Collisional effects on angular momentum orientation in acetylene (X)over-tilde (1)Sigma(+)(g) (nu ''(2)=1,j ''). II. Disorientation by rotationally elastic and multiple inelastic collisions

This is the second of two papers about collisionally induced changes in mol ecular orientation. In the first paper [A. D. Rudert, J. Martin, W. B. Gao, J. B. Halpern, and H. Zacharias, J. Chem. Phys. 111, 9549 (1999)] the orie ntation was measured in the prepared state and in other states populated by collisional transfer from the initially excited one. It was shown that a s ignificant amount of the initial orientation is retained in collisions, eve n for large changes in the rotational quantum number. In this paper the dec ay of the orientation due to elastic and multiple inelastic collisions is i nvestigated. The measurements clearly show that for acetylene self-collisio ns the orientation decay [[k(ori)(total)]=7.6 +/- 1.0 (mu s Torr)(-1)] is m uch slower than the depopulation of the prepared rotational state [[k(tot)] =25 +/- 1.8 (mu s Torr)(-1)]. By using a set of master equations, rate cons tants are derived which describe the effects of both rotationally elastic a nd multiple inelastic collisions. From this model rate constants for orient ation decay due to rotationally elastic collisions, k(ori)(elastic), can be derived. These rate constants decrease from k(ori)(elastic)=10.7 (mu s Tor r)(-1) for j "=1 to k(ori)(elastic)=3.8 (mu s Torr)(-1) for j "=15. The rat e constants for orientation decay are found to be equal to previously measu red rate constants for the alignment decay. A model describing the collisio nally induced change of the direction of the molecular angular momentum vec tor is presented which reconciles both alignment and orientation decay meas urements. It is shown that m(j)-changing, rotationally elastic collisions c ompletely destroy any orientation or alignment and probably occur perpendic ular to the plane of molecular rotation. This is in contrast to rotationall y inelastic collisions which occur primarily in the plane of rotation. (C) 2000 American Institute of Physics. [S0021-9606(00)00122-7].