Contributions of the two conformers to the microwave spectrum and scattering cross-section of the He–Cl2 van der Waals system, evaluated from an ab initio potential energy surface

Abstract

A high-level ab initio PES has been calculated for the ground state He–Cl2 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 ab initio HeCl potentials of equivalent accuracy. Distortions of the He–Cl interaction due to perturbations of the electronic structure of the Cl atoms by strong intramolecular forces in Cl2 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 stable than the T-shaped configuration when zero-point vibrations are taken into account. Although the lowest rovibrational levels of each conformer lie above the potential barrier separating the two wells, the associated wavefunctions reflect the presence of the other well only weakly, 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 affect the order of the rotational levels associated with the T-well. The microwave spectra originating from each of the two conformers are simulated, and suggest a possibility for direct confirmation of the existence of the L-well. The positions of spectral lines associated with the T-shaped conformer are found to be quite sensitive to the binding energy for the linear conformer. The total differential cross section is 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 weak. The additional anisotropy of the He–Cl2 interaction, introduced with the L-well, gives rise to additional oscillations in the cross section behavior as a function of the scattering angle.