The dynamics of oxygen atom formation in the UV photodissociation of nitromethane

The dynamics of oxygen atom formation in the gas-phase photolysis of nitrom ethane (CH3NO2) was studied using the pulsed laser photolysis/laser-induced fluorescence (LIF) pump-and-probe technique. Room-temperature CH,NO molecu les were excited at two UV photodissociation laser wavelengths of 248 and 2 66 nm. Nascent O(P-3) photo-fragments were detected via LIF in the vacuum u ltraviolet spectral region under collision-free conditions. Narrow-band pro be laser light, tunable over the wavelength range 130.2-130.6 nm, was used to monitor the fine-structure state distribution of nascent O(P-3(J=2,1,0)) atom product. From Doppler profiles of the O atom, the fraction of the tot al available energy channeled into product translational energy was determi ned to be <f(T)> = 0.28 +/- 0.02 at 248 nm and <f(T)> = 0.23 +/- 0.04 at 26 6 nm. These f(T) values are considerably lower than the value of <f(T)> = 0 .63 Obtained by dynamical simulation of the soft impulsive model for single N-O bond cleavage. The population ratio of the three finestructure states of the oxygen atoms was found to be close to the statistical ratio at both photolysis wavelengths. The product fine-structure state population distrib ution measured for the O atoms and the <f(T)> values indicate that at both photodissociation wavelengths the 0 atoms are produced mainly via an indire ct predissociation mechanism, but at 248 nm there is an additional contribu tion from a direct predissociation mechanism. The absolute quantum yields f or O(P-3) atom formation were phi (O) = 0.18 +/- 0.03 at 248 nm and phi (O) = 0.13 +/- 0.04 at 266 nm; these values were obtained using a photolytic c alibration method that employed NO: photodissociation as a reference source of well-defined O atom concentration. (C) 2001 Elsevier Science B.V. All r ights reserved.