A quantum mechanical view of molecular alignment and cooling in seeded supersonic expansions

Experimental investigations on the collisional alignment of the rotational angular momentum, occurring in supersonic seeded beams and in drift tubes, have recently documented a strong dependence of the observed effects on the final molecular velocity. The present investigation aims at elucidating the possible mechanisms at the molecular collision level. Quantum state-to-state differential scattering cross sections, calculated for the prototype system O(2)-He, for an interaction potential previously obtained in this laboratory, exhibit propensities relevant to reveal nature and selective role of the elastic and inelastic scattering events, participating in the overall mechanisms which lead to molecular alignment and cooling. The present analysis shows that the dynamics of such phenomena crucially depends on the initial and final rotational state, on the collision energy, on the involved orbital angular momentum and therefore alternative routes are possible for molecular polarization and relaxation. These routes lead to scattering into specific angular cones and therefore observations from different experiments provide complementary pieces of information which, exploiting studies of various molecular systems under diverse experimental conditions, can be correlated in a single mosaic. (C) 1999 American Institute of Physics. [S0021-9606(99)01729-8].