COLLISION-INDUCED INTERSYSTEM CROSSING OF CH-2 FROM (A)OVER-TILDE (1)A(1) TO (X)OVER-TILDE (3)B(1) - A CASE-STUDY OF THE MIXED-STATE MODEL

The rate coefficients for collision-induced intersystem crossing (CIISC) of methylene from the a1A1 first excited to the X3B1 ground electronic state, CH2 (a1A1) + M --> CH2 (X3B1) + 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 a manifold and allowed transitions from the a to the X manifold originating via ''gate'' states of a which are states that contain some triplet character due to spin-orbit coupling with nearby X rovibrational states. The perturbed a and perturbing X 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 (X) 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 (a) and build up of (X) 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 (a) rotation vibration state with energies up to E(vr) greater-than-or-equal-to 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.