Theoretical investigation of rotationally inelastic collisions of CH2(X) with helium.

Following our earlier work on collisions of He with the methylene radical in its excited ã¹A₁ state [L. Ma, M. H. Alexander, and P. J. Dagdigian, J. Chem. Phys. 134, 154307 (2011)], we investigate here the analogous relaxation of CH₂ in its ground X³B₁ electronic state. The molecule is treated as semi-rigid, with fixed bond lengths but a varying bond angle. We use an ab initio potential energy surface (PES) which is averaged over the CH2 bending angle weighted by the square of the bending wave function. The PES for the interaction of He with CH2 in the X state is considerably less anisotropic than for interaction with the ã state since the two 2p electrons on the C atom are evenly distributed among the bonding and non-bonding molecular orbitals. We report quantum scattering calculations of state-to-state and total removal cross sections as well as total removal rate constants at room temperature. Because of the less pronounced anisotropy, these cross sections and rate constants are considerably smaller than for collisions of CH₂(ã) with He. Finally, we investigate the dependence of rotational inelasticity on the bending vibrational quantum number. [ABSTRACT FROM AUTHOR]