Long-path equilibrium IR spectra of weakly bound complexes at low temperatures

Abstract

Infrared spectroscopy of Van der Waals complexes is commonly performed using supersonic jet expansions, with the advantages of generating many complexes and achieving low internal temperatures. However, a lowtemperature equilibrium gas cell has certain complementary advantages, including the ability to achieve very long absorption paths and to interface efficiently with a Fourier-transform spectrometer. The necessarily higher temperatures in a gas cell are useful if the additional spectral lines can be assigned, which may require preliminary analysis of a jet spectrum and/or good theoretical calculations. In our laboratory, 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 special cryogenic absorption cells. Two new, large-aperture, long-path cells will be especially useful for the difficult far-infrared region (20–500 cm^–1) which includes important intermolecular vibrations. The work is illustrated here with observations on (H₂)₂, (D₂)₂, H₂–Ar, CO–H₂ and CO–rare-gas complexes. Spectra of the methane-containing complexes CH₄–Ar, –Kr, –H₂ and –D₂ are reported here for the first time. They occur close to the R(0) line of the CH₄v₃ band, and their interpretation poses an interesting theoretical challenge.