Fine structure-resolved rotational energy transfer of SH (A2Σ+, v′ = 0) state by collisions with Ar

Abstract

Rotational energy transfer (RET) by Ar collisions within the v′ = 0 level of the SH A²Σ⁺ state is probed using a laser-induced dispersed fluorescence technique, following photodissociation of H₂S at 248 nm. The Ar pressure is adjusted appropriately to allow for significant observation of the single-collision induced RET process. The spin-resolved and spin-averaged rate constants are then evaluated with the aid of a kinetic model under single-collision conditions. The theoretical counterparts are calculated using a quantum scattering method, in which a newly fitted potential energy function is based on ab initio potential energy surface reported previously. The experimental and theoretical kinetic data are essentially consistent in the trend of N and ΔN dependence. Several propensity rules are found in the RET collisions. For instance, for ΔN = 1, 2, and 3, the rate constants decrease with increasing N or ΔN. Given a fixed ΔN, the rate constants of the same initial N in the downward transition appear to be larger than those in the upward transitions. In ΔN = 0, the F₂ → F₁ transitions prevail over the F₁ → F₂ transitions (F₁ = N + 1/2, F₂ = N − 1/2), whereas in ΔN ≠ 0, the fine-structure-conserving collisions are more favored than the fine-structure-changing collisions. The principle of microscopic reversibility is also examined for both experimental and theoretical kinetic data, showing that translational energies of the RET collisions are close to thermal equilibrium at room temperature. The propensity rules may be rationalized according to this principle.