Polarized emission spectroscopy of photodissociating nitromethane at 200 and 218 nm

Abstract

We report the polarized emission spectra from photodissociating nitromethane excited at 200 and 218 nm. At both excitation wavelengths, the emission spectra show a strong progression in the NO2 symmetric stretch; at 200 nm a weak progression in the NO2 symmetric stretch in combination with one quantum in the C–N stretch also contributes to the spectra. We measure the angular distribution of emitted photons in the strong emission features from the relative intensity ratio between photons detected perpendicular to versus along the direction of the electric vector of the excitation laser. We find the anisotropy is substantially reduced from the 2:1 ratio expected for the pure CH3NO2 X(1A1)→1B2(ππ*)→X(1A1) transition with no rotation of the molecular frame. The intensity ratios for the features in the NO2 symmetric stretching progression lie near 1.5 to 1.6 for 200 nm excitation and 1.7 for 218 nm excitation. The analysis of the photon angular distribution measurements and consideration of the absorption spectrum indicate that the timescale of the dissociation is too fast for molecular rotation to contribute significantly to the observed reduction in anisotropy. The detailed analysis of our results in conjunction with electron correlation arguments and previous work on the absorption spectroscopy and final products’ velocities results in a model which includes two dissociation pathways for nitromethane, an electronic predissociation pathway and a vibrational predissociation pathway along the 1B2(ππ*) surface. Our analysis suggests a reassignment of the minor dissociation channel, first evidenced in photofragment velocity analysis experiments which detected a pathway producing slow CH3 fragments, to the near threshold dissociation channel CH3 + NO2(2 2B2).