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

Starting from an ab initio symmetry-adapted perturbation theory potent ial energy surface we have performed converged variational and close-c oupling calculations of the bound rovibrational states and of the posi tions and widths of rotationally predissociating resonances of HeHF an d HeDF van der Waals complexes. The energy levels were used to compute transition frequencies in the near-infrared spectra of these complexe s corresponding to the simultaneous excitation of vibration and intern al rotation in the HF(DF) subunit in the complex. The computed transit ion energies and other model independent characteristics of the near-i nfrared spectra are in excellent agreement with the results of high-re solution 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 an d Nesbitt results of 7.35 and 7.52 cm(-1). The agreement of the observ ed and calculated linewidths is less satisfactory. We have found, howe ver, that the linewidths are very sensitive to the accuracy of the sho rt-range contribution to the V-1(r,R) term in the anisotropic expansio n of the potential. By simple scaling of the latter component we have obtained linewidths in very good agreement with the experimental resul ts. We have also found that this scaling introduces a very small (2%) change in the total potential around the van der Waals minimum.