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Dynamics of Collisional Alignment in Supersonic Expansions: Trajectory Studies of He + Co, O2 and CO2



J R. Fair, David Nesbitt


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 qualiatively 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 fo 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 + N+2 drift tube studies of Anthony et al. [J. Chem. Phys. 106, 5413 (1997)].
Journal of Chemical Physics
No. 15


chemical trajectory calculations, collisional alignment, supersonic expansion


Fair, J. and Nesbitt, D. (1999), Dynamics of Collisional Alignment in Supersonic Expansions: Trajectory Studies of He + Co, O<sub>2</sub> and CO<sub>2</sub>, Journal of Chemical Physics (Accessed December 11, 2023)
Created September 30, 1999, Updated October 12, 2021