Ultraviolet (UV) photodissociation dynamics of jet-cooled allyl radical via the B̃(2)A1(3s), C̃(2)B2(3py), and Ẽ(2)B1(3px) electronically excited states are studied at the photolysis wavelengths from 249 to 216 nm using high-n Rydberg atom time-of-flight (HRTOF) and resonance-enhanced multiphoton ionization (REMPI) techniques. The photofragment yield (PFY) spectra of the H atom products are measured using both allyl chloride and 1,5-hexadiene as precursors of the allyl radical and show a broad peak centered near 228 nm, whereas the previous UV absorption spectra of the allyl radical peak around 222 nm. This difference suggests that, in addition to the H + C3H4 product channel, another dissociation channel (likely CH3 + C2H2) becomes significant with increasing excitation energy. The product translational energy release of the H + C3H4 products is modest, with the P(ET) distributions peaking near 8.5 kcal/mol and the fraction of the average translational energy in the total excess energy, ⟨fT⟩, in the range 0.22-0.18 from 249 to 216 nm. The P(ET)'s are consistent with production of H + allene and H + propyne, as suggested by previous experimental and theoretical studies. The angular distributions of the H atom products are isotropic, with the anisotropy parameter β ≈ 0. The H atom dissociation rate constant from the pump-probe study gives a lower limit of 1 × 10(8)/s. The dissociation mechanism is consistent with unimolecular decomposition of the hot allyl radical on the ground electronic state after internal conversion of the electronically excited state.

Ultraviolet (UV) photodissociation dynamics of jet-cooled phenyl radicals (C(6)H(5) and C(6)D(5)) 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 + C(6)H(4) 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, , 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 10(7)-10(8) s(-1) with internal energy from 30 to 46 kcal/mol above the threshold of the lowest energy channel H + o-C(6)H(4) (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-C(6)H(4), as the main channel from unimolecular decomposition of the ground electronic state phenyl radical following internal conversion of the electronically excited state.