Electronic quenching of sulfur induced by argon collisions.

An accurate potential energy model, explicitly designed for studying scattering and treating the spin-orbit and nonadiabatic couplings on an equal footing, is proposed for the S + Ar system. The model is based on the Effective Relativistic Coupling by Asymptotic Representation (ERCAR) approach, building the geometry dependence of the spin-orbit interaction a diabatisation scheme. The resulting full diabatic model is used in close-coupling calculations to compute inelastic scattering cross sections for de-excitation from the S(D) fine structure level into the P multiplet. The energy grid is tuned to resolve the many resonances present and to guarantee converged thermal rates from 1 to 300 K. At temperatures above 100 K, the computed thermal rate coefficients for quenching of S(D) are in good agreement with results from an earlier experimental and theoretical study. The branching ratio at 296 K for de-excitation into the S(P) level agrees well with the value obtained by a different experiment. A discrepancy however remains between theory and experiment at lower temperatures. This is discussed in light of the interference mechanisms at play during this quenching process.