Dynamics of Methyl Radical Formation Following 266 nm Dissociative Photoionization of Xylenes and Mesitylene.

The 266 nm dissociative photoionization of three xylene isomers and mesitylene leading to the formation of methyl radical was examined. The total translational energy distribution profiles [ P ( E T )] for the methyl radical were almost identical for all of the three isomers of xylene and mesitylene, while a substantial difference was observed for the corresponding P ( E T ) profile of the co-fragment produced by loss of one methyl group in m -xylene. This observation is attributed to the formation of the methyl radical from alternate channels induced by the probe. The P ( E T ) profiles were rationalized based on the dissociation of {sp 2 }C-C{sp 3 } bond in the cationic state, wherein the {sp 2 }C-C{sp 3 } bond dissociation energy is substantially lower relative to the neutral ground state. The dissociation in the cationic state follows a resonant three-photon absorption process, resulting in a maximum translational energy of about 1.6-1.8 eV for the photofragments in the center-of-mass frame. Fitting of the P ( E T ) profiles to empirical function reveals that the dynamics of {sp 2 }C-C{sp 3 } bond dissociation is insensitive to the position of substitution but marginally dependent on the number of methyl groups.