Comparison of modified infinite-order sudden theory with experimentally measured state-to-state cross sections for R↔T energy transfer in Ar+HF

The results of an infinite-order sudden theory modified by the inclusion of an explicit exit-channel velocity dependence of the scattering cross section (IOSAM) are compared with standard IOSA results having a scattering cross section independent of exit-channel velocity and with the experimental data reported by Barnes et al. for the HF–Ar system. Calculations of σ(j′←1) and σ(j′←2) are reported at three relative translational energies using two different potential-energy surface formulations, a pairwise additive Lennard-Jones (12,6) potential, and an SCF ab initio surface augmented by an attractive van der Waals term. In general, the IOSAM results on the LJ (12,6) surface are found to be in good to excellent accord with experiment for transitions with ΔJ≳0. For transitions with Δj<0, the agreement between theory and experiment is poor. The results obtained on the unadjusted SCF surface have the correct qualitative trends, but the degree of agreement with experiment is significantly less than that obtained on the LJ (12,6) surface. For virtually all state-to-state transitions, the IOSAM cross section ratios are found to be in better accord with experiment than is the case for IOSA calculations. It is therefore concluded that the improvement in the infinite-order sudden theory achieved by the inclusion of an explicit exit-channel velocity dependence into the scattering cross section is a general feature of the theory that is not dependent upon the detailed topography of the potential-energy surface.