Quantum scattering studies of inelastic collisions of NH(A 3Π) with helium: Fine-structure and Λ-doublet propensities

The results of full close-coupled calculations of state-to-state cross sections for rotationally inelastic collisions of NH in its A 3Π electronic state with helium, based on the recently calculated ab initio potential energy surfaces of Jonas and Staemmler [Z. Phys. D 14, 143 (1989)], are presented. The calculated Λ-doublet resolved cross sections have been compared with predictions based on formal analyses of the scattering equations both in the Hund’s case (a) and (b) limits. For transitions involving low J levels, a strong propensity toward conservation of the e/f label was found, as expected in the case (a) limit. For higher J, the cross sections connecting related pairs of Λ-doublet levels were found to be unequal, reflecting a quantum mechanical interference between the two potential energy surfaces arising from the interaction of a molecule in a Λ>0 state with a perturber. For transitions connected by even l terms in the expansion of the potentials, a simple analysis, based on the relative strengths of the l=2 coupling matrix elements of the electrostatic potential, was found capable of explaining the relative ordering of the cross sections in most cases. A similar success for predicting transitions coupled by odd l terms in the potential was not found; this reflects the fact that the l=3 terms are relatively small for the NH(A 3Π)–He interaction. The calculated cross sections for large J also exhibit a propensity for conservation of the fine-structure label, as expected in the case (b) limit. As an indication of the reliability of the calculated interaction potential and our treatment of the collision dynamics, appropriately summed calculated cross sections reproduce well the experimental rates for transitions from selected f levels into all e levels, as measured by Stuhl and co-workers.