A theoretical Study of Spectroscopic Properties of van der Waals Trimers

Abstract

A method for performing calculations on the lower bound states of van der Waals trimers is developed, which models atom-atom-diatom trimers with basis functions in all five degrees of freedom. Spherical harmonic and distributed Gaussian functions and solutions of one-dimensional adiabatic Hamiltonians are used as basis functions. Arg was examined as a precursor system. No spectroscopy has been performed on Ara, nor is this currently feasible. For the systems considered, most experimental data exists for (^v)HCI = 0 Ar(_2)HCl so this is the main target of the work. Predictions are made for Ar(_2)DCl, for (_v)HCI = 1 Ar(_2)HCl, and for (^v)HF =0,1 Ar(_2)HF ; experiments are currently in progress on some of these systems. The current state of knowledge of the pair potentials of the Ar-Ar, and Ar-HF/CI systems is summarised. Physical models for important three body potential terms are suggested; these arise from dipoles induced on the argon atoms, dispersion effects, orbital deformation and the Ar(_2) overlap-induced field. The parameters in the models come from the literature, where possible, and otherwise from a fit to some ab-initio data points for the Ar(_3) and Ar(_2)HCl trimers (Chalasinski et al.).Calculations on Ar(_s) with various two- and three-body potentials are presented and discussed in the context of earlier work. For Ar(_2)HCl a comparison is made with earlier, approximate, work (Hutson, Halberstadt and Beswick). The possible effects of Hamilto- nian approximations are discussed before addressing the effects of individual three-body components. Two sets of three-body parameters are assessed, and indicate that the physical models used are substantially appropriate, although deficient in detail; agreement with experiment is good, with changes in frequencies of about 1.5cm(^-1) arising from the best three-body model. The most important three-body component is found to be the interaction of the overlap-induced field with the HCI permanent multipoles, with the dispersion effects slightly less important and other terms much less so.