Collisional effects on angular momentum orientation in acetylene (X)over-tilde 1Σg+ (ν"2=1,j").: II.: Disorientation by rotationally elastic and multiple inelastic collisions

This is the second of two papers about collisionally induced changes in molecular 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 orientation was measured in the prepared state and in other states populated by collisional transfer from the initially excited one. It was shown that a significant amount of the initial orientation is retained in collisions, even for large changes in the rotational quantum number. In this paper the decay of the orientation due to elastic and multiple inelastic collisions is investigated. The measurements clearly show that for acetylene self-collisions the orientation decay [[k(ori)(total)]=7.6 +/- 1.0 (mu s Torr)(-1)] is much 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 constants are derived which describe the effects of both rotationally elastic and multiple inelastic collisions. From this model rate constants for orientation decay due to rotationally elastic collisions, k(ori)(elastic), can be derived. These rate constants decrease from k(ori)(elastic)=10.7 (mu s Torr)(-1) for j "=1 to k(ori)(elastic)=3.8 (mu s Torr)(-1) for j "=15. The rate constants for orientation decay are found to be equal to previously measured rate constants for the alignment decay. A model describing the collisionally induced change of the direction of the molecular angular momentum vector is presented which reconciles both alignment and orientation decay measurements. It is shown that m(j)-changing, rotationally elastic collisions completely destroy any orientation or alignment and probably occur perpendicular to the plane of molecular rotation. This is in contrast to rotationally inelastic collisions which occur primarily in the plane of rotation. (C) 2000 American Institute of Physics. [S0021-9606(00)00122-7].