Collisional effects on angular momentum orientation in acetylene (X)over-tilde (1)Sigma(+)(g) (nu(2)''=1, j ''). I. Preparation, detection and conservation in single collisions

The effect of rotationally inelastic collisions on the orientation of the a ngular momentum is investigated for C2H2 (nu(2) = 1, j). The orientation of the angular momentum in initial rotational states j(i) = 7 to 13 is prepar ed by stimulated Raman pumping using circularly polarized light and S-branc h transitions. After allowing an appropriate time for collisions to occur t he orientation is probed by laser-induced fluorescence in the (A) over tild e (1)A(u)<--(X) over tilde (1)Sigma(g) transition. For acetylene the theore tically calculated and the measured initial orientation, A(0)((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 \Delta j\ = 6 with sufficient signal-to-noise ratio. These results are compared with previous measurements on the conservation of alignment in collisions. Rotat ionally inelastic collisions with \Delta j\ = 2 result in an average rotati on of the orientation vector by about 30 degrees. The amount of orientation that remains after a rotationally inelastic collision is found to depend p rimarily on \Delta j\. A Delta m(j) = 0 propensity rule often used in model s of rotational energy transfer is found to be inconsistent with the measur ements. Also a model which proposes a minimum change of the classical angle phi between j and the quantization axis does not reproduce well the experi mental data. Finally, a microscopic model for Delta m(j)-transitions is pre sented which reconciles both alignment and orientation measurements. (C) 19 99 American Institute of Physics. [S0021-9606(99)01445-2].