NONADDITIVE INTERMOLECULAR FORCES FROM THE SPECTROSCOPY OF VAN-DER-WAALS TRIMERS - THE EFFECT OF MONOMER VIBRATIONAL-EXCITATION IN AR-2-HF AND AR-2-HCL

Calculations on the vibration-rotation energy levels of the Van der Waals trimers Ar-2-HF and Ar-2-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, upsilon. 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 Ar-2-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.