The rotational relaxation of NH(c 1Π) in collisions with Ar: A combined theoretical and experimental investigation

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We report full quantum close-coupling and coupled-states calculations of cross sections and rate constants for the rotational relaxation of NH(c 1Π) in collisions with Ar, based on a new ab initio description of the potential energy surface for this system. Single-collision, state-to-state thermal rate constants were extracted from measured fluorescence intensities of relaxing NH(c 1Π,v=0,J) distributions in the presence of Ar starting with singly populated rotational states. Overall, the calculated and experimental rate constants agree well, although the dependence on the initial J is somewhat different. At high J the relaxation is dominated by J→J−1e/f changing transitions. This process is facilitated by approach on the more attractive ArNH(c)A′ PES followed by a curve crossing to the more repulsive A″ PES which correlates asymptotically to the next lower rotational level. For approach in a ‘‘helicopterlike’’ orientation, this process results in Jf→J−1,e transitions, while for approach in a ‘‘pinwheellike’’ orientation, this process results in Je→J−1,f transitions. Thus, similar to what we found earlier for the relaxation of high rotational levels of the CH(X) radical [M. H. Alexander and P. J. Dagdigian, J. Chem. Phys. 101, 7468 (1994)], a strong v, J correlation in the reactant channel results in a strong Λ, J correlation in the product channel. A full kinetic simulation, based on the calculated cross sections, reproduces nearly quantitatively the experimental observations of the pressure dependence of the equilibration of NH(c) Λ doublet populations monitored some years ago by Quinton and Simons.