Evidence for the deep potential well of Li+HF from backward glory scattering

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Abstract
Angular distributions of Li scattered from HF have been measured for various collision energies Etr ranging from 64 to 372 meV. At Etr=88 and 64 meV a clear rainbow structure at small angles is observed. At Etr=296 meV a faint peak emerges at large angles which becomes a prominent feature for all lower energies. The position of the peak always corresponds to backward scattering in the center-of-mass frame. This fact and the rather sudden rise of the backward intensity with decreasing Etr clearly identify this phenomenon as a backward glory. Based on the infinite order sudden approximation both the rainbow and the backward glory as well as the Etr dependence of these features could be rationalized using a simple empirical potential energy surface. The surface consists of two only moderately anisotropic extended regions which differ drastically in the well depths. The shallow well region has a maximal depth of ≊33 meV and is essentially determined by the rainbow scattering. The deep well region features a maximal depth of ≊300 meV and is associated with the backward glory. The well depths are in excellent to fair agreement with recent ab initio results.