Dynamics of collisional alignment in supersonic expansions: Trajectory studies of He+CO, O2, and CO2

Classical trajectory calculations have been performed on experimentally determined intermolecular potentials for He–O2, He–CO, and He–CO2 in order to simulate the collisional formation of rotationally aligned molecular distributions in a supersonic expansion. These calculations verify that multiple collisions between the light “diluent” gas and heavier “seed” rotor molecules result in a distribution of rotor molecules with negative alignment (a2<0), i.e., a preference for j perpendicular to the expansion axis. These rotational alignment effects are found to be robustly insensitive to collision energy and qualitatively similar for all three collision systems, thereby providing a useful basis for comparison with experimental studies. The asymptotic alignment is observed to depend strongly on the angular momentum, increasing monotonically with j. When analyzed on a collision-by-collision basis, this j dependence can be traced to gyroscopic stability, i.e., higher j states are classically more resistant to the collisional loss of alignment. In addition, collisional formation of the alignment is found to reflect comparable contributions from both elastic (mj-changing) and inelastic (j-changing) collisions. Finally, the calculations indicate that molecules 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+N2+ drift tube studies of Anthony et al. [J. Chem. Phys. 106, 5413 (1997)].