Quantum mechanics of Van der Waals complexes: rare gas-hydrocarbon systems

Abstract

The coupled channel approach has been used to study the Van der Waals complexes Ar-C(_2)H(_2), Ar-CH(_4) and Ar-C(_2)H(_4). The Ar-C(_2)H(_2) study employs a pairwise additive, atom-atom potential energy surface first without, and then with, angular anisotropy in some of the carbon atom parameters. The complex is found to be a nearly free internal rotor and the correlation between the complex and the acetylene monomer energy levels is clear. The Ar-CH(_4) study uses two potentials. The first includes the isotropic V(_o) dependence and the first angularly anisotropic V(_3) term. This is then modified to give a second potential which includes a V(_4) term as well. The role of these anisotropic terms in splitting the triply degenerate bending states of the complex, when the methane monomer is in the ground vibrational state, is discussed. The energy level pattern is found to be best described in terms of a hindered rotor model. For Ar-C(_2)H(_4) a pairwise additive, atom-atom potential including angular anisotropy in the carbon atom parameters is again used. The ethylene monomer in the complex is nearly free to rotate about the C-C axis but steric considerations make end-over-end rotation restricted. A method has been implemented to extract wavefunctions from coupled channel calculations. The utility of this technique has been illustrated by providing insights into the Ar-HF, He-CO(_2) and Ar-C(_2)H(_4) systems via calculated spectra and direct visualisation of the wavefunctions.