Ultraviolet photodissociation dynamics of the benzyl radical

Abstract

Ultraviolet (UV) photodissociation dynamics of jet-cooled benzyl radical via the 4²B₂ electronically excited state is studied in the photolysis wavelength region of 228 to 270 nm using high-n Rydberg atom time-of-flight (HRTOF) and resonance enhanced multiphoton ionization (REMPI) techniques. In this wavelength region, H-atom photofragment yield (PFY) spectra are obtained using ethylbenzene and benzyl chloride as the precursors of benzyl radical, and they have a broad peak centered around 254 nm and are in a good agreement with the previous UV absorption spectra of benzyl. The H + C₇H₆ product translational energy distributions, P(E_T)s, are derived from the H-atom TOF spectra. The P(E_T) distributions peak near 5.5 kcal mol⁻¹, and the fraction of average translational energy in the total excess energy, 〈f_T〉, is ∼0.3. The P(E_T)s indicate the production of fulvenallene + H, which was suggested by recent theoretical studies. The H-atom product angular distribution is isotropic, with the anisotropy parameter β ≈ 0. The H/D product ratios from isotope labeling studies using C₆H₅CD₂ and C₆D₅CH₂ are reasonably close to the statistical H/D ratios, suggesting that the H/D atoms are scrambled in the photodissociation of benzyl. The dissociation mechanism is consistent with internal conversion of the electronically excited benzyl followed by unimolecular decomposition of the hot benzyl radical on the ground state.