Photofragmentation of nitromethane in a molecular beam at 193 nm

We report the cross section for dissociation of nitromethane at 193 nm. It is (1.7±1.0)×10−17 cm2 measured relative to that for H 79Br. From this, we infer that the quantum yield for the process is nearly unity. Photofragments of the dissociation were observed at masses 15, 16, and 30 corresponding to CH3, O, and NO, but only a very small amount of fragments at mass 46 (NO2) was measured. Time-of-flight spectra were obtained for all fragments except for mass 46. Our data is explained by assuming that two sequential absorptions of photons occur. The first photon dissociates CH3NO2 to CH3 and NO2. The second photon is absorbed by the NO2 fragment to give NO+O. About 30% of the NO2 participate in the second absorption and the rate determining step is the primary absorption. The primary absorption fragments are characterized by having a most probable kinetic energy of 7% of the maximum available energy with an average of 19%. The remainder of this energy appears as internal energy of the fragments which can include electronic excitation of NO2 to the A 2B2 state or even to further dissociation of NO2 to NO+O.