Steric effects in the state specific reaction Li+HF (v=1, j=1, m=0)→LiF+H

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Abstract
Results of a crossed beam study on the state specific reaction Li+HF (v=1, j=1, m=0)→LiF+H at a translational collision energy of Etr=0.42 eV are reported. Angular distributions of LiF have been measured for three different distributions of the internuclear axis of HF, namely an isotropic one, one where the axis is aligned with the relative velocity of reagents V and one where the axis is aligned perpendicular to both V and the scattering plane. We find a marked influence of these collision geometries (steric effects) on (i) the angular distributions; (ii) the partition of available energy; and (iii) the integral reaction cross sections. The ratio of the latter for preferred side-on and end-on collisions with HF amounts to 1.76. From the angular distributions of products, double-differential cross sections in the center-of-mass frame are determined which exhibit in all three cases preferred backward scattering of LiF. They provide three out of four accessible moments of the orientation-dependent double-differential reaction cross section. The results are compared to quasiclassical trajectory calculations based on the potential energy surface of Chen and Schaefer [J. Chem. Phys. 72, 4376 (1980)] and to predictions of a modified direct interaction with product repulsion (DIPR) model. The latter suggests that the stringent correlation between the electric dipole moment d (the synonym for molecular axis) and the direction in which the products are ejected is relaxed and both a reorientation of the molecular axis during the approach of reagents and an interaction between the products during separation play an important role. These conclusions are supported by trajectory calculations.

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