Theoretical investigation of rotationally inelastic collisions of CH2(${\tilde{X}}$X̃) with helium

Abstract

Following our earlier work on collisions of He with the methylene radical in its excited ã(1)A(1) state [L. Ma, M. H. Alexander, and P. J. Dagdigian, J. Chem. Phys. 134, 154307 (2011)], we investigate here the analogous relaxation of CH(2) in its ground X(3)B(1) 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 CH(2) bending angle weighted by the square of the bending wave function. The PES for the interaction of He with CH(2) 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 CH(2)(ã) with He. Finally, we investigate the dependence of rotational inelasticity on the bending vibrational quantum number.

Exact quantum scattering calculations of transport properties: CH2($\tilde{X}^3$X̃3B1, $\tilde{a}^1$ã1A1)–helium

“…depth, and other properties of these two full PESs are quite different. 30,31 The near equality of the collision integrals for the spherically averaged CH 2 (X,ã)-He PESs follows from the fact that these potential energy curves are nearly identical. We also observe in Fig.…”

“…30,31 Rotational levels are labeled j k a ,k c , where j is the rotational angular momentum, k a is its projection along the principal axis in the prolate limit, and k c is its projection along the principal axis in the oblate limit. 40 Each rotational level exists for only one of the nuclear spin modifications, ortho and para, and these manifolds cannot be interconverted in collisions with helium.…”

“…30,31 The calculations for both theX andã electronic states employed a coupled-cluster method that included all single and double excitations, as well as perturbative contributions of connected triple excitations. For theã state the molecular geometry was frozen at its equilibrium geometry, while for thẽ X state the PES was averaged over the CH 2 bending angle, weighted by the square of the bending wave function.…”

“…29 Our group has recently carried out theoretical investigations of collisional rotational energy transfer in these electronic states in collision with helium, using computed highquality ab initio PESs. 30,31 The anisotropies of the PESs were found to be quite different because of the differing orbital occupancies of theX andã states. The PES for CH 2 (ã)-He is quite anisotropic because of the strong interaction of the electrons on the helium atom with the unoccupied CH 2 orbital perpendicular to the molecular plane.…”

Collision dynamics of symmetric top molecules: A comparison of the rotationally inelastic scattering of CD3 and ND3 with He

“…The transitions directly coupled by terms other than the large V 20 and V 33 terms could be enabled by higher-order transitions involving these large terms. For example, transitions directly coupled by the V 66 or V 53 terms can also be enabled by transitions involving the V 33 term twice or a combination of V 33 and V 20 terms, respectively.…”