PHOTODISSOCIATION OF METHANOL AT 193.3 NM - TRANSLATIONAL ENERGY-RELEASE SPECTRA

Center-of-mass translational energy distributions of the dominant prim ary products resulting from 193.3 nm excitation of jet-cooled CH3OH, C H3OD, and CD3OH were obtained by using the high-n Rydberg time-of-flig ht (HRTOF) technique. The appearance threshold in the HRTOF spectrum y ields a bond dissociation energy, D-0(CH3O-H), of 105+/-1 kca mol(-1), in agreement with recent literature values. Translational energy rele ase spectra from the three isotopomers exhibit progressions of 950+/-1 00 cm(-1), which are attributed to excitation in the upsilon(3) O-CH3 stretch of the methoxy product. The progressions peak at upsilon=1, wi th population out to at least upsilon=5. This differs from the results of a recent wave packet dynamics study on a calculated excited state potential energy surface [Marston et al., J. Chem. Phys. 98, 4718 (199 3)], which predicted no O-CH3 stretch excitation in the methoxy fragme nt following photolysis of ground state methanol. The spatial anisotro py of the fragments (beta similar to-0.7) implies a dissociation time less than or equal to 1 ps. The impulsive model for rotational excitat ion is compared to the unresolved rotational contour of-the vibrationa l peaks in the translational energy release spectra and is found to un derestimate the extent of rotational excitation, though the model corr ectly predicts the increase in contour width observed for the O-deuter ated species. The unresolved rotational contours are fit empirically. The inferred vibrational energy distributions are discussed in terms o f a simple Franck-Condon model for the pseudotriatomic, Me-O-H. Implic ations of the vibrational and rotational photofragment distributions f or the full 1 (1)A'' surface are discussed.