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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°K. The observed spectrum is the usual transmission measurement 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 temperature work.
McMahon et al. (1949) studied this question.