Collision‐Induced Intersystem Crossing in CH2 (ã 1A1): A Quantitative Analysis Using the Mixed‐State Model

Abstract

AbstractThe mechanism of collision‐induced intersystem crossing (ISC) in CH2 from the ã 1A1 first excited singlet to the X̃ 3B1, triplet ground electronic state, CH2 (ã) + M → CH2 (X̃) + M, is investigated. Collisional ISC rate constants are calculated for four selected collider gases, M = He, Ar, N2, and CH4, using the “mixed‐state” model of Freed and Gelbart. The most important triplet‐perturbed rotational levels in the ã‐state are identified from the spectroscopic data reported in the literature. The rates of rotational relaxation processes, which according to the model effectively govern the collision‐induced ISC, are determined by studying the collision broadening of CH2 (X̃) Far Infrared Laser Magnetic Resonance (FIR‐LMR) transitions. The collisional ISC rate constants for CH2 in the ortho nuclear spin modification are calculated to be kISC = 1.4 · 1012, 4.0 · 1012, 7.8 · 1012, and 6.6 · 1012 cm3/mol · s for M = He, Ar, N2, and CH4, respectively. The results are in good agreement with published experimental rate constants.