State-resolved inelastic collisions of single rotational, fine-structure, and Λ doublet levels of NH(A 3Π) with helium: A combined experimental and theoretical study

A comprehensive set of single-collision state-to-state rate constants for the relaxation of NH(A 3Π, v=0,N,Fi,e/f ) levels in collisions with helium are presented. These rate constants were extracted from measured fluorescence intensities of isolated A→X rotational lines in the presence of He subsequent to laser excitation of individual levels in the A state. There is no indication of the collisional propensities predicted for a Hund’s case (a) 3Π state, most likely because NH(A) represents an intermediate coupling case. With increasing N, NH(A) rapidly approaches the Hund’s case (b) limit. For high initial N, rotational relaxation proceeds predominantly through ΔN=−1 fine-structure conserving transitions to Λ doublet levels with Π(A′) reflection symmetry. In addition, a propensity to conserve the Π(A′)/Π(A″) symmetry of the initial level is found for ΔN=0 fine-structure changing transitions. The observed propensities have been interpreted by comparison with full quantum close-coupling and coupled-states calculations of cross sections based on ab initio potential energy surfaces (PES’s). The propensity for forming Π(A′)Λ doublet levels in ΔN=−1 collisions of high N initial levels is found to be facilitated by approach in a ‘‘helicopterlike’’ orientation on the more attractive HeNH(A)A′ PES, followed by curve crossing to the more repulsive A″ PES which correlates adiabatically to the next lower rotational manifold. In addition, thermal rate constants derived from the calculated cross sections agree extremely well with those obtained from a deconvolution of the experimental spectra.