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 W hite cell using mirrors with a focal length of 10 cm is the central fe ature of the,system. Nearly all optical elements, including those in t he 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 Li mited to four passes in this application because of refraction of the infrared radiation at the sample surface. Twelve to 20 passes are poss ible for a sample with an index of refraction close to I and with smal ler aperture settings.