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 via 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(¹D₂) fine structure level into the ³P 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(¹D₂) 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(³P₀) 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.