Collision-induced intersystem crossing of CH2 fromã 1A1 toX̃ 3B1: A case study of the mixed-state model

Abstract

The rate coefficients for collision-induced intersystem crossing (CIISC) of methylene from the ã 1A1 first excited to the X̃ 3B1 ground electronic state, CH2 (ã 1A1)+M→CH2 (X̃ 3B1)+M, were investigated within the framework of the mixed-state mechanism [see, e.g., K. F. Freed, in Potential Energy Surfaces, edited by K. P. Law (Wiley, New York, 1980)]. Accordingly, the overall electronic relaxation was assumed to proceed via a sequence of rotational transitions within the ã manifold and allowed transitions from the ã to the X̃ manifold originating via ‘‘gate’’ states of ã which are states that contain some triplet character due to spin–orbit coupling with nearby X̃ rovibrational states. The perturbed ã and perturbing X̃ levels and relevant interaction matrix elements were identified from the available spectroscopic data. Rate coefficients for rotational relaxation processes were obtained from collision broadening measurements of CH2 (X̃) far-infrared laser magnetic resonance (FIR-LMR) transitions. Taking these data, thermal CIISC rate constants corresponding to experimental results for the overall depletion of rotationally thermalized CH2 (ã) and build up of (X̃) by M were evaluated for twelve interesting collision partners (M=He, Ne, Ar, Kr, Xe, N2, SF6, H2, D2, CH4, C2H6, H2O), taking into account every single CH2 (ã) rotation vibration state with energies up to Evr≤900 cm−1. The results were found to be in good agreement with reported room-temperature experimental data. Temperature dependencies, which were predicted for M=Ar, N2, and CH4, also agree with measured values.