ROTATIONAL AND VIBRATIONAL COLLISIONAL PROCESSES IN $CH_{3}Cl$

$^{1}$ H. O. Everitt and F. C. De Lucia, J. Chem. Phys. 90, 3520 (1989).""Author Institution: Department of Physics, Duke University; Department of Physics, Ohio State University; U.S. Army Research Office, Ohio State UniversityThe rates of several collisional processes have been measured in $CH_{3}Cl$ using time-resolved infrared/mm-wave double resonance. This study was performed to characterize further collisional energy transfer processes found in other methyl halides previously studied at Duke: $^{13}CH_{3}F$ and $^{12}CH_{3}F$. A process was $discovered^{1}$ in $CH_{3}F$ that thermalizes rotational levels of the same symmetry type within a vibrational state. The cross section for this process is large enough that it makes a significant contribution to the pressure broadened linewidth. This process has been observed to have a very large cross section in $CH_{3}Cl$. Fundamental energy conservation considerations place constraints on the range of states that can be thermalized by this process. $CH_{3}Cl$ data which addresses this issue will be presented and discussed. In addition, our previous work in $CH_{3}F$ showed a large vibrational swapping rate which effectively circumvented the $\Delta K=3n$ selection rule. This rate is expected to be much smaller in the pumped vibrational state of $CH_{3}Cl$ due to the smaller vibrational transition moment. This has been quantitatively confirmed by our $CH_{3}Cl$ measurements