A Long Path, Low Temperature Cell Roger P. Blickensderfer, George E. Ewing, and Rex Leonard Design and performance are given of a multiple traversal low temperature absorption cell that operates routinely at a path length of 230 m. The system is comprised of an f/30-aperture White cell surrounded by an aluminum Dewar. The cell has been used in the temperature range 300-77 K and may be pres- surized to 3 atm. Vacuum feedthrough adjustments are employed in order to correct for defocusing of the mirrors during the cooling process. Introduction Long path, low temperature cells have been described recently for the spectroscopic study of intermolecular interactions and dimerization in the gas phase. Watan- abe and Welsh' have used a multiple reflection cell that gives an effective path length of 13.6 m in the study of hydrogen dimers, (H 2 ) 2 , at 20.4 K. Blickens- derfer and Ewing 2 have used a 0.75-m cell at forty tra- versals, yielding a total path of 30 m, to observe cluster- ing in oxygen gas at 87 K. In both cases, optically for- bidden transitions were investigated, necessitating the use of long path lengths for gases of moderately low density. The samples were cooled to low temperatures in order to increase the probability of dimer formation during collisions. In addition to their importance for the study of intermolecular interactions, long path, low temperature cells are suitable for the investigation of op- tical properties of air, since the conditions of the upper atmosphere can be simulated. We describe here a 3.8- m cell that operates routinely at variable path lengths up to 230 m and may be cooled to 77 K. The cell con- sists of a multiple traversal optical system enclosed in an aluminum Dewar. The Dewar Figure 1 is a schematic of the dewar, designated Cryogenic Associates model SD-51, before it was modi- fied to accommodate the optical system. It consists of three concentric 3.2-mm wall aluminum shells: a double wall jacket for coolant and an outer vacuum con- tainer. The aluminum is reinforced by outside vacuum casing supports and inside nitrogen chamber supports, to prevent cylinder collapse over the extended length. R. P. Blickensderfer and G. E. Ewing are with the Chemistry Department, Indiana University, Bloomington, Indiana 47401; R. Leonard is with Cryogenic Associates, Inc., 1718 N. Luett Avenue, Indianapolis, Indiana 46222. Received 26 April 1968. The inlet ports for sample gas and coolant are located at the top center of the outer vacuum casing. Since the inner cell shrinks during cooling, bellows transitions are inserted between the outer vacuum container and cool- ing jacket. The cooling jacket floats on low thermal conductivity nylon supports, in both the vertical and horizontal directions. The sample port and coolant port are both equipped with relief valves, and the vac- uum container is protected from high pressure by a rup- ture disk. The sample compartment may be safely pressurized to 3 atm absolute. Convection currents be- tween cold gas inside the cell and warm gas in the inlet port, are minimized with a double baffle arrangement, as shown. The aluminum inner end plates are sealed to the flanges with 32-cm Teflon coated metal 0-rings. * Neoprene 0-rings are used for the outer end plates. The over-all length of the cell is 4.2 m, and the outside diameter is 42 cm. The total weight of the cell is ap- proximately 90 kg. A filling tube extending to the bottom of the coolant space (not shown), improves cooling efficiency by forc- ing liquid to the bottom and allowing the effluxing vapor to cool the walls. The coolant space holds approxi- mately 50 liters, and a total of about 100 liters of liquid nitrogen is required to cool the cell and fill the coolant space. Fifty layers of superinsulationt wound around the coolant jacket reduce radiation losses significantly. The space between the end plates is insulated with superinsulation enclosed in a porous bag. Before refrigerant is added, the pressure around the inner cell is reduced to less than 5 with a rotary oil pump. After the cell is cooled to 77 K, the evacuation valve is closed, and the dewar is cryo pumped by acti- vated charcoal in packets glued to the outer wall of the coolant jacket. Hold times of a week or more are possi- ble when the cell is cooled with liquid nitrogen. * United Aircraft Products, Inc., Dayton, Ohio. t National Research Corporation, Cambridge, Mass. 2214 APPLIED OPTICS / Vol. 7, No. 11 / November 1968
Transcript
A Long Path, Low Temperature Cell
Roger P. Blickensderfer, George E. Ewing, and Rex Leonard
Design and performance are given of a multiple traversal low temperature absorption cell that operatesroutinely at a path length of 230 m. The system is comprised of an f/30-aperture White cell surroundedby an aluminum Dewar. The cell has been used in the temperature range 300-77 K and may be pres-surized to 3 atm. Vacuum feedthrough adjustments are employed in order to correct for defocusing ofthe mirrors during the cooling process.
IntroductionLong path, low temperature cells have been described
recently for the spectroscopic study of intermolecularinteractions and dimerization in the gas phase. Watan-abe and Welsh' have used a multiple reflection cell thatgives an effective path length of 13.6 m in the studyof hydrogen dimers, (H2)2, at 20.4 K. Blickens-derfer and Ewing2 have used a 0.75-m cell at forty tra-versals, yielding a total path of 30 m, to observe cluster-ing in oxygen gas at 87 K. In both cases, optically for-bidden transitions were investigated, necessitating theuse of long path lengths for gases of moderately lowdensity. The samples were cooled to low temperaturesin order to increase the probability of dimer formationduring collisions. In addition to their importance forthe study of intermolecular interactions, long path, lowtemperature cells are suitable for the investigation of op-tical properties of air, since the conditions of the upperatmosphere can be simulated. We describe here a 3.8-m cell that operates routinely at variable path lengthsup to 230 m and may be cooled to 77 K. The cell con-sists of a multiple traversal optical system enclosed in analuminum Dewar.
The DewarFigure 1 is a schematic of the dewar, designated
Cryogenic Associates model SD-51, before it was modi-fied to accommodate the optical system. It consists ofthree concentric 3.2-mm wall aluminum shells: adouble wall jacket for coolant and an outer vacuum con-tainer. The aluminum is reinforced by outside vacuumcasing supports and inside nitrogen chamber supports,to prevent cylinder collapse over the extended length.
R. P. Blickensderfer and G. E. Ewing are with the ChemistryDepartment, Indiana University, Bloomington, Indiana 47401;R. Leonard is with Cryogenic Associates, Inc., 1718 N. LuettAvenue, Indianapolis, Indiana 46222.
Received 26 April 1968.
The inlet ports for sample gas and coolant are locatedat the top center of the outer vacuum casing. Since theinner cell shrinks during cooling, bellows transitions areinserted between the outer vacuum container and cool-ing jacket. The cooling jacket floats on low thermalconductivity nylon supports, in both the vertical andhorizontal directions. The sample port and coolantport are both equipped with relief valves, and the vac-uum container is protected from high pressure by a rup-ture disk. The sample compartment may be safelypressurized to 3 atm absolute. Convection currents be-tween cold gas inside the cell and warm gas in the inletport, are minimized with a double baffle arrangement, asshown. The aluminum inner end plates are sealed tothe flanges with 32-cm Teflon coated metal 0-rings. *Neoprene 0-rings are used for the outer end plates.The over-all length of the cell is 4.2 m, and the outsidediameter is 42 cm. The total weight of the cell is ap-proximately 90 kg.
A filling tube extending to the bottom of the coolantspace (not shown), improves cooling efficiency by forc-ing liquid to the bottom and allowing the effluxing vaporto cool the walls. The coolant space holds approxi-mately 50 liters, and a total of about 100 liters of liquidnitrogen is required to cool the cell and fill the coolantspace. Fifty layers of superinsulationt wound aroundthe coolant jacket reduce radiation losses significantly.The space between the end plates is insulated withsuperinsulation enclosed in a porous bag.
Before refrigerant is added, the pressure around theinner cell is reduced to less than 5 with a rotary oilpump. After the cell is cooled to 77 K, the evacuationvalve is closed, and the dewar is cryo pumped by acti-vated charcoal in packets glued to the outer wall of thecoolant jacket. Hold times of a week or more are possi-ble when the cell is cooled with liquid nitrogen.
* United Aircraft Products, Inc., Dayton, Ohio.t National Research Corporation, Cambridge, Mass.
The Optical SystemThe optical system used is that originally described
by White.3 It consists of a set of three concave spheri-cal mirrors adjusted to give multiple images of the lightsource, and hence a large number of traversals. Twomajor problems are associated with the optical perfor-mance of a long path low temperature cell: (1) tempera-ture gradients arising from heat losses set up densityfluctuations which generate schlieren patterns introduc-ing noise into the spectra; (2) dimension changes dur-ing cooling cause defocusing of the mirrors. The ab-sence of schlieren pattern noise from the spectra ob-tained with the cell attests to the high efficiency of thedesign.
The second problem was met by adjusting the mirrorsexternally to correct for defocusing after cooling to lowtemperatures. The mirrors, * with a 3.8-m radius ofcurvature, have an evaporated gold surface on a 9.5-mmVycor substrate. They are mounted in spring loadedaluminum holders. Defocusing is correctable by ad-justing only one of the two focusing mirrors, and thusonly two feedthroughs are necessary. Figure 2 illus-trates the adjustment for the case of eight traversals.The entrance beam is focused at (0) and it diverges tofill Ml. Mi forms the first image at (1). M2 images(1) at (2), Mi images (2) at (3), and finally, M2 images(3) at the exit (X). Defocusing of 11 causes a displace-
ment of its center of curvature from A to A'. The feed-through adjustments for M2 at z and y are used to relo-cate the center of curvature of M2 and B'. The cell isnow set for the original number of traversals (8), al-though the images now have different positions (1', 2',3').
Figure 3 shows the feedthrough arrangements, em-ploying the valve stem from a Veeco L-50-S valve.tThe brass pin on the end of the bellows travels forwardor backward against the mirror holder as the bellowsscrew is turned. The flange of the valve stem is sealedto the inside end plate with a Teflon coated metal 0-ring. A 12.7-mm diam thin wall stainless tube that en-gages the bellows screw is rotated against the greased
neoprene 0-ring of a Veeco vacuum coupling. The ad-justment is followed through a window facing the oppo-site end of the cell where the images are displayed. Themirrors were spaced an additional 17 mm apart at roomtemperature to compensate for contraction of the cellover the range 300-77 K. Without this correction,the images broaden considerably and begin to overlap atabout forty traversals. The mirrors must be free fromstrain as they are cooled, to prevent distortion of theimages. The aluminum holder is about an eighth of aninch larger than the mirror to compensate for the differ-ence in thermal expansion coefficient. The mirrors areheld in place by Teflon clamps which are bolt loadedslightly to maintain contact during cooling. When themirrors are held directly by set screws, the holders con-tract against the mirrors causing strain and enlargementof the images by a factor of three at 77 K. With theTeflon mounts and the 17-mm spacers, the image qualityat 77 K is as good as at room temperature, and the cellcan be used routinely at sixty traversals.
The observation window (Fig. 3) and windows for thesource light entering and exiting the cell (not shown),are mounted to the cold inner end plates by indiumseals. These are formed by laying a loop of 0.76-mmindium wire* against the flat surface of the aluminumplate. The window is evenly bolted against the loop,using a torque screwdriver gradually increased to0.05 kg-m. Indium seals formed in this way are reliableat low temperatures.
The Cell PerformanceThe low temperature, long path cell rests on two con-
crete pillars and is shown in Fig. 4. The two adjust-ment feedthroughs and the observation window can be
* Indium Corporation of America, Utica, N.Y.
seen on the outer end plate. Near the center of the cellis the gas handling equipment (compressed gas cylinder,manometer, etc.). At the far end are the source, mono-chromator, and detector. A tungsten ribbon source ismagnified by a factor of three into the cell and then de-magnified by the same factor to match the f/8.7 aper-ture of a Hilger-Engis model 1000 grating spectrometer.The beam intensity is attenuated to about 10% of itsoriginal value after sixty traversals, but good signal tonoise is obtained by means of PbS phase sensitive detec-tion at 13 Hz.
In a typical experiment, the spectrum of oxygen gasat 87 K is examined. First, helium at a pressure of 500Torr is added to the sample space. The helium serves asa gas thermometer to monitor the cell temperature andalso as a heat transfer agent to cool the mirrors. Thecell is precooled to -100 K with liquid nitrogen, and theliquid argon (87 K) is added until the coolant compart-ment is half full. Since the cell is well insulated, it isnot necessary to fill the cell completely with coolant tomaintain uniform low temperature of the sample com-partment. The helium is removed and oxygen gasadded to a pressure of 500 Torr.
Fig. 5. Spectrum of oxygen gas near 12,600 A. The cell tem-perature is 87 K, with the mirrors set for fifty-two traversals,giving an effective path length of 200 m. The spectral slit widthis 1 cm-'. Curve A is the spectrum of the evacuated cell and
curve B is the oxygen spectrum at a pressure of 500 torr.
After the sample has reached equilibrium (24 h), themirrors are refocused and set for fifty-two to sixtytraversals. The spectral region examined is the 0-0band of the 'A, -- 32- system of molecular oxygen.The estimated oscillator strength of this transition isabout 10-11 for monomeric oxygen,4 showing the ex-
treme weakness of bands that may be observed with thecell. At low temperatures, weak intermolecular forcescause oxygen to cluster and dimerize.2 The observedspectrum (Fig. 5) is a superposition of a broad con-tinuum owing to (02)2 and sharp absorption features,characteristic of rotational fine structure of the monomertransition.
The noise levels of the low temperature spectra are nohigher than for room temperature spectra taken undersimilar conditions, showing that schlieren noise has beeneffectively eliminated.
We wish to thank John Dorsett, whose machine workand design suggestions have been an important contri-bution to the success of this project.
This paper represents Contribution 1604 from theChemical Laboratories of Indiana University. Thework was supported by the National Science Founda-tion.
R. P. Blickensderfer is a National Defense EducationAct Fellow.
References1. A. Watanabe and H. L. Welsh, Can. J. Phys. 45, 820 (1965).2. R. P. Blickensderfer and G. E. Ewing, J. Chem. Phys. 47, 331
(1967).3. J. U. White, J. Opt. Soc. Amer. 32, 285 (1942).4. V. I. Dianov-Klokov, Opt. Spectrosc. 16, 225 (1964).
Max Garbuny of Westinghouse Research and DevelopmentCenter will be New York State Visiting Professor of Optics at the
University of Rochester for the academic year 1968-69.
