JOURNAL OF TIE OPTICAL SOCIETY OF AMERICA

The Preparation and Optical Properties of Gold Blacks* tLoUIs HARRIS AND ROSEMARY T. MCGINNIES **

M1assachusetts Institute of Technology, Cambridge, Massachusetts

AND

BENJAMIN M. SIEGEL***Weizinann Institute of Science, Rehovoth, Palestine

The optical properties of gold smokes deposited on cellulose nitrate films under differentexperimental conditions have been studied. The conditions of pressure of the inert atmosphere,purity of the gas, rate of evaporation, and the distance between source and deposition surfacegiving the highest infra-red absorption per unit mass have been found. The thermal massrequired for high infra-red absorption is small compared to the thermal masses of other receiversused for infra-red measurements. The gold "blacks" turn yellow and have lower infra-redabsorption when heated above 1100C.

Gold " blacks" with very high infra-red transmission (3-15A) and low transmission at shorterwave-lengths are prepared when oxygen is present in the "inert" atmosphere.

The particle size and particle distribution of the gold smokes deposited under differentexperimental conditions have been investigated with the electron microscope at high resolution.

INTRODUCTION

THE use of deposits of metallic smokes forthermal receiverswassuggested byPfundl 2

and by Burger and Van Cittert. 3 Gold "blacks"for absorbing thermal radiation have been re-ported by several investigators." Measurementsin this laboratory with a number of metallicsmokes indicated that gold "blacks" might havea high ratio of (infra-red absorption/thermalmass). This is an important property for infra-reddetectors designed to respond to rapid variationsof incident radiation. The quality of the blackobtained by evaporating gold in an inert atmos-phere depends among other things on the gaspressure, the purity of the gas, the distancebetween the source and the surface of deposition,the mass of the receiver, the rate of gold evapora-tion, and the rate of heat conduction from thereceiver. We have studied the influence of each

* This work has been supported by the Navy Depart-ment under Contract NObs 25391.

t Contribution from Department of Chemistry, Massa-chusetts Institute of Technology, Cambridge, Massa-chusetts.

** Present address: University of Alabama, Tuscaloosa,Alabama.

*** At the Polytechnic Institute of Brooklyn, New York,by special arrangement.

'A. H. Pfund, Rev. Sci. Inst. 1, 397 (1930).2 A. H. Pfund, J. Opt. Soc. Am. 23, 375 (1933).3 Burger and Van Cittert, Zeits. f. Physik 66, 210 (1930).'L. C. Roess and E. N. Dacus, Rev. Sci. Inst. 16, 170

(1945).S C. B. Aiken, W. H. Carter, Jr., and F. S. Phillips,

Rev. Sci. Inst. 17, 380 (1946).

of the parameters mentioned upon the qualityof the gold "black" and correlated them withmeasurements of the mass/unit area of the golddeposit.

Absorption properties of the evaporated gold"blacks" were obtained from transmission andreflectance measurements of the smoke depositedon cellulose nitrate films. The transmission tothe radiation of a 50'C blackbody, the specularreflection of the 50'C blackbody radiation inci-dent at an angle of 450, the spectral transmissionfrom 0.4 to 15, and the diffuse reflection in thevisible spectrum, were all measured on depositsmade under different conditions.

The fraction of incident radiation absorbed bya blackened film is given by

A=1-T-S, (1)

where T is the fraction transmitted and S is thefraction scattered. By measuring the transmis-sion of the blackened film, only an upper limitto the amount of radiation absorbed is obtained.However, it has been found here that gold"blacks" prepared under certain conditions havea small back scattering and therefore the absorp-tion of such "blacks" can be calculated forcomparative measurement from the transmissiondata alone. The absorption by the cellulosenitrate films used here has been found to benegligible.

582

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VOLUME 38, NUMBER 7 JULY, 1948

GOLD BLACKS

Electron microscope studies were made on thegold smoke deposits in order to correlate theobserved optical properties with the particlesize and distribution of the deposits. They willbe reported in detail elsewhere.

EXPERIMENTAL PROCEDURE

The technique employed in producing thegold "blacks" and measuring their propertieswill be described in detail.

Preparation of Specimens

1. Cellulose nitrate films. The films were madeby spreading a solution of thirty-second-viscositycellulose nitrate, containing plasticizer, on- awater surface.

The films weighed -8 X 10-6 gram/cm 2 , corre-sponding to a thickness of -5 X 10-6 cm.

The nitrocellulose films were mounted on brassframes 5 cmX5 cmXO.105 cm thick, having ahole 3 cm in diameter, in the center of the frame.A 5-cm square of 0.8-mm thick copper sheet wasfastened to the back side of the frame. Thiscopper backing served to dissipate the heat fromthe cellulose nitrate film during the gold evapo-ration. The front (film) side of the frame facedthe filament from which the gold was evaporated.

2. Evaporation of gold. The "blacks" were pre-pared in an atmosphere of nitrogen to permitcontinued use of a tungsten filament. A 5-cmlength of 0.5-mm diameter gold wire was woundaround the "V" of a 30-mil tungsten filament,8.7 cm long. The filament was heated in a highvacuum until the gold melted to form a dropat the bottom of the "V," and then the tem-perature was quickly raised well above thatto be used in the preparation of the gold"blacks." The current through the filament wasreduced as soon as the "spitting" of impuritiesceased. The gold was kept just molten until afterthe nitrogen was introduced and the pressureadjusted; then the current through the filamentwas increased to the desired value. A filamentcan be used twelve to twenty times, if it is notbroken while applying the gold wire. It is ad-visable to examine the filaments carefully undera microscope before inserting them in the elec-trodes. A crack '-2X10-4 cm wide in the "V"of the filament produces a streaked gold "black"deposit.

A shield 1.3 cm from the filament screened thefilm from most of the tungsten filament.

A solenoid-operated-shutter, between theshield and film, permitted the film to be exposedto the gold-"black" smoke for a desired lengthof time, and only when the current through thefilament was constant.

The apparatus for preparing gold "blacks" isshown in Fig. 1.

In all the experiments, the drop of gold wascentered opposite the center of the film; theposition of the filament was held fixed at 7 cmfrom the nearer side of the bell jar, and theposition of the film was varied to give therequired distance for the given experiment.

Description of Measurements

1. Weighings. The central area of the cellulosenitrate film has a uniform "black" deposit on acircle of diameter of 2.4 cm. A circle of 2-cmdiameter was cut from this central area bymeans of a cork borer.

The circles of cellulose nitrate plus the gold"black" weighed from -0.05 X 10-3 gram to0.9 X 10-3 gram. Since a cellulose nitrate film ofthis area weighed only 0.025 X 10-3 gram, varia-tions of 20 percent in the thickness of the cellu-lose nitrate films could be tolerated withoutaffecting the required accuracy of the goldweighings. The variations in thickness of cellu-lose nitrate film were, however, less than 20percent. The weighings, made with an Ainsworth

|| H ~m(iloment tosh u tt e r Age f c e of W. m)

I ps to h Id a e to shied)V beII- c k,

4 g .0 Ad - tt tu~~ngten

l film h~~~~~~~~~~~olde_ igc to be .

vaporized

FIG. 1. Apparatus for the preparation of Gold Black.

583

HARRIS, MIcGINNIES, AND SIEGEL

micro-balance, were reproducable to i0.002X10-3 gram. To achieve this accuracy it wasnecessary to maintain the films in desiccatorsbetween operations and measurements. In humidweather, the films were also dried at 60'C forthirty minutes before weighing.

2. Infra-red transmission. A blackbody as de-scribed by J. D. Hardy6 was used as a sourceof infra-red radiation in the 10A region. Itconsisted of a negative cone painted black andimmersed in a constant temperature water bathmaintained at 50.0040.05'C. An aluminummirror was located at 450 to the optical axis and23.4 cm from a limiting aperture in front of theradiating cone. This mirror was mounted on arotatable holder so that the mirror could bereplaced exactly by one of our experimentalfilms and its function will be described under thereflection measurements. A thermopile connectedto a General Motors d.c. Breaker Amplifier waslocated 2.9 cm from the mirror and adjusted tointercept the radiation reflected at 45°. Theamplified response of the thermopile was indi-cated on a microammeter.

A shutter in which the blackened films weremounted was placed 7 cm from the aluminum

10 _

0.7-- - - - 1

06 _

0.4 Rote o p.ti. 33 10'gm sec.

Distance eceiver t ngsten itomert, 7Cm

03 _ _ quols fraction Transmitted

0. Nitlrogen Pressure

O - T 0.3m - -0~~~~~~~o0m

0 20 40 60 80 100 120 140DENSITY OF DEPOSIT

gram I/ceNimeter'

FIG. 2. Absorption of infra-red radiation by gold "blacks"as a function of density of deposit.

6 J. D. Hardy, J. Clin. Investigation 13, 593, 615 (1934).

mirror so that the film could be placed in thepath of the radiation or raised out of the way.A limiting stop was placed in front of the filmso that only the central portion of the depositwas irradiated in these measurements. Thetransmission ratio was the quotient of the meterdeflection with the blackened film in the radia-tion path and the deflection with the filmremoved.

3. Infra-red reflection. Relative reflections wereobtained bv mounting blackened films on theopposite side of the rotatable aluminum mirrordescribed above and measuring the deflection ofthe microammeter when the radiation was re-flected from the film and from the aluminummirror. Thus the angle of incidence for all theinfra-red reflectance measurements was 450. Theabsolute reflection ratio was somewhat lowerthan the measured value because the aluminummirror was not a perfect reflector. Its reflectionratio with respect to a new gold mirror wasmeasured and found to be 0.9. The reflectionratios reported below are corrected to be relativeto the reflection from the gold mirror. Blackpaint and black cardboard had reflection ratiosof -0.025.

A cellulose nitrate film had an infra-redtransmission of 99 percent while the infra-redreflection for an angle of incidence'of 45° variedbetween 4 percent and 7 percent. Reflectionmeasured as described here gives a value inexcess of the diffuse reflection.

4. Infra-red spectral transmission. The infra-red spectral transmissions were made at theM.I.T. Spectroscopy Laboratory with a Beck-man infra-red spectrometer. The measurementsextended from 1.0 to 15 A. A Beckman ultravioletspectrometer was used for the measurementsfrom 0.6,u to 1.0,.

S. Visible spectral transmission and visiblespectral diffuse reflectivity. Spectral diffuse trans-missions and reflections in the visible spectrumwere made at the M.I.T. Color MeasurementLaboratory, using a Hardy clorimeter.

OBSERVATIONS AND MEASUREMENTS

1. Infra-red transmission for different thick-nesses of gold "blawk." Figure 2 shows the infra-red transmission of gold "blacks" prepared atthe optimum distance, 7 cm, between tungsten

584

GOLD BLACKS

filament and cellulose nitrate film, and at anintermediate rate of gold evaporation, for thisdistance, and at different pressures of nitrogen.

2. The effect of distance between filament andfilm on the nature of the deposit. Gold "blacks"were prepared at different rates of evaporationand at different pressures at distances betweentungsten filament and cellulose nitrate film of3.8 cm, 7.0 cm, and 14 cm.

2a. Deposits prepared at a distance of 3.8 cm'The deposits prepared at a rate of evaporationof 3.3 X 10-3 gram of gold per second and atnitrogen pressures of from 0.5 to 3.0 mm reflected15 percent of infra-red radiation. The infra-redreflection increased for the deposits prepared atincreased rates of evaporation; at the evapora-tion rate of 16X10-3 gram of gold per second,the reflection was 77 percent for a deposit havinga transmission of 22 percent. The gold "blacks"prepared at a lower evaporation rate (-1 X 10-3gram of gold per second) and at a pressure of2.0 mm gave the best absorption coefficient atthis distance (reflection -8 percent). The ab-sorption "coefficient" was less, however, thanthat obtained for the 7.0 cm distance.

2b. Deposits prepared at a distance of 7.0 cm.The deposits prepared at a rate of evaporationof 3.3 X 10-3 gram of gold per second showed aninfra-red reflection of 15 percent for a pressureof 0.3 mm. The deposits formed at a pressure of0.3 mm had a yellowish appearance. Of thedeposits formed at. a pressure of 0.5 mm, thedeposits of lower mass reflected 5 percent of the

TABLE 1. Infra-red absorption for different rates ofgold evaporation.

Fila-ment

current(am-peres)

21.223252730

Approx.rate of

t evaporationof gold

(gram/sec.)

.0.5X10- 3

0.9X 10-31.5X 10-32.3X 10-33.3 X 10-3

Time(sec-onds)

3001601206050

Infra-red(BB)trans-mission,T frac-tion

0.080.190.210.210.225

Infra-red450

reflectionfraction

<0.01<0.01

0.030.030.04

weightof gold

(gram/cm2 )

83X10-658 X 10-656 X 10-657 X'10-655 X 10-6

k*

13,00012,00012,00012,00012,000

k0 =(I/gram/cm2 ) logol/ T

incident radiation; the reflection increased to asmuch as 15 percent for the heavier deposits.The deposits prepared at a pressure of 1 mmhad the lowest reflections. The infra-red reflec-tion increased from less than 1 percent to 4percent as the evaporation rate increased from0.5X10-3 gram of gold/second to 3.3 X 10-3

gram/second.Deposits formed at an evaporation rate of

-12 X 10- gram of gold per second, and atpressures below 1.0 mm, were yellowish. Thedeposits formed at pressures of 1.0 to 3.0 mm,at this rate, reflected -1 1 percent of the incidentradiation.

2c. Deposits prepared at a distance of 14 cm.The optimum pressure for the deposition of goodinfra-red absorbers, at this distance, was also1 mm. The infra-red absorption per unit mass forthe deposits formed at this distance was uni-formly lower than for the 7.0-cm distance,except for light deposits.

FIG. 3. Infra-red transmissionof gold "blacks."

z

E(0*

z0

toa.L

WAVELENGTHMicrons

585

HARRIS, McGINNIES, AND SIEGEL

rABLE II. Change in transmission of gold "black" depositson being heated.

Before heating After heatingRate of Density A A

evapora- of gold = I = Pres- tion "black" -(7' -(7'sure (g/sec.) (g/cn5 ) 7' R* +R*) 7' R* +R*)a

0.5 nsi 3.3 XI0-3 41 X10-6 0.52 0.13 0.35 0.31 0.35 0.340.5 0.0X10-3 66X10'- 0.37 0.15 0.48 0.14 0.45 0.41Lo ° 3.3Xl0-3 106X10-6 0.06 0.03 0.91 0.03 0.09 0.88

R* =infra-red reflectance at 45 degrees.

3. The variation of infra-red absorption ithrate of gold evaporation. The data obtained for apressure of 1 mm and for a distance of 7 cm aregiven in Table 1. There is, thus, very littlechange in absorption with rate of gold evapora-tion up to rates as high as 3.3 X 10-3 gram/second;at still higher rates the absorption becomes less.The deposits prepared at the lowest rate havethe lowest reflection.

4. Effect of purity of the gas. Gold blacks withthe largest k* (such as those represented inTable I) were made with the purest gas used,nitrogen containing less than 0.3 percent ofoxygen, or a partial pressure of 0.003 mm ofoxygen. When the nitrogen was enriched inoxygen, the k* decreased, so that for a gasmixture containing 5.5 percent oxygen the k*decreased to a value of approximately 1 percentof that for the deposits prepared with the purernitrogen. For gas mixtures containing between0.3 percent and 5 percent the k's had valuesintermediate between that for the purest gasand the low value given.

In order to be certain of obtaining a "black"

having a high absorption in the visible and lowabsorption in the infra-red, the inert gas shouldcontain at least 2 percent oxygen and the rate ofgold evaporation should be approximately 0.5X 10-3 gram/second. (We have obtained depositshaving such characteristics with gas mixturescontaining even less than 1 percent oxygen, atrates less than 0.5 X 10-3 gram/second.)

5. Spectral transmission in the infra-red. Thetransmission of radiation of a number of depositsof gold smoke prepared under different conditionsis given as a function of wave-length from O.S,to 151i in Fig. 3. The percent transmission ofeach of these deposits to radiation from the500 C blackbody is given in the first column onthe right and their respective masses in thesecond column on that side. All the depositswere prepared at a distance of 7.0 cm.

Curves 1 and 3 are given by deposits of differ-ent mass produced at 1 mm nitrogen, minimumamount of oxygen, and intermediate rate. Thetransmission can be seen in both cases to decreaseslightly to a minimum just beyond 2 and thento rise gradually a few percent, remaining con-stant beyond 8 to lO,. Curve 2, obtained froma deposit made in 1.0 mm of nitrogen but at afast rate of evaporation (approximately 12 X 10-3g/sec. of gold) has similar characteristics but isa poorer absorber per unit mass.

Curves 4 and 5 show the very interestingcharacteristics of deposits produced when oxygenpresent. These curves have a pronounced mini-mum at 0.8 yu and then a sharp rise to 2 ,u so that

80 -- = 450 ____

30 _ -

7 - -

40

400 450 500 550 0

WAVELENGTH

FIG. 4. Visible diffuse trans-mission and reflection of gold"blacks."D! y

I ba

"I

400 450 500 550 600

WAVELENGTH

100, . . . . .

586

GOLD BLACKS

TABLE III. Summary of electron microscope studies of gold smoke deposits.

Rate of gold Nitrogen Size of unit colloid particle Number of particles Type ofevaporation pressure limits average in initial aggregates aggregation

0.5 to 3.3 0.5 to 1.0 mm* 10-150A 50A many open networkX 10-3 gram/sec. 3.0 mm* 250A many open network

3.3X 10-3 gram/sec. 0.3 mm 15 to 100A 50A few uniform0.5 mm 25 to 200A 100A intermediate sponge-like1.0 mm 75 to 250A 150A sponge-like

lo lO-3 gram/sec. 0.5 to 1.0 mm 10 to 200A 40A few uniform

* One percent oxygen present.

the deposits are almost completely transparentbetween 3 and 10,4 and falling to approximately90 percent at 15,u. The deposit for curve 5 wasmade at a total pressure of 1 mm and at a lowrate of gold evaporation. Such a curve is char-acteristic of deposits made at 1 mm pressure,slow rate, and oxygen partial pressure of 0.02mm. Curve 4, which has the same opticalcharacteristics as curve 5, was obtained for adeposit made at a total pressure of 3.0 mm andat an intermediate rate. It is characteristic ofintermediate rates of deposit if the partialpressure of oxygen is greater than 0.06 mm.

If care is exercised that no oxygen is introducedwith the introgen, that the bell jar system istight, and that there is a minimum of adsorbedoxygen, good quality deposits such as illustratedby curves 1, 2, and 3 and in Table I can beobtained at 1-mm pressure with low rates ofevaporation or at 3.0 mm and intermediate rates.

The transmissions indicated in Fig. 3 showthat the transmissions obtained with the 50'Cblackbodv are satisfactory measures of the trans-mission between 2 and 15,4.

6. Spectral properties in the visible. The diffusereflection of visible radiation was measured as afunction of wave-length on several deposits todetermine whether there is a characteristicscattering in the visible region which might beused as a criterion for the character of the"black" as an absorber of infra-red radiation.These data are shown in Fig. 4.

All the gold "blacks" show low reflection,nearly all show slightly less reflection in thered than the blue.

All the deposits are characterized by a maxi-mum transmission at 480 mu. Bright gold depositshave the maximum transmission at 500 m1u.

The gold "black" deposits with the highestinfra-red absorption per unit mass (No. 1 andNo. 3) have greater transmission in the red thanin the blue, while gold "black" deposits with lowinfra-red absorption coefficient (No. 5 and No. 4)have nearly the same or slightly lower trans-mission in the red than in the blue. In a numberof cases, deposits prepared under the conditionsused for No. 5 and No. 4 appeared blue to theeye by transmitted light (incandescent source).

7. Effects of heating gold deposit. There areseveral other factors which influence the char-acter of the gold deposit and one of them is thetemperature of the receiving surface, or thesubsequent heat treatment to which the depositis subjected. In our study the deposits whosetransmission and reflection had been measured,were heated to 120'C for two to three hours andthen the changes in transmission and reflectionwere observed. (The cellulose nitrate film uponwhich the gold was deposited limited the temper-ature for heating to a maximum of 120'C.) The"black" deposits took on a yellowish appearanceafter this heat treatment. Table II lists theobservations which were made on such deposits.

It is immediately evident that there is a lossin transmission upon heating and that this lossis caused by increased reflection. There is atendency for the amount absorbed to decreaseand in some cases the reflectivity increased

TABLE IV. Comparison of gold "blacks" and carbon asinfra-red absorbers.

Percentabsorption Mass Thermal mass

Gold "black" 80 56 X10-6 g/cm2

1.7 X10-6 cal./0

C/cm2

Carbont 72 180 X10-6 g/cm2 36 X10 cal./oC/ m2

t Clemens Schaefer and F. Matossi, Das tJltrarote Spektruim (VerlagJulius Springer, Berlin, 1930).

587

HARRIS, McGINNIES, AND SIEGEL

considerably above the loss in transmission,indicating that elevated temperatures introducea definite limitation in gold "black" as anabsorber.

At higher temperatures there is apparently asintering of the metal deposit, and since golddiffuses readily at relatively low temperatures,this sintering effect already occurs to a consider-able extent at 120'C. Beeck, Smith, and Wheeler7

found in studying the sintering of nickel de-posited in this manner that the same effects wereobserved if the original deposit was made on amassive surface heated to a given temperatureduring deposition or if the metal were depositedon the same surface at room temperature andthen heated to the elevated temperature.

8. Effect of backing. The metal backing behindthe cellulose nitrate film has been describedabove and its function pointed out. Without abacking only a very small amount of golddeposits on the cellulose nitrate film and thesame effect has been observed in the case of verythin metal foils. The distance between the filmand the backing has a pronounced effect on thequality of the gold deposits obtained.

The rate of deposition and the optical proper-ties of the black deposit are determined, inaddition, by the nature of the receiving surface.A few observations have been made on surfacesof other materials. For example much lightergold deposits are formed on thin strips of nickel,having masses of 86 X 10-6 g/cm2 , than on thecellulose nitrate films having masses of 8 X 10-6g/cm2 , as used in our experiments, for the sametime and rate of evaporation. It was necessaryto bring the metal backing in contact with thenickel strips in order to obtain a good deposit ofgold "black." On massive surfaces the gold smokedeposits readily.

9. Electron microscope studies. The gold smokesdeposited under varying conditions show pro-nounced differences in particle size and distribu-tion. We shall only summarize the results' ofthese studies, since they will be treated in detailelsewhere.

The electron micrograph images of thin de-posits indicate that the gold deposits in smallspheroidal particles, and the broadening of the

7 Otto Beeck, A. E. Smith, and Ahlborn Wheeler, Proc.Roy. Soc. A177, 62 (1941).

diffraction rings indicate crystal sizes of about50A. The unit spheroids observed in the electronmicroscope could appear to be crystallites.

The electron micrographs clearly show anincrease in size of the unit colloidal particle andincreasing tendency to cluster with increasingpressure of nitrogen in which the evaporationtakes place.

The results of the electron microscope studiesare summarized in Table III.

Electron diffraction patterns of all gold de-posits show randomly distributed crystallites.The effect of oxygen could not be detected in theelectron diffraction patterns; only the latticespacing of pure gold was apparent.

DISCUSSION OF RESULTS

Gold smoke deposits upon thin cellulosenitrate films have the best properties for theabsorption of infra-red radiation when producedunder conditions which are in an intermediaterange of nitrogen pressure, rate of evaporation,and distance between source and surface ofdeposition. The partial pressure of oxygen in thegas should be less than 0.003 mm. The optimumrange of experimental conditions for gold"blacks" having the highest infra-red absorptionper unit mass is: (a), an atmosphere about 1.0-mm pressure of nitrogen; (b), evaporation rate of0.5 X 10-3 to 3 X 10-3 gram of gold per second;(c), distance of about 7 cm between the sourceof evaporation and the surface on which thegold is depositing; (d), copper backing 0.1 cmor closer behind the film being deposited.

The "blacks" produced under these conditionshave excellent characteristics for infra-red ab-sorbers of low thermal mass. Deposits having amass of only 45 X 10-6 gram/cm2 can be preparedwhich will absorb 68.4 percent (90 percentfor a double traversal) of the incident radiationto 15t.

It is of interest to compare the absorption ofinfra-red radiation (average for 9 ) of the bestgold "blacks" with deposits of carbon, as isdone in Table IV. Thus, these gold "blacks"have a thermal mass of approximately 1/20ththat of carbon.

An important application of the gold "blacks"deposited in an atmosphere of oxygen-enrichednitrogen is as a filter for transmitting infra-red

588

GOLD BLACKS

radiation beyond 2. Such deposits have a sharpcut-off at about 1y and are good absorbers of theshorter wave-lengths. A triple filter of this typewill transmit less than 1 percent of the radiationbelow 1,u and more than 90 percent of the radia-tion beyond 3,g.

SUMMARY

We have studied the optical properties of goldsmokes deposited on cellulose nitrate films underdifferent experimental conditions. The conditionsof pressure of the inert atmosphere, the rate ofevaporation, and the distance between sourceand deposition surface, and the backing for the

film giving the highest infra-red absorption perunit mass have been found. The particle sizeand particle distribution of the gold smokesdeposited under the different experimental condi-tions have been investigated with the electronmicroscope at high resolution.

ACKNOWLEDGMENTS

We are indebted to Miss Beatrice Alexanderand to Miss Alice M. Pearson for their excellenttechnical assistance, to Dr. Charles W. Youngfor his many helpful discussions and suggestions,and to Mr. David Jeifries for his able assistancewith many of the measurements.

589