Intermolecular potential of anisotropic systems: LiF–Ar and LiF–Kr

Abstract

High resolution differential cross sections for LiF scattering from Ar and Kr were measured in the thermal velocity range with varying LiF rotational temperature. An extremely narrow velocity distribution in the LiF beam, which reduced the equivalent temperature of the experiment to typically below 1 K, made it possible to resolve the high frequency oscillations of the differential cross section. Rainbow oscillations were identified and measured. Effects of experimental convolution were discussed and treated. Average potential well depths and radii were determined. It was inferred from the breadth of the primary rainbows that the potential well is not pathological in possessing extremely deep or shallow depths (ε) for certain orientations: standard deviation of ε is about 50%. The persistency of the high frequency oscillations with angle is evidence of a quite spherical locus of rm, although the locus is not necessarily concentric with the center of mass. The high frequency oscillations and in part the rainbow oscillations are damped by rotational transitions. The differential cross section is largely independent of the rotational state distribution. Calculations using an optical model for the scattering and assumed angular dependent potentials permitted a quantitative test of a theoretical long range potential model and also permitted a fit of an empirical angular dependent model at the radius of the onset of these transitions. The quantitative test indicated that neglected repulsive terms are important in the theory. The fitted empirical model was consistent with the measured dispersion in ε. The radius at which rotational transitions become significant has been determined and lies outside rm, approximately at the rainbow radius, but inside the radius probed by total scattering cross section measurements. The potential well parameter rm=3.8 Å, ε=4.1×10−14 erg and rm=3.7 Å, ε=5.3×10−14 erg were obtained for LiF–Ar and LiF–Kr, respectively.