NEAR-INFRARED SPECTRUM AND ROTATIONAL PREDISSOCIATION DYNAMICS OF THE HE-HF COMPLEX FROM AN AB-INITIO SYMMETRY-ADAPTED PERTURBATION-THEORY POTENTIAL

Starting from an ab initio symmetry-adapted perturbation theory potential energy surface we have performed converged variational and close-coupling calculations of the bound rovibrational states and of the positions and widths of rotationally predissociating resonances of HeHF and HeDF van der Waals complexes. The energy levels were used to compute transition frequencies in the near-infrared spectra of these complexes corresponding to the simultaneous excitation of vibration and internal rotation in the HF(DF) subunit in the complex. The computed transition energies and other model independent characteristics of the near-infrared spectra are in excellent agreement with the results of high-resolution measurements of Lovejoy and Nesbitt [C. M. Lovejoy and D. J. Nesbitt, J. Chem. Phys. 93, 5387 (1990)]. In particular, the ab initio potential predicts dissociation energies of 7.38 and 7.50 cm(-1) for HeHF and HeDF respectively, in very good agreement with the Lovejoy and Nesbitt results of 7.35 and 7.52 cm(-1). The agreement of the observed and calculated linewidths is less satisfactory. We have found, however, that the linewidths are very sensitive to the accuracy of the short-range contribution to the V-1(r,R) term in the anisotropic expansion of the potential. By simple scaling of the latter component we have obtained linewidths in very good agreement with the experimental results. We have also found that this scaling introduces a very small (2%) change in the total potential around the van der Waals minimum.