JOURNAL OF TI-IE OPTICAL SOCIETY OF AMERICA

A Low Temperature Infra-Red Transmission Cell*

H. 0. McMAHON, R. M. HAINER, AND GILBERT W. KINGA rtlhur D. Little, Inc., Cambridge 42, Massachusetts

(Received May 26, 1949)

A versatile apparatus is described which allows the infra-red absorption spectra of materials to be obtained

at temperatures down to that of liquid helium 4.2 0K. The observed spectrum is the usual transmission meas-urement obtained in the normal way without disturbing the optics of the spectrometer.

Samples can be observed which are solid, liquid, or gas at ordinary temperatures. Solids and liquids are

suitably supported and are surrounded by helium gas at the temperature of liquid helium in order to provide

for heat removal from the sample by both gaseous conduction and convection. The temperature rise of the

sample due to absorbed radiation is thus very small. Condensible gases may be deposited onto one side of the

vacuum tight infra-red transparent windows bathed on the inner side by cold helium gas. Observations have

been made for periods of forty-five minutes using a single charge of approximately 200 cc of liquid helium.Detailed features of construction and operation conform to the exacting requirements of very low tempera-ture work.

THE possibility of obtaining new information con-cerning the molecular properties of certain ma-

terials by observing their infra-red absorption spectra atvery low temperatures has led to the construction of atransmission cell which maintains a specimen at tem-peratures down to 4.20K, the boiling point of helium.For several reasons it is desirable that the sample beinterposed simply and directly into the infra-red beamwithout modification of the excellently designed opticsof the present-day infra-red spectrometer. This is de-sirable partly because of the difficulties involved in themodification of infra-red optics and partly because of theimportant advantages which exist in favor of a simpletransmission type of measurement. An alternative tech-nique that has been used in the past for low tempera-ture measurements has followed the practice of Rubensand Hertz' for measuring reststrahlen reflections fromcold solids. This method consists of reflecting the infra-red beam from a cold metallic mirror upon which the

LIQUID HELIUM CONTAINER

LOCK WASHER

SPRING WASHERS

SILVER CHLORIDEWINDOW-* SAMPLE lNFRE D-SAMPLEBE~~~AM

FIG. 1. Cross section of low-temperature infra-redtransmission cell: sample chamber.

* This work was done in part under a Grant-in-Aid from theAmerican Cancer Society pon recommendation of the Com-mittee on Growth of the National Research Council.

' H. Rubens and G. Hertz, Ber. d. Ber. Akad. (1912) p. 256.

sample has been deposited and has been adopted 2 4primarily, because of the difficulty of devising a trans-mission cell with assurance that the sample would bemaintained at the low temperature while being irradi-ated by an infra-red beam.

Obviously the reflection-double-transmission meas-urement is unable to distinguish between radiationwhich penetrates and passes through the sample twiceand that which is reflected from the sample surfacewith no penetration. Strong reflection is therefore inter-preted as high transmission. It is well known thatpowerful absorption is accompanied by strong reflec-tion. This of course may lead to very serious errors.Thus, in the case of a silica glass film, for example, a50 percent reflectivity occurs at 9.6b& which would beinterpreted as high transmission although the true trans-mission is substantially zero even for exceedingly thinfilms. If low temperatures sharpen absorption bands thiseffect will increase.

Reflections also play a role in simple transmissionmeasurements, but the effects are in general minor andare never such as to obscure structure.,

A transmission cell is, therefore, desirable for lowtemperature work, but means must be provided forsatisfactorily maintaining the sample at the low tem-perature. This difficulty can be overcome by devisinginfra-red transparent windows capable of holding ahigh vacuum while at the same time being able to with-stand cooling to very low temperatures.

Considerable effort has been expended in finding asuitable solution to this problem and has finally resultedin the described apparatus. The principal element is avacuum-tight cell containing the sample bathed inhelium gas, and fitted with infra-red transparent win-dows. It is surrounded by a very high vacuum for goodthermal isolation, a requirement which is essential

2 Conn, Lee, Sutherland, and Wu, Proc. Roy. Soc. A176, 484(1940)..

3W. H. Avery and C. F. Ellis, J. Chem. Phys. 10, 10 (1942).4W. H. Avery and J. R. Morrison, J. App. Phys. 18, 960 (1947).5W. C. Price and K. S. Tetlow, J. Chem. Phys. 16, 1157 (1948).

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SEPTEMBER, 1949VOLUME 39, NUMBER 9

INFRA-RED TRANSMISSION CELL

for liquid helium experimentation and desirable alsoin order to minimize drift problems which would beintroduced if large temperature gradients were allowedto develop in the spectrometer housing.

DESCRIPTION OF APPARATUS

Figures 1 and 2 show the construction of the low-temperature transmission cell. The cell itself has theform of a truncated cone aligning axially with the beamof infra-red energy and having the same cross section.The cell is sealed through a cylindrical container intowhich liquid helium or any other refrigerant may bepoured. The two ends of the sample chamber are fittedwith transparent silver chloride windows6' t compressedaround their rims by means of a suitable arrangementof spring loaded washers and nuts. It is necessary thatthe closures formed by these windows be absolutelyvacuum tight at all temperatures since the interiorof the cell is filled with helium gas to establish a heattransfer medium, and the exterior is highly evacuatedto provide thermal isolation. Fortunately the physicalproperties of rolled silver chloride7 windows make thismaterial almost ideally suited to the application athand. At room temperature silver chloride has proper-ties very similar to those of metallic lead. It can bedeformed readily and will flow without cold workingand without the necessity of frequent annealing. Itretains its high degree of infra-red transparency evenafter considerable cold working. Its undesirable proper-ties are its high coefficient of thermal expansion, its easeof marring, and its ready corrosion when in contactwith other than noble metals. This large thermal ex-pansion requires stiff back-up springs to hold the win-dow against its seat and at the same time to allow forthe necessary differential thermal expansion betweenthe silver chloride and metal sample cell in the directionof change of thickness of the window. Differential ex-pansion in the plane of the window is also troublesomesince this tends to cause the silver chloride to slide onits seat as the temperature is lowered. If sliding isallowed to occur, the two sealing surfaces no longermate exactly and sufficient leakage occurs to spoil thevacuum. If satisfactory seating is obtained by com-pression on rounded lands, the circumference cannotmove on cooling and circumferential or radial cracksdevelop as the result of the bulk shrinkage. Failures ofthis type have been eliminated by using a domedrather than a flat window, so that as a temperaturechange occurs the height of the dome changes but thestrains developed do not exceed the elastic limit of thematerial. During this development a number of differ-

6A vacuum tight infra-red transparent cell for use near roomtemperature has been used by R. M. Fuoss, Rev. Sci. Inst. 16, 154(1945).

t Note added in proof: Recently quartz windows held betweensilver chloride gaskets have been used successfully at liquidnitrogen temperature; this extends the use of the apparatus tothe visible and ultraviolet region.

7 H. C. Kremers, J. Opt. Soc. Am. 37, 337 (1947).

SALT

WHOLEASSEMBLY

FIG. 2. Cross section of low temperature infra-redtransmission cell: total assembly.

ent shapes of windows and seats were tried before acompletely successful combination was found; for awindow of 1.25 in. diameter a radius of curvature of8 in. for the window is found to be satisfactory. Thewindows were formed in a hand-lapped, high qualitymonel metal die on a hydraulic press capable of 15,000lbs. per square inch. The use of monel metal for all com-ponents in contact with silver chloride reduces markedlythe effects of corrosion. Silver has been used for thewasher in contact with the silver chloride window in orderto eliminate corrosion but the use of silver for the wholesample container is needlessly expensive.

The interior of the sample cell communicates directlywith the outside of the apparatus by means of a thin-walled stainless-steel capillary tube. This tube servesto fill the sample cell with helium gas and is also at-tached to a pressure-vacuum gauge so that the resultingcombination serves as a helium gas thermometer en-abling the operator to know the temperature of thesample quite accurately. The function of the heliumgas is to remove energy absorbed by the sample andtransfer it to the walls of the sample cell, which in turnare immersed in the liquid helium or other refrigerant.

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McMAHON, HAINER, AND KING

FIG. 3. Low temperature infra-red transmission cell separated intomajor components with radiation shield removed.

In general, it is extremely difficult to remove energyfrom a thin film which is absorbing radiation and issurrounded by a vacuum. Even if the film is a goodconductor of heat, which it generally is not, and it isin good contact with the metal sink, it is usually so thinthat extremely large temperature gradients would berequired to conduct the absorbed energy any appreci-able distance, e.g., along a radius, or to radiate suffi-cient energy to the surroundings. When the film is sur-rounded by helium gas, however, it is easy to show thatthe quantity of energy absorbed could not raise the filmtemperature above that of the ambient by more thanone degree at most. The heat transfer rate for freeconvection around a vertical plane, such as the absorb-ing film, depends primarily upon the volumetric expan-sion coefficient, the thermal conductivity and the vis-cosity of the ambient medium. For helium gas at 4.2 0Kan unusually favorable condition prevails since the ex-pansion coefficient is of course enormous, the thermalconductivity is high and the viscosity is very low.Thus a slight rise in temperature of the film sets uppowerful convection currents which effectively preventany further rise. The energy to be dissipated can beestimated from the temperature of the infra-red sourceand aperture of the system, times the area of the sourcevisible at the sample. It was measured directly in ourspectrometer, for an average absorbing sample (poly-styrene) by the increased rate of evaporation of heliumon opening the shutter exposing the sample to radiation.The increased rate was 2.0 liters per minute of gas atroom temperature, corresponding to 0.13 watts inagreement with calculation.

In order to protect the liquid-helium pot and itsassociated sample chamber from the thermal radiationof the room temperature surroundings, a copper radia-tion shield which completely envelops the very lowtemperature region has been soldered to an auxiliarypot which is cooled with liquid nitrogen. This liquid-nitrogen pot is mounted directly above the liquid-helium pot, coaxially with it, and is of a somewhatlarger diameter. The radiation shield is silver plated inorder to reduce the emissivity. The helium pot is sup-ported by means of two very thin-walled stainless steeltubes which serve as filler tubes and, on account of theirlow heat conductivity as partial thermal isolators.These tubes are run through the liquid-air pot whichthen acts as a heat station for the liquid-helium regionmaking the problem of thermal isolation somewhateasier. The liquid-nitrogen pot is also suspended bytwo filler tubes and these, as well as the helium-fillertubes, pass up through the covering plate of the wholevacuum-jacketed assembly. The area where the fourfilling tubes are brought through the lid is thermallyisolated from the rest of the lid area by means of a raisedstainless steel hat of low thermal conductivity. Thisdesign avoids transferring excessive amounts of re-frigeration to the neoprene gasket or the outer shellof the vacuum jacket during refrigerant transfer opera-tions. The entire assembly is inserted into a brassshell which has a connection to a vacuum system.

The inner assembly rests on a specially designedneoprene gasket which is recessed into a suitable flangeattached to the upper rim of the brass jacket. The innerassembly aligns the sample cell with two rocksaltwindows provided in the brass shell so that the infra-redbeam may traverse the entire apparatus. The rocksaltwindows are sealed with neoprene gaskets. The liquid-helium pot acts as an extremely effective trap for all gasesexcept helium. Thus, in the absence of gaseous helium,the only heat loss from the outer jacket is to the radiationshield at 770 K and the sample cell at 40 K. This beingnegligible, the salt windows and outer jacket remain atroom temperature and do not cause temperature driftsin the spectrometer or in the detector.

Nearly all the component parts of this apparatus aresilver brazed together to give strength and durability.Several parts, e.g., the sample cell and the radiationshield, however, are soft soldered in place in order toallow for easy modification from time to time. Materialsfor construction are chosen for their thermal properties,chemical properties, or ease of fabrication. They include:brass for most pieces, copper for the radiation shield,stainless steel for heat insulation, and monel for thesample cell to reduce the corrosion with silver chloride.

Figure 2 shows the cross-section of the assembledcell indicating clearly: the sample cell of approxi-mately 10 cc volume; the liquid-helium pot of 220 ccvolume; the liquid-nitrogen pot of 240 cc volume; theradiation shield; the four filling tubes for the two con-tainers, one for the lower pot being large enough to

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INFRA-RED TRANSMISSION CELL

accept the liquid-helium transfer tube from the Collinshelium cryostat; the external shield with its salt win-dows; and the connections to the vacuum system. Fig-ure 3 shows a photograph of the low temperature cell,separated into the major components with the radia-tion shield removed. Figure 4 is a photograph of theassembled cell in the operating position of a Perkin-Elmer spectrograph and attached to the 20 mm Pyrexglass stopcocks of the high vacuum system. Thesestopcocks allow for the removal of the low temperaturecell from the pumping system in order to fill it withliquid helium without loss of vacuum and for quickresumption of the dynamic vacuum on returning thecell to the spectrograph.

SAMPLES

The design of this low temperature apparatus allowsthe mounting of the sample in the sample cell either as asolid film or as a liquid (or solution) in an appropriateliquid cell made of two silver chloride plates; rock saltcannot be used because of the high probability ofcracking on cooling. This liquid cell can be much likethe conventional liquid cell made with salt windowsand a spacer, or it can be made from silver chlorideplates with a shallow recess pressed into them, andsealed face to face by fusing them together around theperiphery to form a fixed thickness cell without spacer.Such a cell can be filled by a hollow needle inserted intoa hole drilled through the fused rim. In addition, thelow temperature dewar allows the study of spectra ofgases and liquids with appreciable vapor pressure.These samples are prepared by letting the gas or vaporinto the vacuum system from which it deposits on thecold silver-chloride windows of the lower pot, the upperpot and shield being maintained at room temperatureor at a temperature such that the vapor pressure of thesample is high. In addition the way is open for furthergeneralization of the sampling by vacuum sublimationor molecular distillation of the sample onto the windows.

OPERATION

The apparatus can be operated at the temperatureof liquid nitrogen as long as desired by occasional fillingof the lower pot from a small portable Dewar. For runsat liquid-helium temperatures transfer of helium ismade directly from the cryostat. The heat of vaporiza-tion of liquid helium is only 5 cal. per gram, so inter-mediate transfer equipment would waste considerablequantities of liquid helium during the cooling. The timefor observation of spectra, without refilling, is limitedto the dissipation of the stored refrigeration by lossesfrom radiation, metallic conduction and gaseous conduc-tion by the residual helium in the vacuum system. Thelatter factor can be quite large if the pressure of thehelium in the vacuum system is not sufficiently lowbecause of small leaks, a poor pumping rate, a poorultimate vacuum, or a. combination of these troubles.

Since liquid helium has a small heat of vaporization anda density of only 0.13 grams per cc, the 220 cc capacityof the liquid helium container provides only 150 caloriesof refrigeration at 40 K. The heat losses either with orwithout the infra-red beam's falling on the samplecan be accurately measured by metering the heliumvaporized and exhausted through the filling tubes.Approximately 160 liters of helium gas is exhausted atstandard conditions from a full charge of liquid. Losseswith the infra-red's falling on a sample mounted beforethe entrance slit amount to approximately 4 liters perminute, twice the rate without radiation, so that opera-tion for 40 minutes can be expected. In practice 45-minute runs have been obtained.

The difficulties experienced with this apparatus in themeasurement of the low temperature spectra of a num-ber of materials have been due to unsatisfactory vacuumconditions or failure of the silver chloride windows.Minor vacuum troubles can be tolerated so long as therate of loss of coolant does not become so great thatthe time of observation is cut severely. However, thecracking of a silver chloride window immediatelyterminates low temperature operation. With the presentapparatus this failure has not been frequent. Forsamples introduced in the gas phase, where the silverchloride windows do not need to be removed to changesample, eight successive low temperature determina-tions with warming to room temperature in betweenhave been made without difficulties of any sort. Factors

FIG. 4. Assembled low temperature infra-red transmission cell inoperating position of a Perkin-Elmer spectrograph.

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