COLLISIONAL ALIGNMENT OF CO2 ROTATIONAL ANGULAR-MOMENTUM STATES IN A SUPERSONIC EXPANSION

The rotational alignment of CO2 seeded in a supersonic expansion is measured using a general, direct absorption method based on fast (75 kHz) polarization modulation and phase sensitive detection with of a narrow band tunable IR laser. The anisotropic distribution of \M(J)\ states is created by a directed velocity slip between the carrier gas and a nonspherical seed gas. Strong alignment signals are observed in a pinhole expansion that depend systematically on the carrier gas, stagnation pressure, and J state. In a slit expansion, however, no alignment is detected for comparable conditions. The observed effects are quite significant and occur at rather modest expansion conditions (e.g., 2.5% CO2 in He at 1000 Torr). As much as 60% enhancement of n(perpendicular to) /n(parallel to), i.e., the ratio of molecules with J directed perpendicular vs parallel to the expansion axis is obtained. The data indicate that the observed alignment is correlated with the magnitude of the velocity slip, and that elastic collisions where Delta J=0 and Delta M(j) not equal 0 are primarily responsible for creating and sustaining the alignment.