Ultraviolet Photodissociation Dynamics of the Allyl Radical via the (B)over-tilde2A1(3s), (C)over-tilde2B2(3py), and (E)over-tilde2B1(3px) Electronic Excited States

Ultraviolet (UV) photodissociation dynamics of jet-cooled allyl radical via the (B) over tilde (2)A(1)(3s), (C) over tilde B-2(2)(3p(y)), and (E) over tilde B-2(1)(3p(x)) 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(E-T), distributions peaking near 8.5 kcal/mol and the fraction of the average translational energy in the total excess energy, < f(T)>, in the range 0.22-0.18 from 249 to 216 tim. The P(E-T)'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 beta approximate to 0. The H atom dissociation rate constant from the pump-probe study gives a lower limit of 1 x 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.