ELECTRONIC EXCITATION IN LOW-ENERGY COLLISIONS - A STUDY OF THE COLLISION-INDUCED DISSOCIATION OF NITROMETHANE ION BY CROSSED-BEAM TANDEM MASS-SPECTROMETRY

The dynamics of collision-induced dissociation (CID) of nitromethane ion have been investigated with an angle-resolved crossed-beam tandem mass spectrometer. Kinetic energy and angular distributions of two main products of the dissociation, NO+ and NO2+, were measured at center-of-mass (CM) collision energies from 1.5 to 119 eV. Impulsive mechanisms are involved for NO2+ formation at all energies investigated. At and below 3-eV collision energy, a rebound reaction mechanism is observed, implying small impact parameter collisions dominate the CID process. A transition from backward to forward scattering is observed for NO2+ in the collision energy range of 3-6 eV. At higher energies, NO2+ formation is progressively more forward scattered but never approaches zero scattering angle within the energy range studied. The mechanism for forming NO+ is uniquely different from NO2+ in that two intensity maxima are observed at all energies, suggesting that the dissociation proceeds via at least two reaction pathways. The peak maximum at 0-degrees is attributed to dissociation following isomerization of nitromethane ion to methyl nitrite ion while the back-scattered peak follows an impulsive excitation reaction path similar to NO2+ formation and is attributed to nonisomerized nitromethane cations that require small impact parameter collisions to induce isomerization and dissociation. As the collision energy is increased, a highly endothermic dissociation process involving the transfer of 5.5-5.7-eV energy from translational to internal energy is also observed. Since the threshold for NO+ formation is only 0.64 eV, it is suggested that these low-energy collisions lead to electronic excitation of nitromethane ion to the sixth ionization band of its photoelectron spectrum and that dissociation proceeds on the excited-state hypersurface.