Velocity dependence of absolute total cross sections for LiF scattered by nonpolar targets

Total cross sections accurate to 1–2 percent were measured in the thermal energy range for LiF scattered by rare gas atoms (He, Ne, Xe) and nonpolar molecules (D2, CH4). In all cases, the cross sections were pressure independent, indicating that (1) the scattering was not a discernable function of the internal state of the primary beam molecule, and (2) the degree of inelasticity occuring at the outermost impact parameter regions probed was small. This allowed the observed velocity dependence of the cross sections to be interpreted in terms of spherically averaged monotonic potentials except in the cases of LiF–He, LiF–Ne, and LiF–D2 where the repulsive and well regions are probed. For every system, the slope d lnQ/d ln v of the cross section deviated from the theoretical slope of −0.4 predicted for a pure van der Waals 1/r6 interaction, suggesting the presence of higher order forces. As the size of the impact parameters explored increased, as increasingly large scattering targets were used, the slopes approached −0.4. For systems which sampled attractive regions of the intermolecular potential, cross sections which were calculated from the Schiff-Landau-Lifschitz (SLL) relation using the induction energy and the Slater-Kirkwood dispersion energy were 6–8 percent smaller than the corresponding experimental cross sections. For LiF–Xe, the system which most closely approached pure van der Waals behavior (with a slope of −0.389), the calculated cross section was 8 percent smaller than experiment. The absence of glory oscillations in the cross sections, attributed to rotational transitions at small impact parameters, provided an estimate of the lower limit for opacity at the glory impact parameter.