Brewster angle optical Dewar: construction of an inexpensive model for cryogenic or simple liquids for laser Raman scattering experiments

Syed Ameen Max-Planck-Institiit fur Biophysikalische Chemie, D-3400 Göttingen, Postfach 968 West Germany. Received 20 January 1975.

In SRS spectroscopic experiments there is a need for an inexpensive optical Dewar system for handling samples of cryogenic liquids at stabilized low temperatures without boiling. In stimulated Raman scattering studies and for obtaining high energy laser pulses at frequencies (Stokes and anti-Stokes), other than those of the Q -switched Nd+3

glass and ruby lasers, some cryogenic liquids such as N2O2, He, etc. are used.1-3 For laser-temperature-jump spectro­photometery one also needs such a Dewar for liquid N2 to produce Stokes frequencies to study the chemical relaxa­tion times of reactions,2,4 by rapid vibrational heating of solvent.

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For spectroscopic separation of pulses produced in stim­ulated Raman scattering experiments, at controlled tem­peratures, a spectrographs setup is required where liquid prisms are employed.

A simple, inexpensive glass Dewar with Brewster angles and optical windows has been made that consists of triple quartz tubes (designated by A, B, C in Fig. 1), joined inside one another in such a way that the middle tube B has the opening for filling, at the center of the flask and the inner­most tube A has two openings, one at each end [Fig. 1(a)]. The middle tube B serves as a cooling jacket for the inner­most tube A, which is used for filling the liquid under in­vestigation. When the middle jacket B is filled with cryo­genic liquid of the same kind, evaporation cooling stabilizes the temperature of the innermost tube A. When the mid­dle tube B is filled with cryogenic liquid of a lower boiling point than the one in tube B, stabilized temperature is again obtained; and spectroscopic studies can be carried on in this part A of the Dewar. The tubes A and B are joined together by common Brewster angle spectrosil windows at either end. The middle tube B is connected with small, vertically standing, evacuated double-walled inlet tubes as shown in Fig. 1(a). . Liquids are poured into one of these inlets, while applying a slight suction at another to avoid condensation of air and vapor. Tube C is evacuated and sealed, which makes this system a complete optical Dewar.

There are four optical windows, two on either side of the flask, one attached to tube C and the other to both the inner tube A as well as to the middle tube B. Between the windows of either side there is vacuum. On the lateral side a double-walled evacuated tube serves as a reservoir, which is connected to the middle tube B as shown in Fig. 1(b). This open-mouthed tube is used to pour cryogenic liquid into the coolant tube B. Since the liquid evaporates faster in this middle tube B, this reservoir is required.

For operational purpose first the middle tube B is filled through the reservoir, while applying gentle suction to one of the inlets of the innermost tube A to avoid condensation of air then finally the innermost tube A is filled with the cryogenic liquid under study. This tube A holds a very stable temperature for a long time. Instead of cryogenic liquids in tube A, any other liquid can be studied at very low temperatures. A thermostat can also be conveniently installed in the lateral reservoir of central tube A to regu­late the temperature for very long times and making this a close system.

In case of stimulated Raman scattering, this Brewster angle optical Dewar serves additionally as a big liquid prism from which the Stokes and anti-Stokes pulses come out already separated; no extra spectograph is required. The reflection losses are also minimized because of the Brewster angle. (We have also constructed another Dewar but without the Brewster angle.)

Fig. 1. Brewster angle (prism type) dispersion Dewar for Raman scattering experiments with cryogenic or ordinary liquids.

This whole construction is simple, inexpensive, and yields a perfect optical Dewar (with or without Brewster angles) for cryogenic liquids systems, which can also be used for the study of simple liquids at low stable tempera­tures.

Thanks are due Leo De Maeyer and Manfred Eigen for extending my doctoral fellowship, of which this project is a part. I want to thank K. H. Grellman of our Institute and Herman Linne for extending the facilities of the glass workshop to me.

References 1. N. Sutin et al. J. Am. Chem. Soc. 94, 1554 (1972). 2. N. Sutin et al. J. Am. Chem. Soc. 92, 413 (1970). 3. B. P. Stoicheff, A. K. McQuillan, and J. B. Grun, Phys. Rev.

180, 61 (1969). 4. M. Eigen and L. De Maeyer, in Techniques of Organic Chemis­

try A. Weissberger, Ed. (Vol. 8, 1963), Part 2, p. 89595. 5. S. Ameen, Ph.D. dissertation (Katholiek University of Leuven,

Belgium, and Max-Planck Institüt für Biophysikalische Chem-ie, Göttingen, West Germany, 1975.

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