The dynamics of oxygen atom formation in the UV photodissociation of nitromethane Original Research Article

The dynamics of oxygen atom formation in the gas-phase photolysis of nitromethane (CH3NO2) was studied using the pulsed laser photolysis/laser-induced fluorescence (LIF) pump-and-probe technique. Room-temperature CH3NO2 molecules were excited at two UV photodissociation laser wavelengths of 248 and 266 nm. Nascent O(3P) photofragments were detected via LIF in the vacuum ultraviolet spectral region under collision-free conditions. Narrow-band probe laser light, tunable over the wavelength range 130.2–130.6 nm, was used to monitor the fine-structure state distribution of nascent O(3PJ=2,1,0) atom product. From Doppler profiles of the O atom, the fraction of the total available energy channeled into product translational energy was determined to be 〈fT〉=0.28±0.02 at 248 nm and 〈fT〉=0.23±0.04 at 266 nm. These fT values are considerably lower than the value of 〈fT〉=0.63 obtained by dynamical simulation of the soft impulsive model for single N–O bond cleavage. The population ratio of the three fine-structure states of the oxygen atoms was found to be close to the statistical ratio at both photolysis wavelengths. The product fine-structure state population distribution measured for the O atoms and the 〈fT〉 values indicate that at both photodissociation wavelengths the O atoms are produced mainly via an indirect predissociation mechanism, but at 248 nm there is an additional contribution from a direct predissociation mechanism. The absolute quantum yields for O(3P) atom formation were φ(O)=0.18±0.03 at 248 nm and φ(O)=0.13±0.04 at 266 nm; these values were obtained using a photolytic calibration method that employed NO2 photodissociation as a reference source of well-defined O atom concentration.