ON THE GENERATION OF PREFERENTIAL LAMBDA-DOUBLET POPULATIONS IN THE COLLISIONAL RELAXATION OF HIGHLY ROTATIONALLY EXCITED CH(X(2)PI)

By means of full quantum close-coupling and coupled states calculations based on an ab initio potential energy surface for the Ar-CH system, we confirm a propensity seen experimentally by Hancock, Stuhl, and their co-workers. During the rotational relaxation of high rotational levels of the CH(X (2) Pi) radical, produced by photolysis of a suitable precursor, there appears a clear population imbalance in favor of the Lambda-doublet levels of Pi(A'') symmetry. A full kinetic simulation, based on the calculated cross sections, reproduces nearly quantitatively the experimental observations of both the temporal evolution and the pressure dependence of this h-doublet asymmetry. This asymmetry is a consequence of both an enhanced depletion of high N Pi(A') levels and the enhanced formation of Pi(A'') levels in the next lower (N-1) manifolds. The physical origin of this propensity involves a crossing between two adiabatic bender potentials which follow, respectively, the A' and A'' potential energy surface (PES). This crossing occurs only for the ''helicopter-like'' approach of the CH molecule, in which its rotational angular momentum is aligned along the initial relative velocity vector. Thus, a strong upsilon, N correlation in the reactant channel results in a strong Lambda, N correlation in the product channel.