Long-range intermolecular potentials for the metastable rare gas-rare gas systems Ar*, Kr*( 3P 0,2)+Ar, Kr, Xe

Abstract

In a crossed beam experiment with a calibrated supersonic secondary beam the absolute value (rms accuracy 2.5%) and the velocity dependence of the total cross section Q have been measured in a wide range of relative velocities 500<g<5000 m s −1 for the title systems. The data have been analysed with the ion-atom Morse-Morse-spline-van der Waals potential of Gregor and Siska for the Ne*-Xe system as a reference, resulting in accurate values for the well depth ϵ, well position R m and long-range pure van der Waals constants C 6 (without the influence of C 8 and C 10). These long-range pure C 6 values are in excellent agreement with the calculated values of Dalgarno for the corresponding alkali atom-rare gas systems when scaled with the ratio of polarisabilities. The major difference in comparison with the alkali systems is the larger influence of the higher-order dispersion terms C 8 and C 10 (due to the ( np) −1 core hole), as reflected in the velocity dependence of the attractive contribution Q a ∝ (2ϵ R m/ ħg) 2/( s−1) with s=6.5–7 for high velocities and in the large amplitude of the glory contribution Q gl. For all systems the interesting N=1 glory maximum has been observed, with the glory range extending to the N=1.5 minimum for Ar*-Ar and to the N=6 maximum for Kr*-Xe. For the homonuclear systems only the glory oscillations due to the van der Waals-type potentials are observed. The glory damping due to the scattering on multiple potentials is described in terms of a rms spread Δ(ϵ R m) in the ϵ R m product, e.g., for Ar*-Ar we find Δ(ϵ R m)/ϵ R m=0.13. These measurements are fully complementary to the high-energy differential cross section measurements of Gillen et al., which only probe the chemical type potential with a deep well. For the heteronuclear Kr*-Xe system a strong velocity-dependent damping of the glory oscillations is observed, which can be described quantitatively by an excitation transfer process to a near-resonant short-lived Xe**((5p) 5 (6p)) state. The characteristic velocity dependence is used to calculate the radial coupling matrix element for this transition.