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

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.