Vibrational Quenching of Glory Undulations in Scattering of Na from Polyatomic Gases

Abstract

The velocity dependence of total cross sections has been measured for the molecular‐beam scattering of Na atoms from Ar, Kr, and eight polyatomic gases. The glory undulations observed for Ar and Kr agree well with other recent work. The molecular systems studied fall into three categories with regard to the appearance of undulations: (a) strong: CF₄, SiF₄, C₂F₆, and BF₃; (b) weak: GeH₄ and C₄F₈; and (c) very weak or absent: C(CH₃)₄ and CF₃Cl. It seems plausible that damping of the undulations occurs when the polarizability of the molecule changes appreciably during a relatively slow vibration. A semiempirical quench parameter q is defined for molecules of the form AB_n to give a relative measure of the effectiveness of spherically symmetric vibrations in damping the glory undulations. q is proportional to (1) an estimate of the derivative of the polarizability with respect to the symmetric stretching coordinate; (2) a semiclassical measure of the vibration amplitude; and (3) the inverse of the symmetric stretching frequency. The values of q for molecules studied here are in qualitative agreement with our experimental results.