Collisional effects on angular momentum orientation in acetylene X̃ 1Σg+ (ν2″=1,j″). I. Preparation, detection and conservation in single collisions

The effect of rotationally inelastic collisions on the orientation of the angular momentum is investigated for C2H2 (ν2=1, j). The orientation of the angular momentum in initial rotational states ji=7 to 13 is prepared by stimulated Raman pumping using circularly polarized light and S-branch transitions. After allowing an appropriate time for collisions to occur the orientation is probed by laser-induced fluorescence in the à 1Au←X̃ 1Σg transition. For acetylene the theoretically calculated and the measured initial orientation, A0(1), ranges between 0.7 and 0.9. A substantial part of the orientation is conserved in rotationally inelastic collisions, which could be measured up to |Δj|=6 with sufficient signal-to-noise ratio. These results are compared with previous measurements on the conservation of alignment in collisions. Rotationally inelastic collisions with |Δj|=2 result in an average rotation of the orientation vector by about 30°. The amount of orientation that remains after a rotationally inelastic collision is found to depend primarily on |Δj|. A Δmj=0 propensity rule often used in models of rotational energy transfer is found to be inconsistent with the measurements. Also a model which proposes a minimum change of the classical angle φ between j⃗ and the quantization axis does not reproduce well the experimental data. Finally, a microscopic model for Δmj-transitions is presented which reconciles both alignment and orientation measurements.