Cryogenic multiple reflection absorption cell and Fourier transform spectrometer system for the far infrared

A far-infrared system used to measure weak absorption spectra of gases at p ressures up to a few atmospheres and at temperatures as low as 20 K is desc ribed. The principal component is a multiple reflection mirror cell, cooled either with liquid nitrogen or with the boil-off gas from liquid helium. T he cell, of 35 l volume, has an f/10 optical beam and a pathlength which ca n be varied from 4 to 60 m. At the longest pathlength diffraction limits th e lowest usable frequency to 20 cm(-1). The cell is coupled to a Fourier tr ansform spectrometer and a 1.6 K bolometer. The apparatus has been used in a detailed study of the collision-induced pure translational band of H-2, a nd the translation-rotation bands of N-2 and N-2-Ar mixtures. (C) 1999 Amer ican Institute of Physics. [S0034-6748(99)02001-8].