Dynamics of collisional alignment in supersonic expansions: Trajectory studies of He+CO, O-2, and CO2

Classical trajectory calculations have been performed on experimentally det ermined intermolecular potentials for He-O-2, He-CO, and He-CO2 in order to simulate the collisional formation of rotationally aligned molecular distr ibutions in a supersonic expansion. These calculations verify that multiple collisions between the light "diluent" gas and heavier "seed" rotor molecu les result in a distribution of rotor molecules with negative alignment (a( 2) < 0), i.e., a preference for j perpendicular to the expansion axis. Thes e rotational alignment effects are found to be robustly insensitive to coll ision energy and qualitatively similar for all three collision systems, the reby providing a useful basis for comparison with experimental studies. The asymptotic alignment is observed to depend strongly on the angular momentu m, increasing monotonically with j. When analyzed on a collision-by-collisi on basis, this j dependence can be traced to gyroscopic stability, i.e., hi gher j states are classically more resistant to the collisional loss of ali gnment. In addition, collisional formation of the alignment is found to ref lect comparable contributions from both elastic (m(j)-changing) and inelast ic (j-changing) collisions. Finally, the calculations indicate that molecul es with j aligned parallel to the expansion axis are correlated with faster average velocities than molecules with j perpendicular to the axis, which is consistent with the He+CO experimental studies of Harich and Wodtke [J. Chem. Phys. 107, 5983 (1997)], as well as the He+N-2(+) drift tube studies of Anthony [J. Chem. Phys. 106, 5413 (1997)]. (C) 1999 American Institute o f Physics. [S0021-9606(99)02438-1].