LONG-PATH EQUILIBRIUM IR-SPECTRA OF WEAKLY-BOUND COMPLEXES AT LOW-TEMPERATURES

Infrared spectroscopy of Van der Waals complexes is commonly performed using supersonic jet expansions, with the advantages of generating ma ny complexes and achieving low internal temperatures. However, a low-t emperature equilibrium gas cell has certain complementary advantages, including the ability to achieve very long absorption paths and to int erface efficiently with a Fourier-transform spectrometer. The necessar ily higher temperatures in a gas cell are useful if the additional spe ctral lines can be assigned, which may require preliminary analysis of a jet spectrum and/or good theoretical calculations. In our laborator y, infrared spectra of complexes have been studied from 20-9000 cm(-1) at high resolution (0.150-0.002 cm(-1)), using long paths (20-200 m) and low temperatures (20-140 K) with Bomem FTIR spectrometers and spec ial cryogenic absorption cells. Two new, large-aperture, long-path cel ls will be especially useful for the difficult far-infrared region (20 -500 cm(-1)) which includes important intermolecular vibrations. The w ork is illustrated here with observations on (H-2)(2), (D-2)(2), H-2-A r, CO-H-2 and CO-rare-gas complexes. Spectra of the methane-containing complexes CH4-Ar, -Kr, -H-2 and -D-2 are reported here for the first time. They occur close to the R(0) line of the CH4 nu(3) band, and the ir interpretation poses an interesting theoretical challenge.