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Submitted on 1 Jan 1964

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The optical properties of evaporated gold in the vacuumultraviolet from 300 Å to 2 000 Å

L.R. Canfield, G. Hass, W.R. Hunter

To cite this version:L.R. Canfield, G. Hass, W.R. Hunter. The optical properties of evaporated gold in the vac-uum ultraviolet from 300 Å to 2 000 Å. Journal de Physique, 1964, 25 (1-2), pp.124-129.�10.1051/jphys:01964002501-2012401�. �jpa-00205721�

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a la transition de la bande « d ) au niveau de Fermi.Du reste des r6sultats r6cents de Ehrenreich etPhilipp [4] ont montre que des bandes d’absorptionpar effet photoélectrique interne sont les memespour le nickel et le cuivre qui occupent des positionssimilaires compar6es a celles occupees par I’argent-palladium dans le tableau p6riodique.

11 semble que ce soit donc aussi le cas pour lasequence palladium-argent.Nous allons poursuivre cette 6tude en examinant

les propri6t6s du palladium et de 1’argent puis enobservant ce qui se produit lorsque nous etudieronsles propri6t6s de l’alliage argent-palladium.

Discussion

M. ELLIS. - 1o Were these measurements ofreflectance and transmittance taken in air or

vacuum.

20 If in vacuum, do the values change uponadmission of air ?

M. LosTIs. - Values were taken in air.

M. BLANC. -- Distance approximative supportcreuset ?

M. LOSTIS. - 35 cm.

BIBLIOGRAPHIE

[1] LOSTIS (P.), Thèse Ing.-Docteur, juin 1958 (Rev. Opt.,1959, 38, 1).

[2] ABELÉS (F.), J. O. S. A., 1957, 47, 473.

[3] MARÉCHAL (A.), DUPUY (O.) et RENAULT (M.), Opt.Acta, 1962, 9, 47.

[4] EHRENREICH et PHILIPP, à paraître dans PhysicalReview.

THE OPTICAL PROPERTIES OF EVAPORATED GOLDIN THE VACUUM ULTRAVIOLET FROM 300 Å TO 2 000 Å (1)

By L. R. CANFIELD and G. HASS,U. S. Army Engineer Research and Development Laboratories, Fort Belvoir, Virginia.

and

W. R. HUNTER,E. O. Hulburt Center for Space Research, U. S. Naval Research Laboratory, Washington, D. C.

Résumé. 2014 On a mesuré les facteurs de réflexion de couches d’or, obtenues par évaporation,dans l’ultraviolet de 300 à 2 000 Å. On a déduit les constantes optiques des mesures de facteurde réflexion effectuées pour différents angles d’incidence. Contrairement à ce qui se passe pourl’aluminium, le facteur de réflexion de couches d’or préparées sous vide varie peu au cours de leurexposition à l’air. Des couches semi transparentes, d’une épaisseur voisine de 150 Å, présententdes maximums de réflexion, dus à des phénomènes d’interférence, pour de nombreuses longueursd’onde de l’ultraviolet lointain.

Abstract. 2014 The reflectance and optical constants of evaporated gold films were measured inthe vacuum ultraviolet from 300 Å to 2 000 Å. The optical constants were determined fromreflectance measurements made at various angles of incidence. In contrast to aluminium, goldshows little change in reflectance after deposition in vacuum and during exposure to air. Due tointerference, semitransparent films of about 150 Å on glass show the highest reflectance at mostwavelengths in the vacuum ultraviolet.

LE JOURNAL DE PHYSIQUE TOME 25, JANVIER-FÉVRIER 1964,

Introduction. - A study of the optical prop-erties of gold in the vacuum ultraviolet is ofinterest for practical and theoretical reasons. Thereflectance of gold is higher than that of many

(1) This work was supported in part by the GoddardSpace Flight Center, NASA, Greenbelt, Maryland.

other film materials for wavelengths shorter than900 A and exhibits almost no change duringextended use and storage in air. In addition, goldfilms are easy to deposit in any desired thicknessand have proven to be an excellent medium inwhich tojrule diffraction gratings for use in theextreme ultraviolet.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01964002501-2012401

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From the theoretical point of view, measure-ments of the reflectance and the optical constantsof gold in the vacuum ultraviolet are of interestto the solid state physicist since they furnish infor-mation on the absorption spectrum and energyband structure of the metal.

It is the purpose of this paper to present theresults of recent measurements of the reflectanceand optical constants of gold in the wavelengthregion from 300 A to 2 000 A, and to discuss theeffect of film thickness on the optical properties inthis region.

Experimental techniques. - All evaporationswere performed in an evaporator-reflectometercombination in which the reflectance of the filmscould be measured within seconds of the comple-tion of their deposition. The instrument, shownin figure 1, is attached to a one-meter normalincidence monochromator. A detailed description

FIG. 1. - Section of evaporator-reflectometer combination

of the system has been published elsewhere [1],therefore only the salient features of this instru-ment will be mentioned here. Basically the evapo-rator-reflectometer consists of two vacuum cham-bers separated by a barrier with a hole in it. The

vapor source, in this case a tungsten boat con-taining gold, is located in the lower chamber, whilethe upper chamber contains the reflectometer.The substrate holder may be disengaged from thereflectometer mechanism and placed over the holein the barrier between the two chambers. Thus,during an evaporation, the substrate faces the boatwhich is approximately 40 cm away from it. The

substrate and an additional shutter serve to shieldthe upper chamber and its contents from themetal vapor. Immediately after the completionof an evaporation the substrate may be swung upinto position for measurement.The polished glass plates used as substrates were

. given a preliminary cleaning with a detergent and,just prior to the evaporation, a final cleaning witha high-voltage dc glow discharge.The evaporation rates of gold were varied so

that the deposition rates ranged from 2 to1 600 Å/sec; the thickness was monitored bymeasuring the light transmittance at À = 5 000 A.During the evaporations the pressure was approxi-mately 10-6 torr, except when it was deliberatelyincreased to study the effect of pressure on theoptical properties.Two types of light sources were used to cover the

spectral range from 300 A to 2 000 A. One wasa dc glow discharge (1) and the other a pulseddischarge source (2). Figure 2 shows the spectraof the radiation available for measurement. withthe two light sources. The radiation emitted fromthe glow discharge is shown at the top of ( fig. 2).The many-line hydrogen molecular spectrum,which within the limits of resolution of the mono-chromator is a quasi-continuum, and two atomiclines at 1216 A and 1 026 A, cover the spectralregion from approximately 900 A to beyond theupper wavelength limit used in this investigation.The neutral resonance lines of He and Ne producedisolated lines at 584 A and 736 Å, respectively,when those gases were introduced into the lightsource. More complete coverage of the regionbelow 900 A is afforded by using the pulsed dis-charge source. The remainder of figure 2 showsthe radiation available using the pulsed dischargewith different gases. The spectral range down to300 A is. covered rather fully using the four gasesshown. With oxygen, wavelengths as short as246 A may be obtained.The reflectance of the films could be measured

at any angle of incidence between 60 and 860 usinga 1P21 photomultiplier sensitized with sodium sali-cylate as a detector. For the determination of

optical constants by the reflectance method des-cribed by Tousey [3] and Simon [4], reflectancemeasurements were made at several angles of inci-dence at each wavelength. The resulting valuesof reflectance were then used to determine the

possible values of the index of refraction n andof the extinction coefficient k (the complex indexof refraction, N = n - ik) for each angle of inci-dence by means of previously computed curves.The values of n and k common to each determi-nation for several angles of incidence then gavethe optical constants at the wavelength understudy. These initial values were then fed into a

digital computer which was programmed to cal-

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FIG. 2. - Spectra produced by the two light sources.

culate the reflectance for each angle at whichmeasurements were made and to evaluate the de-viation between the measured and calculatedvalues.. The constants were then varied by smallamounts and the calculation repeated until thedeviation was minimized. The program also com-

puted the reflectance at normal incidence which,in all cases, agreed within experimental error withthe measured values.

Results. - Evaporated gold shows little changein vacuum ultraviolet reflectance after its depo-sition in vacuum, and during exposure to and pro-longed storage in air. The marked contrast inbehavior to that of a reactive metal such as alumi-num may be seen in figure 3. In separate exper-iments, the reflectance of freshly deposited alumi-num and gold were studied as a function of timeat 1216 A, before and after exposure to air. Thereflectance of aluminum drops rapidly, particu-larly upon exposure to air, due to the formation

FIG. 3. - Comparison of the effect of aging on the reflec-tance of evaporated films of gold and of aluminium at1 216 A.

of a strongly absorbing oxide layer [5]. The reflec-tance of gold, however, decreases only about onepercent while in vacuum and stays essentially

127

constant after exposure to air. At other wave-lengths the degree of change was about the. same,but at some wavelengths a slight increase could beobserved.. Contamination resulting from storagein air sometimes caused a slight change in reflec-tance at all wavelengths, however, a simple rinsewith ethyl alcohol restored the reflectance prac-tically to its original value. Thus the conclusionwas reached that measurements in situ were unnec-essary, and that the true optical properties of goldcould be studied using films that had been exposedto air.The effect of deposition parameters on the reflec-

tance of gold films was investigated, and it wasfound that neither the pressure during depositionnor the rate of deposition were critical untilextremes were reached. Deposition rates of from50 to 1 600 A per second with pressures of fromless than 10-6 to above 10-5 torr were employedwith practically no effect on the reflectance of theresulting film. Only when a rather thick film wasmade under poor conditions did the reflectancedrop, undoubtedly due to increased surface rough-ness. Ip figure 4 the upper solid curve representsthe reflectance of a film made in a more or lesstypical evaporation ; the lower is the reflectanceof a film made under the relatively poor conditionsgiven. The reflectance values of Robin [6], obtain-ed from visibly opaque films and shown as thedashed curve of figure 4, are considerably lowerthan those measured here.

FIG. 4. - Effect of evaporation conditions on the reflec-tance of evaporated gold as a function of wavelengthfrom 1 000 A to 2 000 A.

If the surface of the evaporated gold film is

rough, it will scatter the incident radiation andcause erroneous reflectance measurements whichwill result in inaccurate determinations of n and k.Since roughness increases with increasing film

thickness, the films should be kept thin to avoidroughness but, at the same time, thick enough tobe opaque. Interference effects and transmissionlosses will be negligible if less than 0.1 % of the

energy incident of the vacuum-film interface istransmitted through the film to the glass substrate.Calculations of the film thickness required to satisfythis condition were made using preliminary valuesfor n and k obtained from a film 1600 A thick.A plot the of thickness necessary to reduce trans-mission to 0.1 % as a function of wavelength from300 Å to 2 000 A is shown in figure 5. The opticalconstants given by Schulz [7] and Schulz andTangherlini [8] were used to calculate the trans-mittance of gold at 5 000 A as a function of thick-ness, and the right-hand scale of figure 5 gives thetransmittance at 5 000 A corresponding to the

FIG. 5. -j- Calculated thickness of gold films required todecrease transmittance to 0.1 % as a function of wave-length from 300 A to 2 000 A. Transmittance data at5 000 A are included for comparison.

thickness given in the left-hand scale..It is seenthat films that are opaque in the vacuum ultra-violet are semitransparent at 5 000 A. Thusmonochromatic light of that ’wavelength may beused to monitor the thickness of gold films thatwill be opaque in the vacuum ultraviolet.Once the desirable thickness for gold films was

determined for the various wavelengths, meas-urements of n and k were made using films of theappropriate thicknesses. The results are shown

FIG. 6. - Optical constants and normal incidence reflec-tance of gold in the vacuum ultraviolet.

128

in figure 6, where n, k, and the measured normalincidence reflectance are plotted as a function ofwavelength from 300 A to 2 000 Å. Cole andOppenheimer [9] have measured the optical cons-tants of gold at the wavelengths 1216, 1048, 920,584 and 304 A. Their values of n and k are,with the exception of the indices at 584 A and304 A, lower than the ones reported here.

Reflectance measurements from 1770 A to 450 Ahave been reported by Walker, Rustgi andWeissler [9a] who obtained the same reflectanceversus wavelength characteristic as shown in

figure 6. Their reflectances were equal to thoseshown in the figure for wavelengths greater than700 A, but somewhat lower for wavelengths shorterthan 700 A.The extinction coefficient, k, cannot be corre-

lated directly with- absorption processes occurringin the metal, however, Frohlich and Pelzer [10]have shown that a phenomenological relation existsbetween absorption processes and the bulk opticalconstants. According to them, the quantity,

when plotted against wavelength, should havemaxima corresponding to absorption processes.

This expression was calculated using the n and kvalues of figure 6 and the results are shown infigure 7. There are distinct maxima ; at 32.6 eV,25.8 eV, 16.3 eV, and 6.7 eV, corresponding towavelengths 380, 480, 760 and 1 880 A.

FiG. 7. - 2nk/(n2 + k2)2 as a function of wavelengthfor gold in the vacuum ultraviolet.

Robins [11] used a reflection technique to studymonoenergetic electrons elastically and inelas-

tically scattered by an evaporated gold surfaceand observed four characteristic losses of aboutthe same energy ; 32.6 eV, 25.8 eV, 16.0 eV and6.3 eV.

Interference effects may be observed when verythin gold films are deposited on glass. Figure 8shows the calculated and measured normal inci-

FIG. 8. - Calculated and measured reflectance of opaqueand semi-transparent (t = 150 A) gold films on glass asa function of wavelength from 1 000 A to 2 000 A.

dence reflectances of opaque and 150 Å thick filmsfrom 1000 A to 2 000 A. The agreement betweencalculated and measured values for the 150 A thickfilm indicates that n and k of the bulk materialare still correct for films this thin. The curve alsoshows that highest reflectance is obtained withfilms that are not opaque in the extreme ultra-violet. The same situation applies to films ofplatinum [12].The calculated reflectance of evaporated gold on

glass as a function of film thickness at severalwavelengths is shown in figure 9. At all wave-

Fic. 9. - Calculated reflectance of gold on glass as a func-tion of film thickness for various wavelengths in thevacuum ultraviolet.

lengths shown except 700 Å there is an increasein reflectance at a gold thickness less than about250 Å. It can be concluded that for use as areflector throughout the vacuum ultraviolet a goldthickness of about 150 A on glass represents a goodcompromise.

129

REFERENCES

[1] MADDEN (R. P.) and CANFIELD (L. R.), J. Opt. Soc.Am., 1961, 51, 838.

[2] HUNTER (W. R.), Proc. Xth Colloquium Spectrosco-picum Internationale, Spartan Books, Washington,D. C., 1963, p. 247.

[3] TOUSEY (R.), J. Opt. Soc. Am., 1939, 29, 235.[4] SIMON (I.), J. Opt. Soc. Am., 1951, 41, 336.[5] MADDEN (R. P.), CANFIELD (L. R.) and HASS (G.),

J. Opt. Soc. Am., 1953, 53, 620.[6] ROBIN (S.), C. R. Acad. Sc.,1953, 236, 674.[7] SCHULZ (L. G.), J. Opt. Soc. Am., 1954, 44, 357.

[8] SCHULZ (L. G.) and TANGHERLINI (F. R.), J. Opt. Soc.Am., 1954, 44, 362.

[9] COLE (T. T.) and OPPENHEIMER (F.), Applied Optics,1962,1, 709.

[9a] WALKER (W. C.), RUSTGI (O. P.) and WEISSLER(G. L.), J. Opt. Soc. Am., 1959, 49, 471.

[10] FROHLICH (H.) and PELZER (H.), Proc. Phys. Soc.,1955, A 68, 525.

[11] ROBINS (J. L.), Proc. Phys. Soc., 1961, 78, 1177.[12] JACOBUS (G. F.), MADDEN (R. P.) and CANFIELD

(L. R.), J. Opt. Soc. Am., 1963, 53, 1084.

THE ANGULAR AND SPECTRAL DISTRIBUTIONOF TRANSITION RADIATION FROM THIN SILVER FOILS

By E. T. ARAKAWA, N. O. DAVIS, L. C. EMERSON and R. D. BIRKHOFF,Health Physics Division, Oak Ridge National Laboratory (1), Oak Ridge, Tennessee.

Résumé. 2014 On a déterminé expérimentalement les distributions angulaire et spectrale, dans ledomaine de longueurs d’onde de 2 500 Å à 5 600 Å, de photons émis par des feuilles d’argent de660 Å et 1 980 A d’épaisseur lorsqu’on les bombarde avec des électrons de 40 keV. La distributionspectrale de la lumière polarisée dans le plan d’incidence a montré un maximum aigu à3 300 Å ± 12 Å. Cette valeur est une moyenne évaluée sur 1’ensemble des directions faisant avecla normale à la feuille des angles allant de 10° à 50°. En plus de ce maximum, le spectre présentaitun fond continu dont l’intensité décroissait lentement lorsque la longueur d’onde augmentait,etun faible minimum à 3 200 A, suivi, pour les longueurs d’onde plus courtes, d’une augmentationd’intensité. La distribution angulaire des photons émis, pour la longueur d’onde correspondantau maximum, présente un maximum à 30° de la normale à la feuille. Aucun photon n’est émisperpendiculairement ou tangentiellement à la feuille. En revanche, la lumière émise pour d’autreslongueurs d’onde du fond continu, par exemple 2 700 Å et 4 500 A, était surtout intense à 50°de la normale. On a déterminé le rendement lumineux absolu après avoir étalonné le spectro-mètre, l’analyseur et Ie photomultiplicateur avec une lampe à filament de tungstène provenantdu U. S. National Bureau of Standards. En ce qui concerne les directions faisant un angle de 10°.à 40° avec la normale, le rendement lumineux était en tout point en accord avec les prévisionsthéoriques pour toutes les longueurs d’onde sauf pour celle correspondant au maximum aigupour laquelle la valeur expérimentale est inférieure de 30 % environ.

Abstract. 2014 The angular and spectral distributions of photons emitted by Ag foils 660 Å and1 980 Å in thickness when bombarded by 40 keV electrons have been determined experimentallyin the wavelength region from 2 500 A to 5 600 Å. The spectral distribution of light polarizedin the plane of incidence showed a sharp peak at 3 300 Å ± 12 Å, this value being an averageover photon directions from 10° to 50° from the foil normal. In addition to the peak, the spectrumshowed a continuum which decreased slowly with increasing wavelength and a deep minimumat 3 200 Å with a rise in the shorter wavelengths. The angular distribution of photons emittedat the peak wavelength showed a maximum at 30° from the foil normal with zero intensity at 0°and near 90°, whereas the photons emitted at other wavelengths in the continuum, e.g., 2 700 Aand 4 500 Å, were most intense at 50° from the foil normal. The absolute photon yield was deter-mined by calibrating the spectrometer, analyzer, and photomultiplier with a tungsten filamentlamp obtained from the U. S. National Bureau of Standards. For photon directions from 10°to 40° the photon yield was found to agree with the theoretical predictions in all respects at allwavelengths except at that of the sharp peak where the experimental values were about 30 %lower.

JOURNAL DE PHYSIQUE TOME 25, JANVIER-FÉVRIER 1964,

Introduction. - In 1945 Frank and Ginsburg [1]predicted that radiation would be emitted when auniformly moving electron passed from one

medium into another. To explain the pheno-(1j Operated by Union Carbide Corporation for the

U. S. Atomic Energy Commission.

menon of this " transition radiation ", they consi-dered an electron passing from a vaccum into aperfect conductor. While approaching the con-ductor the electromagnetic field in the vacuum isequal to the field of the electron and of its imagemoving towards it. From the point of view of