Infrared Spectra of HCl, DCl, HBr, and DBr in Solid Rare-Gas Matrices

Abstract

The infrared absorption spectra of HCl, H³⁵Cl, DCl, HBr, and DBr isolated in solid rare‐gas matrices in the range 4°—20°K are presented. The influence of temperature, dilution, deposition rate variations, as well as the presence of certain impurities, on the spectra are also reported and discussed. In the fundamental region the spectra normally consist of a few relatively narrow bands or lines which show reproducible and fully reversible variation of intensity and bandwidth with change of temperature. The presence of certain dopants, e.g., N₂, in the halide—noble‐gas mixture leads, under suitable deposition conditions, to the replacement of the temperature‐dependent features by much narrower temperature‐insensitive lines at somewhat shifted frequencies. The temperature‐dependent features have been identified as individual rotation—vibration lines of the matrix‐isolated diatomic molecules. Their separations are significantly less than those of the corresponding lines in the free molecule spectra, thereby indicating the existence of hindered molecular rotation in the matrix. The barrier to rotation is interpreted in terms of two different models for the molecule‐lattice interactions and their limitations discussed. The temperature‐insensitive lines show no rotational structure, and are normally obtained when N₂ or certain other dopants are present in the gas mixtures from which the matrices are formed at 20°K. They do not appear if the deposition is carried out at 4°K. It is suggested that the rotational quenching may be attributed to the formation of loose couples between halide and N₂ molecules during the condensation process.