High Resolution Infrared Spectroscopy of Van Der Waals Molecules

Abstract

The high resolution laser infrared spectroscopy of weakly bound complexes is shown to provide a valuable source of information on the structure and dynamics of such species. It also contains important information on the nature of the intermolecular potential energy surface. Some of the most detailed information is contained in the direct absorption spectroscopy of those dimers present in equilibrium with the monomers in low temperature, low pressure, long pathlength static cells. This is exemplified by the results on rare gas atoms bound to hydrogen chloride. The spectra contain a wealth of information on the structure and dynamics of such species. However these equilibrium spectra are limited to clusters containing monomers with open rotational structure in the infrared, otherwise the dimer spectrum can be completely obliterated by nearly continuous absorption of the monomer. Also the dimer spectra can be frequently very dense and difficult to analyse. It is shown that such problems can be overcome in part by direct absorption experiments in pulsed supersonic jets. The spectra for complexes of rare gases bound to each of nitrous oxide and carbonyl sulphides are considered. The derived structural parameters and their implication on the intermolecular potential energy surface are discussed. For these series of rare gas containing complexes (e.g. Rg-HCl, Rg-OCS and Rg-N₂O), it is shown that the long range attraction is increased on exciting the monomer vibration. At the same time the short range repulsion is also increased. As a result, the changes in the structures of the complexes on exciting the monomer vibration and the band origin shift on cluster formation are both determined by a delicate balance between changes in attractive and repulsive forces.