CONTRIBUTIONS OF THE 2 CONFORMERS TO THE MICROWAVE-SPECTRUM AND SCATTERING CROSS-SECTION OF THE HE-CL-2 VAN-DER-WAALS SYSTEM, EVALUATED FROM AN AB-INITIO POTENTIAL-ENERGY SURFACE

A high-level ab initio PES has been calculated for the ground state He -Cl-2 complex. The existence and relative depths of the two wells, one for each of the linear (L-well) and T-shaped (T-well) geometries, are shown to be correctly predicted with a simple atom-atom model using a b initio HeCl potentials of equivalent accuracy. Distortions of the He -Cl interaction due to perturbations of the electronic structure of th e Cl atoms by strong intramolecular forces in Cl-2 are suggested to be responsible for the remaining underestimation of the binding energy. Even though it has a deeper well, the linear configuration is less sta ble than the T-shaped configuration when zero-point vibrations are tak en into account. Although the lowest rovibrational levels of each conf ormer he above the potential barrier separating the two wells, the ass ociated wavefunctions reflect the presence of the other well only weak ly, as indicated by the low local maxima in the probability densities at the other well positions. The presence of the L-well is found to af fect the order of the rotational levels associated with the T-well. Th e microwave spectra originating from each of the two conformers are si mulated, and suggest a possibility for direct confirmation of the exis tence of the L-well. The positions of spectral lines associated with t he T-shaped conformer are found to be quite sensitive to the binding e nergy for the linear conformer. The total differential cross section i s found to be affected significantly by the L-well position in the PES , while the influence of both the well depths appears to be rather wea k. The additional anisotropy of the He-Cl-2 interaction, introduced wi th the L-well, gives rise to additional oscillations in the cross sect ion behavior as a function of the scattering angle. (C) 1998 American Institute of Physics.