The photodissociation of several combustion relevant radical molecules has been carried out. These species behave significantly different from closed shell species and the experiments presented here provide fundamental new insights into the chemical dynamics of free radicals. The experiments were carried out on a molecular beam photofragment translational spectroscopy instrument. Radicals were generated by flash pyrolysis of a suitable precursor and entrained in a pulsed molecular beam. The radicals were then photodissociated and the products were detected with a rotatable mass spectrometer based on electron impact ionization. The benzyl radical photodissociation and the cyclopentadienyl radical photodissociation were investigated at 248 nm. Two dissociation channels were observed for each radical. The benzyl radical dissociates into H + C7H6 and CH3 + C6H4 and the cyclopentadienyl radical dissociates to H + C5H4 and C3H3 + C2H2, where the C5H4 fragment was identified as ethynylallene by its ionization energy. The translational energy distribution determined for each channel suggests that every dissociation mechanism occurs via internal conversion to the ground state followed by intramolecular vibrational redistribution and dissociation. The branching ratio between the two competing channels for each radical was measured experimentally and compared to RRKM (Rice-Ramspberger-Kassel-Marcus) calculations. For both radicals, the dominant experimental channel was corroborated by RRKM calculations.

The main goal of this research program was to obtain accurate thermochemical and spectroscopic data, such as ionization energies (IEs), 0 K bond dissociation energies, 0 K heats of formation, and spectroscopic constants for radicals and molecules and their ions of relevance to combustion chemistry. Two unique, generally applicable vacuum ultraviolet (VUV) laser photoion-photoelectron apparatuses have been developed in our group, which have used for high-resolution photoionization, photoelectron, and photodissociation studies for many small molecules of combustion relevance.

Product imaging has been used to investigate several processes important to a fundamental understanding of combustion. The imaging technique produces a "snapshot" of the three-dimensional velocity distribution of a state-selected reaction product. Research in three main areas is planned or underway. First, product imaging has been used to investigate the reactive scattering of radicals or atoms with species important in combustion. These experiments, while more difficult than studies of inelastic scattering or photodissociation, are now becoming feasible. They provide both product distributions of important processes as well as angular information important to the interpretation of reaction mechanisms. Second, the imaging technique has been used to measure rotationally inelastic energy transfer on collision of closed-shell species with important combustion radicals. Such measurements improve our knowledge of intramolecular potentials and provide important tests of ab initio calculations. Finally, experiments using product imaging have explored the vacuum ultraviolet photodissociation of O2, N2O, SO2, CO2 and other important species. Little is known about the highly excited electronic states of these molecules and, in particular, how they dissociate. These studies provide product vibrational energy distributions as well as angular information that can aid in understanding the symmetry and crossings among the excited electronic states.

The kinetics of several free radical-radical reactions was studied using pulsed laser photolysis coupled to transient UV-vis absorption spectroscopy over the 292 – 714 K temperature range and the 1- 100 bar pressure range. Free radicals such as CH3, OH and Cl are generated by photodissociation of parent molecules; some other radicals such as CH3O2 and HO2 are generated in secondary reactions following the initial photodissociation. Different free radicals are monitored at different wavelengths depending on their UV absorption spectra. Quantitative measurements of the concentrations of free radicals relay upon the absolute intensity of photolysis laser light determined by well characterized in situ actinometry. Experimental temporal absorption profiles are fitted by numerical solutions of a system of differential equations (ODE) which correspond to the reaction mechanism.