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 calculation s based on an ab initio potential energy surface for the Ar-CH system, we confirm a propensity seen experimentally by Hancock, Stuhl, and th eir co-workers. During the rotational relaxation of high rotational le vels 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 quantitativ ely the experimental observations of both the temporal evolution and t he pressure dependence of this h-doublet asymmetry. This asymmetry is a consequence of both an enhanced depletion of high N Pi(A') levels an d the enhanced formation of Pi(A'') levels in the next lower (N-1) man ifolds. The physical origin of this propensity involves a crossing bet ween two adiabatic bender potentials which follow, respectively, the A ' and A'' potential energy surface (PES). This crossing occurs only fo r the ''helicopter-like'' approach of the CH molecule, in which its ro tational angular momentum is aligned along the initial relative veloci ty vector. Thus, a strong upsilon, N correlation in the reactant chann el results in a strong Lambda, N correlation in the product channel.