The photodissociation dynamics of the ethyl radical C(2)H(5) has been investigated by velocity map imaging. Ethyl was produced by flash pyrolysis from n-propyl nitrite and excited to the à (2)A(') (3s) Rydberg state around 250 nm. The energetically most favorable reaction channel in this wavelength region is dissociation to C(2)H(4) (ethene) + H. The H-atom dissociation products were ionized in a [1+1(')] process via the 1s-2p transition. The observed translational energy distribution is bimodal: A contribution of slow H-atoms with an isotropic angular distribution peaks at low translational energies. An expectation value for the fraction of excess energy released into translation of = 0.19 is derived from the data, typical for statistical dissociation reactions. In addition, a fast H-atom channel is observed, peaking around 1.8 eV. The latter shows an anisotropic distribution with β = 0.45. It originates from a direct dissociation process within less than a rotational period. Time-delay scans with varying extraction voltages indicate the presence of two rates for the formation of H-atoms. One rate with a sub-nanosecond time constant is associated with H-atoms with large translational energy; a second one with a time constant on the order of 100 ns is associated with H-atoms formed with low translational energy. The data confirm and extend those from previous experiments and remove some inconsistencies. Possible mechanisms for the dissociation are discussed in light of the new results as well as previous ones.

The photodisscociation dynamics of the alkyl radicals i-propyl (CH(CH3)2) and t-butyl (C(CH3)3) are investigated by H-atom photofragment imaging. While i-propyl is excited at 250 nm, the photodynamics of t-butyl are explored over a large energy range using excitation wavelengths between 347 nm and 233 nm. The results are compared to those obtained previously for ethyl, CH3CH2, and to those reported for t-butyl using 248 nm excitation. The translational energy (ET) distribution of the H-atom photofragments is bimodal and appears rather similar for all three radicals. The low ET part of the distribution shows an isotropic photofragment angular distribution, while the high ET part is associated with a considerable anisotropy. Thus, for t-butyl, two H-atom loss channels of roughly equal importance have been identified in addition to the CH3-loss channel reported previously. A mechanism for the photodissociation of alkyl radicals is suggested that is based on interactions between Rydberg- and valence states.

The photodissociation dynamics of isolated fulvenallene, the most stable C7H6 isomer, is investigated in a free jet. Fulvenallene, an intermediate in toluene combustion, is generated by flash pyrolysis from phthalide and excited between 255 and 245 nm into a ππ* state with high oscillator strengths. We show that loss of a hydrogen atom and formation of fulvenallenyl is an important dissociation channel for fulvenallene. Velocity map images of the hydrogen atom photofragments are recorded. The photofragment angular distribution is isotropic and the translational energy release can be fitted using a simple one-parameter function. Around 9% of the excess energy is released as translation. All data are in agreement with a statistical photodissociation in the electronic ground state after internal conversion. The impact of the results for combustion chemistry is discussed.

We present a joint experimental and computational study of the nonradiative deactivation of the benzyl radical, C7H7, after UV excitation.

We report a joint theoretical and experimental study on the photodissociation of the C3H2 isomer propargylene, HCCCH, combining velocity map imaging with nonadiabatic trajectory surface hopping calculations. Propargylene loses an H-atom, after laser excitation at around 250 nm, presumably to the T6 state. The photofragment angular distribution exhibits only a very small anisotropy, but the H-atom translational energy distribution extends to high energies and shows an expectation value of 〈fT〉, the fraction of excess energy released as translation, of 48%, outside the range expected for a statistical reaction mechanism. The computations suggest a predissociation in the T4-T6 state and lead to a translational energy distribution and photofragment angular distribution that match the experimentally observed ones very well.