title: Theoretical investigation of intersystem crossing between the ã¹A₁ and X³B₁ states of CH₂ induced by collisions with helium.
abstract: Collisional energy transfer between the ground (X³B₁) and first excited (ã¹A₁) states of CH2 is facilitated by strong mixing of the rare pairs of accidentally degenerate rotational levels in the ground vibrational manifold of the [Formula: see text] state and the (020) and (030) excited bending vibrational manifolds of the X state. The simplest model for this process involves coherent mixing of the scattering T-matrix elements associated with collisional transitions within the unmixed ã and X states. From previous calculations in our group, we have determined cross sections and room-temperature rate constants for intersystem crossing of CH2 by collision with He. These are used in simulations of the time dependence of the energy flow, both within and between the X and ã vibronic manifolds. Relaxation proceeds through three steps: (a) rapid equilibration of the two mixed-pair levels, (b) fast relaxation within the ã state, and (c) slower relaxation among the levels of the X state. Collisional transfer between the fine-structure levels of the triplet (X) state is very slow.
authors.affiliation: Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742-2021, USA.
authors.forename: Lifang
authors.initials: L
authors.lastname: Ma
authors.affiliation: Department of Chemistry and Biochemistry and Institute for Physical Science and Technology, University of Maryland, College Park, Maryland 20742-2021, USA.
authors.forename: Millard H
authors.initials: MH
authors.lastname: Alexander
authors.affiliation: Department of Chemistry, The Johns Hopkins University, Baltimore, Maryland 21218-2685, USA.
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Dagdigian