Infrared spectra of hydrogen–rare-gas Van der Waals molecules

Abstract

Previously measured spectra of the Van der Waals molecules formed from H₂ or D₂ molecules and Ar, Kr or Xe atoms have been used to derive detailed potential surfaces for these systems. The success of the analyses has prompted a new program to obtain improved high-resolution spectra which may serve as a basis for refined potentials. The first results of these new studies are reported here. The spectra, which lie in the 2–4 µm wavelength region of the hydrogen stretching vibration, are observed in absorption through a long path in a mixture of hydrogen and the rare gas at low temperature. The new results were obtained with a path of 220 m in a 5.5 m multiple-traversal cell at 77 K for H₂–Ar, HD–Ar and D₂–Ar, and at 97 K for H₂–Kr. A 2 m vacuum infrared grating spectrometer was used with electronic signal averaging to record the weak spectra. The resulting spectral linewidths of ca. 0.1 cm^–1 represent an improvement of from 3 to 6 times over the previous spectra, with the result that much greater detail is revealed in the more crowded regions of the spectra. Furthermore, fully resolved spectra were observed in the Q(0) regions of HD–Ar and D₂–Ar which were previously too weak to be studied in detail. All the spectra studied here are subject to predissociation because the upper states of the observed transitions lie at 3000–5000 cm^–1, far above the 40–70 cm^–1 binding energy of the Van der Waals bond. Predissociation is clearly observed as line broadening in the HD–Ar S(0) region, and also detected for one line of H₂–Kr in the S(0) region. In other regions, the predissociation linewidths must be less than ca. 0.1 cm^–1. These observations are of interest since predissociation in Van der Waals molecules is currently an area of considerable theoretical activity and there are relatively few experimental measurements.