrometer slit. M3has a depth of focus of -100 mm (approximately itshalf-length), but the alignment of the two halves mustbe made to place the image from each half on the slit.Although neither half of M3 can be at its exact focus, thetwo images must intersect at the slit to properly illu-minate the grating. The quality of the horizontal focusis less sensitive since it mainly affects the length of theilluminated area along the slit.

We have also considered the use of a bent mirror 7 inplace of the conventionally ground and polished M3.We constructed a variable curvature test mirror byapplying appropriate torsion to a 50- X 200-mm glassflat mounted in an adjustable jig and inserted the bentcylinder in place of M3 in the system. This method ispractical for radii of the order of 19 m and might proveadvantageous in the future. Such a mirror simplifiesthe focusing of the optical system since the horizontalfocus can be brought to the slit by manipulating the

15 November 1984 / Vol. 23, No. 22 / APPLIED OPTICS 4037

| | |

\\

- . . , . I . . . . I . . . . I , ,

.4 , U. ,

. . . . I , , . , I . . . . I , ,().0

IF AI

,I

height of M2, and then the curvature of M3 can be ad-justed to bring the vertical focus to the same point.Thus a variable curvature mirror could improve theoverall focus and would eliminate the need to adjustseparately two parts of M3. Neither the bending jig northe glass flat employed were of high optical quality, sothe images we obtained with bent mirrors were inferiorto those obtained with the conventional M3 illustratedin Fig. 4. While the bent mirror approach has consid-erable potential, the simplicity and production qualityof conventionally produced mirrors has led us to employfixed radius mirrors in the initial implementation of theoptical system.

Figure 8 is a ray-trace spot diagram at S2 for a simu-lated (see Fig. caption) synchrotron source of 1 X 0.1mm for the optical system in Table I. The image is notideal because of the aberrations discussed above, withthe broadened vertical dimension being most signifi-cant. This imaging defect is not a serious fault in ourapplication because a source image on the slit of rea-sonable width (say ten slit widths) prevents the opticalsystem from being excessively sensitive to small vibra-tions of the mirrors, to thermal drifts, and to electronbeam position variations.

IV. Coatings, Reflectances, and Polarization

The choice of coating material for the mirrors is themost important factor in determining the flux and po-larization of the beam entering the spectrometer. Ofthe numerous possibilities, we consider here only stablesubstances which are well characterized optically in theVUV and which can be utilized over wide wavelengthranges. Fresh aluminum, for example, gives extremelyhigh reflectance but is inappropriate for our purposesbecause its reflectance properties are rapidly altered byoxidation. Likewise, special multilayer coating werenot considered because their reflectivities generally canbe optimized only for limited wavelength ranges.

Mirror M2 is the most interesting surface since it hasthe smallest angle of incidence (68°). All other thingsbeing equal, this mirror generally will have the smallestreflectance and the highest polarization selectivity ofthe three. Thus it is the properties of the M2 coatingthat largely determine the overall performance of thesystem. The 68° angle is near the pseudocritical angle(the angle of minimum reflectance for light polarizedin the plane of incidence) for many coatings, and largedifferences between the reflectance for light polarizedin the plane of incidence (Rp) and for light polarizedperpendicular to the plane of incidence (Rs) are ob-served near this angle. These effects are wavelengthand coating dependent, so no single system can be bestfor all experimental applications.

For a three-mirror system, the reflectances aremultiplicative. We have computed the reflectiveproperties of the three-mirror system in Table I as afunction of wavelength using the optical constants forseveral coating materials. The calculations were per-formed using the following relations8:

(a - cosa) 2 + b2

(a + cosa) 2 + b2

I0.C

5.C

E

E0.0

-5.C

-10.0

IJ ' ' ' '_T I . . . . I ' I ' ' I I I 1

I- I -" 1I" lv. : e

) . . .l , l I . . . I l l I I I-5.0 0.0 5.0 10.0

y (mm)Fig. 8. Ray-trace spot diagram at S2 for a simulated synchrotronsource. The source was simulated by Gaussian distributions ofrandom source points with 1- and 0.1-mm standard deviations in thehorizontal and vertical directions, respectively. The horizontal di-vergence was from a uniform distribution over 60 mrad, and the ver-tical divergence was from a Gaussian distribution with a 6-mrad

standard deviation.

Rp = Rs (a - sina tana) 2 + b2Rp=Rs . .-. (4

whereka + sina tana)y + bz

2a2 = [(n2 - k2- sin2 a)2 + 4n 2 k 2]'/ 2 + (n 2- k2- sin2 a),

2b2= [(n2

- k2- sin2a)2 + 4n 2k 2]1 /2- (n 2

- k2- sin2a).

(5)

(6)

The optical constants n and k are the real and imagi-nary parts of the complex refractive index of the coating,and a is the angle of incidence. Values of the opticalconstants as functions of wavelength were obtainedfrom the literature.9 -'2 It should be recalled that, inthis discussion, the parallel polarized component of thelight beam from the synchrotron is the componentpolarized in the plane of the electron orbit. Since theorientation of the mirrors is such that this componentis always perpendicular to the plane of incidence (theplane containing the incident ray and surface normal),we use the reflectance Rs for the parallel component ofthe beam and Rp for the perpendicular component.

Figures 9 and 10 are plots of the three-mirror system'stransmittance and resultant polarization at S2 for sev-eral coatings for the wavelength range 200 < X < 2000A.The plot of resultant polarization assumes that thesource emitted light 80% s and 20% p polarization at allwavelengths. This is approximately the case for SURFII in the wavelength range above; however, the actualsource polarization factor is known to be wavelengthdependent, with the s component fraction becominglarger at shorter wavelengths. As can be seen from Figs.9 and 10, platinum provides the least transmittanceexcept at the very shortest wavelengths, while itspolarizing effects are generally less than that of gold butbetter than osmium. Gold has the highest polarizationenhancement but falls below osmium in reflectance,while osmium coatings gave the highest throughput.Figures 9 and 10 also include results for an interestingmixed system in which Ml and M3 are osmium coatedand M2 is gold coated. This Os-Au-Os system takes

4038 APPLIED OPTICS / Vol. 23, No.22 / 15 November 1984

0.0

c 0.7a)

° 0.6E0U 0.5

c 0.4a

0.3a)C

C 0.24-

E 0.10I- 0l- 0

500 1000

Wavelength (A)

1500 2000

Fig. 9. Calculated transmittance of the three mirror optical systemfor the s polarized component of the synchrotron beam for severalcoating materials. Solid curves represent systems with all threemirrors coated with the indicated material, while the dotted curve is

for M2 gold coated and Ml and M3 osmium coated.

100

C0C,

N

-

00L

-a

00_

601

0O 500 1000 1500 2000

Wavelength (A)

Fig. 10. Effective polarization of the beam transmitted to thespectrometer entrance slit by the three-mirror system for severalcoating materials. The solid and dotted curves are as defined for Fig.9. The radiation entering the system at S1 was assumed (see text)to be 80% parallel (s) polarized and 20% perpendicular (p) polarized

at all wavelengths.

advantage of the polarizing power of gold at M2 and thereflecting power of osmium at Ml and M3. Shouldpolarization purity be of great importance, the allgold-coated system is best, particularly below 600 A.Not all materials will enhance the polarization; for ex-ample, calculations for fresh aluminum coated mirrorsshowed a resultant polarization almost identical withthe source beam since aluminum's reflectance for bothpolarizations was very high.

V. Discussion and Summary

The relatively low divergences of the light in thetransfer lines between the mirrors makes it feasible toinsert molecular flow impeding devices13 to improve thevacuum system's differential pumping performance.Many of the experiments planned for the facility imposesignificant gas loads on the system and any sample orcarrier gas must be pumped out in stages so that thepressure at Ml (see Fig. 1) is below 10-9 torr. The op-tical system in Table I allows the insertion of deviceswhich limit the molecular flow conductances to a few

liters/sec between Ml and M2 and between M2 and M3,allowing two stages of differential pumping between M3and the storage ring, at a sacrifice of <20% of theavailable radiation.

In summary, an efficient, polarization enhancingoptical system has been designed and constructed fora high-resolution VUV spectroscopic facility utilizingsynchrotron radiation. The system uses simple cylin-drical optics which match the divergences from the lightsource to the spectrometer's apertures. By selectingcoatings or combinations of coatings, the performanceof the system can be tailored to meet many differentexperimental requirements. The imaging quality of thesystem is adequate for the application intended, but ifhigher quality is required, a similar system could beemployed which uses elliptical cylinders to reduce sig-nificantly the aberrations.

We thank W. R. Hunter for many helpful discussionson coatings and on the optical design and D. K. Prinzfor the use of her ray-tracing subroutines and for helpfuldiscussions on their use. This work was supported inpart by the National Science Foundation under grantPHY 8302871 to the University of Maryland.

References1. D. L. Ederer and S. C. Ebner, "A User Guide to Surf," National

Bureau of Standards, Washington, D.C. (undated report).2. T. Namioka, "Design of High-Resolution Monochromator for the

Vacuum Ultraviolet: An Application of Off-Plane EagleMounting," J. Opt. Soc. Am. 49, 961 (1959).

3. P. G. Wilkinson, "A High Resolution Spectrograph for the Vac-uum Ultraviolet," J. Mol. Spectrosc. 1, 288 (1957).

4. D. K. Prinz, Naval Research Laboratory; Subroutine RTMOS,private communication (1983).

5. G. H. Spencer and M. V. R. K. Murty, "General Ray-TracingProcedure," J. Opt. Soc. Am. 52, 672 (1962).

6. Supplied by Tucson Optical Research Corp., Tucson, Ariz.(1984).

7. J. H. Underwood, "Generation of a Parallel X-Ray Beam and ItsUse for Testing Collimators," Space Sci. Instrum. 3, 259(1977).

8. W. K6nig, Handbuch der Physik, Vol. 20 (Springer, Berlin, 1928),p. 141.

9. L. R. Canfield, G. Hass, and W. R. Hunter, "The Optical Prop-erties of Evaporated Gold in the Vacuum Ultraviolet from 300A to 2000 A," J. Phys. 25, 124 (1964).

10. W. R. Hunter, "On the Optical Constants of Metals at Wave-lengths Shorter than their Critical Wavelengths," J. Phys. 25, 154(1964).

11. J. T. Cox, G. Hass, J. B. Ramsey, and W. R. Hunter, "Reflectanceand Optical Constants of Evaporated Osmium in the VacuumUltraviolet from 300 to 2000 A," J. Opt. Soc. Am. 63, 435(1973).

12. W. R. Hunter, D. W. Angel, and G. Hass, "Optical Properties ofEvaporated Platinum Films in the Vacuum Ultraviolet from 2200A to 150 A," J. Opt. Soc. Am. 69, 1695 (1979).

13. T. B. Lucatorto, T. J. McIlrath, and J. R. Roberts, "CapillaryArray: A New Type of Window for the Vacuum Ultraviolet,"Appl. Opt. 18, 2505 (1979).

15 November 1984 / Vol. 23, No. 22 / APPLIED OPTICS 4039

IM)