Quenching of Glory Undulations in Scattering of Na Atoms from Polyhalide Molecules

Abstract

The velocity dependence of total cross sections has been measured for the scattering of Na atoms from CBr₄, Br₂, SnCl₄, CCl₄, CH₃I, SF₆, and SiCl₄ (listed in order of decreasing chemical reactivity). All these systems show the ν^−2/5 velocity dependence expected for the long‐range r⁻⁶ dispersion force term in the intermolecular potential. However, the usual undulatory structure from ``glory scattering'' associated with the van der Waals potential well appears in only one case, Na+SF₆. For the most reactive systems, Na+CBr₄, Br₂, SnCl₄, this absence of undulations may be attributed to quenching by reactive scattering, which is important even at impact parameters outside the van der Waals radius. For CH₃I, the quenching is probably due to the anisotropy of the intermolecular potential. For CCl₄ and SiCl₄, it seems plausible that the quenching may be due to the presence of low‐frequency vibrations which distort the molecule appreciably during the collision.