Knowledge Bank

Several time-resolved experiments have been performed on the $J=0-1$ and $J=1-2$ transitions of OCS and on the $J=0-1$ transitions of $CH3CI$. The analytical evaluation of decay curves due to wall-collisions and the Doppler effect developed in this work allows an analysis of non-exponential collision-induced relaxation. The speed dependence of $T2$ relaxation rates and of some $T1$ rates has been determined to first order by assuming a relaxation rate changing linearly with the speed, including in the analysis the resultant changes in wall-collision and Doppler effects due to changes in the average speed of the monitored population. Non-zero speed dependences imply non-Lorentzian pressure-broadened lineshapes (observed in $OCS1$, OCS $T2$ and $T1$ decays have a significant speed dependence, reflecting contributions from non- (dipole-dipole) processes. $CH3C1$ $T2$ decays for the $J=0-1F=3/2-3/2$ transition show a zero speed dependence. In OCS, the results of two 2-level $T1$ experiments and of one 3-level double resonance experiment imply that the $T1$ rates for the three individual levels ($M=0,J=0,1,$ and 2) are identical. Therefore, the $T1$ decays for these three experiments should be exponential in the absence of a speed dependence. The $T1$ speed dependence is significant but smaller than the $T2$ speed dependence for these transitions. We have also found in the OCS $J=1-2$ that $T1(M=0)$ is shorter than $T2(M=\pm 1)$, which implies the occurrence of significant $△J=0,\Delta M=\pm 2$ transitions. Also, for all OCS $T1$ times, $T1$ (Stark voltage on) is longer than $T1$ (Stark voltage off). That is, reducing the number of level degeneracies reduces the collision $efficiency.1$R. A. Creswell, S. R. Brown, and R. H. Schwendeman, J. Chem. Phys. 64, 1820 (1976).