Non-additive intermolecular forces from the spectroscopy of Van der Waals trimers: the effect of monomer vibrational excitation in Ar2–HF and Ar2–HCl

Abstract

Calculations on the vibration–rotation energy levels of the Van der Waals trimers Ar₂–HF and Ar₂–HCl are carried out, in order to investigate the role of three-body (non-pairwise-additive) forces. The present calculations focus on the lowest Van der Waals vibrational state formed from HX molecules in different intramolecular vibrational states, v. The calculations use both pairwise-additive potentials, derived from the accurately known Ar–Ar, Ar–HF and Ar–HCl potentials, and various different contributions to the three-body forces. All five intermolecular degrees of freedom are included. The red shift observed in the Ar₂–HF fundamental band is fairly well predicted (within 0.5 cm^–1) by the pairwise-additive potential, but the agreement with experiment is improved when non-additive forces are included. The rotational constants are found to be very sensitive to non-additive terms, and especially to the ‘exchange quadrupole’ term. Non-additive forces of the type usually used in studies of atomic forces, such as triple dipole forces, are found to be inadequate to reproduce the observed spectra.