PREDISSOCIATION DYNAMICS OF THE METHYL RADICAL $\widetilde{BT}$ STATE

$^{1}$ L.D. Ziegler, Y.C. Chung, P. Wang and Y.P. Zhang, in Time Resolved Spectroscopy, edited by R.J.H. Clark and R.E. Hester (Wiley, New York, 1989) and references therein. $^{2}$ H.T. Yu, A Sevin, E. Kassab and E.M. Evieth, J. Chem. Phys. 80 2049 (1984).Author Institution: Department of Chemistry, University of CaliforniaQuantum level specific dynamics that occur on a subpicosecond timescale have been probed using resonance Raman spectroscopy. Excited state dissociation rates and lifetimes have been determined for the [0000] vibrational level of the B state of $CH_{3}$. The mean lifetimes varied from 69 femtoseconds for $J^{\prime} = 4$ to 84 femtoseconds for $J^{\prime}= 11$. The role of rotational forces in the unimolecular dissociation dynamics of CH3 has been examined by modeling the observed dissociation rates assuming a centrifugally modified potential energy function. Estimates for the height and curvature of the barrier to dissociation were compared to Ziegler's results for $ammonia.^{1}$ The results were consistent with predissociation of the excited states of the methyl radical and ammonia by hydrogen tunneling through a potential barrier. The $CH_{3}$ barrier was found to be approximately the same height rates the barrier for NH3, but was considerably narrow. The distance from the inner minimum, $r_{0}$, to the barrier, rb was also found to be smaller for $CH_{3}$. The theoretical work2 of Yu et al. was in qualitative agreement with our work, however their calculations predicted $r_{b} = 1.55 {\AA}$, which was much larger than that found by our value of $r_{b} = 1.35 {\AA}$. Experiments to examine the predissociation dynamics of the B state of $CD_{3}$ are in progress. The combined analysis of $CH_{3}$ and CD, dissociation rates will provide a more accurate characterization of the B state potential surfac