Theoretical investigation of rotationally inelastic collisions of CH2 (($)over-tilde-$a) with helium

Rotationally inelastic collisions of the CH2 molecule in its ($) over tilde $a(1)A(1) electronic state have been investigated. We have determined a potential energy surface (PES) for the interaction of rigid CH2(($) over tilde $a), frozen at its equilibrium geometry, with a helium atom, using a coupled-cluster method that includes all single and double excitations, as well as perturbative contributions of connected triple excitations [RSSCD(T)]. The PES is quite anisotropic, due to lack of electron density in the unoccupied CH2 non-bonding orbital perpendicular to the molecular plane. Quantum scattering calculations have been carried out to compute state-to-state rotational energy transfer and elastic depolarization cross sections at collision energies up to 2400 cm(-1). These cross sections were thermally averaged to derive room-temperature rate constants. The total removal and elastic depolarization rate constants for the ortho k(a) = 1 levels agree well with recent experimental measurements by Hall, Sears, and their co-workers. We observe a strong even-odd alternation in the magnitude of the total rate constants which we attribute to the asymmetry splitting of the k(a) = 1 levels. (C) 2011 American Institute of Physics. [doi:10.1063/1.3575200]