Inelastic collisions of fine structure and Λ-doublet resolved rotational states of PH(A 3Π, v=0) with helium

A state-resolved experimental and theoretical investigation of inelastic collisions of PH(A 3Π) rotational/fine-structure levels with helium is presented. Experimental state-to-state rate constants were extracted from PH A→X fluorescence spectra recorded upon laser excitation to individual levels in the A state in the presence of helium. These rate constants have been compared with and interpreted through theoretical rate constants computed with full quantum close-coupling and coupled-states calculations of cross sections based on ab initio PH(A)–He potential energy surfaces determined by Kolczewski et al. [J. Chem. Phys. 106, 7637 (1997)]. The low-J rotational levels of PH(A) are close to the Hund’s case (a) limit, and the cross sections exhibit a number of the predicted case (a) collisional propensities. For low initial N, there is a strong propensity for fine-structure conserving transitions. With increasing initial N, fine-structure changing transitions occur with increasing probability. This tendency appears to be related to the transition of the coupling scheme away from pure case (a).