Anisotropic translational cooling: Velocity dependence of collisional alignment in a seeded supersonic expansion

Whether or not the collisional alignment resulting from seeded supersonic expansions may be utilized for chemical studies of aligned reactants will depend on our ability to understand this phenomenon. We have performed quantum-state-specific neutral time-of-flight measurements on molecular beams of dilute CO seeded in helium using polarized probe light, revealing the collisional alignment of individual rotational states of CO as a function of the terminal velocity. The derived peak velocities and translational temperatures depend on the probe-light polarization. CO molecules with their rotational angular momentum vectors, J⃗, aligned parallel to the flow velocity are more effectively accelerated by the light carrier gas and reach a lower terminal translational temperature than molecules with J⃗ aligned perpendicular to the flow velocity. The potential for using collisional alignment is discussed.