Multipass optical system equipped with thin films for the study of the spectra of cryogenic solids: measurement of the infrared spectrum of solid H_2

A multipass optical system external to the optical head of a cryostat has been designed for use with a Bruker IFS 120 HR interferometer. A White cell using mirrors with a focal length of 10 cm is the central feature of the system. Nearly all optical elements, including those in the cryostat, have been optimized with thin-film technology for maximum signal throughput in the 6000 to 7000 cm(-1) range. The extension of the path length of the sample, in a cell limited to 3.8 cm in length, is illustrated with spectra of solid para-H(2) at 12 K. The cell was limited to four passes in this application because of refraction of the infrared radiation at the sample surface. Twelve to 20 passes are possible for a sample with an index of refraction close to 1 and with smaller aperture settings.