Ultraviolet photodissociation dynamics of the phenyl radical

Ultraviolet (UV) photodissociation dynamics of jet-cooled phenyl radicals (C₆H₅ and C₆D₅) are studied in the photolysis wavelength region of 215–268 nm using high-n Rydberg atom time-of-flight and resonance enhanced multiphoton ionization techniques. The phenyl radicals are produced from 193-nm photolysis of chlorobenzene and bromobenzene precursors. The H-atom photofragment yield spectra have a broad peak centered around 235 nm and are in good agreement with the UV absorption spectra of phenyl. The H + C₆H₄ product translational energy distributions, P(E_T)’s, peak near ∼7 kcal/mol, and the fraction of average translational energy in the total excess energy, 〈f_T〉, is in the range of 0.20–0.35 from 215 to 268 nm. The H-atom product angular distribution is isotropic. The dissociation rates are in the range of 10⁷–10⁸ s⁻¹ with internal energy from 30 to 46 kcal/mol above the threshold of the lowest energy channel H + o-C₆H₄ (ortho-benzyne), comparable with the rates from the Rice–Ramsperger–Kassel–Marcus theory. The results from the fully deuterated phenyl radical are identical. The dissociation mechanism is consistent with production of H + o-C₆H₄, as the main channel from unimolecular decomposition of the ground electronic state phenyl radical following internal conversion of the electronically excited state.