Photodissociation dynamics and spectroscopy of free radical combustion intermediates

Abstract

The photodissociation spectroscopy and dynamics of free radicals is studied by the technique of fast beam photofragment translational spectroscopy. Photodetachment of internally cold, mass-selected negative ions produces a clean source of radicals, which are subsequently dissociated and detected. The photofragment yield as a function of photon energy is obtained, mapping out the dissociative and predissociative electronic states of the radical. In addition, the photodissociation dynamics, product branching ratios, and bond energies are probed at fned photon energies by measuring the translational energy, ET), and angular distribution of the recoiling fragments using a time-and positionsensitive detector. Ab initio calculations are combined with dynamical and statistical models to interpret the observed data.The photodissociation of three prototypical hydrocarbon combustion intermediates forms the core of this work. The methoxy radical (CH30), the vinoxy radical (CH2CHO) 2 and the ketenyl radical (HCCO), are representative examples of alkanoxy, alkenoxy, and alkynoxy radicals, respectively. The dominant channel in methoxy dissociation following ultraviolet excitation is CH30 + CH3 + 0. Vibrational structure is resolved in the P(&) distribution, and a new heat of formation is obtained for CH30. Two primary channels are active in vinoxy photodissociation: CH2CHO + CH3 + CO, and CH2CHO + CH2CO + H. The dissociation dynamics in vinoxy occur by internal conversion to the ground state followed by dissociation over a barrier to products. The ketenyl radical is observed for the first time in the ultraviolet, dissociating via two distinct channels: HCCO + CH X(213) + CO, and HCCO -+ CH u (~C -) + CO. The dynamics in this case occur by internal conversion and intersystem crossing, followed by dissociation on potential surfaces with negligible barriers to products. The branching ratio is a function of photon energy, and a heat of formation is determined for ketenyl.In addition, the photodissociation of a negative ion radical, N202-, is also explored.This ion photodissociates via two channels: N20F + 0-+ N20, and N20; + NO-+ NO.Product vibrational structure is observed in the N20 bending mode, and a heat of formation is obtained for N202-. However, even in the relatively well-defined limit of a crossed molecular beam experiment, in which the reactants approach and the products are detected along defined trajectories, the results are inherently averaged over a range of impact parameters describing the difference between head-on and glancing collisions of the reactants. One way to avoid averaging over the impact parameter in the bimolecular "full collision" described above is to simpllfv the system by studying the unimolecular "half collision" of Is there more than one product channel participating?What is the bond strength, of AHm for each channel?How fast ,does dissociation take place?How is the excess energy distributed among the products?Can we determine the dissociation mechanism from this information?For the s...