Pathways and kinetic energy disposal in the photodissociation of nitrobenzene

Abstract

Vacuum-ultraviolet photoionization molecular-beam mass spectrometry is a means of identifying primary photodissociation products and determining their recoil energies. At several photolysis wavelengths between 220 and 320 nm, we have observed three primary photodissociation pathways for nitrobenzene. Two of the pathways are C6H5NO2 →C6H5+NO2 and C6H5NO2→C6H5NO+O. The third pathway produces NO by one or both of the processes C6H5NO2→C6H5O+NO and C6H5NO2→C5H5+CO+NO. The relative yield of the pathways producing NO2 and NO varies strongly with the photolysis wavelength. The production of NO2 exceeds that of NO by about 50% for the 280 nm photolysis, but increases to almost a sixfold excess in 222 nm dissociation. The second pathway has a threshold energy that is about 0.50 eV greater than the thermodynamic limit for the formation of nitrosobenzene (C6H5NO) and an oxygen atom from nitrobenzene, probably reflecting the energy required to produce triplet nitrosobenzene and, perhaps, a barrier to dissociation on the triplet surface. The distribution in arrival times for a fragment provides an estimate of the recoil energy at each photolysis wavelength in these experiments. The channel producing nitric oxide (NO) radicals releases a relatively large amount of kinetic energy. Assuming the channel producing nitric oxide (NO) also produces phenoxy (C6H5O), we calculate a linear increase in kinetic energy from 0.29 eV at 320 nm to 1.1 eV at 220 nm. By contrast, the other two channels release only a small amount of kinetic energy (≊0.1 eV) at all wavelengths. An impulsive model does not describe the observed kinetic energy release for these low energy channels, suggesting that the energy release is more nearly statistical. The recoil energy predicted by an impulsive model for the channel producing nitric oxide and phenoxy radicals is closer to the observed kinetic energy release.