QUANTUM SCATTERING STUDIES OF THE LAMBDA-DOUBLET RESOLVED ROTATIONAL ENERGY-TRANSFER OF OH(X(2)PI) IN COLLISIONS WITH HE AND AR

Three dimensional potential energy surfaces for the collision systems OH(X(2) Pi)+He and OH(X (2) Pi)+Ar have been calculated using the coupled electron pair approximation (CEPA) and large basis sets. The asymptotically degenerate (2) Pi(x) and (2) Pi(y) states split into two states of (2)A' and (2)A '' symmetry, respectively, when the C-infinity v symmetry is lifted by the approach of the noble gas atom. The average and half difference of the calculated points on the A '' and A' potential energy surfaces were fitted to analytical functions, which were then vibrationally averaged. These potential energy surfaces have been used in quantum scattering calculations of cross sections for collision induced rotationally inelastic transitions. Test calculations showed that the cross sections obtained from exact close-coupling calculations (CC) and within the coupled states approximation (CS) are in close agreement for these systems, and therefore the CS approximation has been used in all further calculations. Rotational transitions with Lambda doublet resolution show, within the same spin-orbit manifold and at low collision energies, a propensity to populate preferentially the e final levels in the F-1((2) Pi(3/2)) State and an elf conserving propensity in the F-2((2) Pi(1/2)) state, while transitions between the two spin-orbit manifolds show a parity conserving propensity. For the v=2 vibrational level kinetic rate coefficients were calculated for a large range of temperatures. The calculated cross sections are in excellent agreement with recent measurements of Schreel, Schleipen, Epping, and ter Meulen. (C) 1995 American Institute of Physics.