Fragmentation of three isotopic toluene monocations in the 15-100 eV photon energy range

The dissociative photoionization of toluene-h(8), toluene-alpha-d(3) and to luene-alpha-d(8) was studied within the 15-100 eV photon energy range using monochromatized synchrotron radiation as excitation source and a reflectro n time-of-flight mass spectrometer in both electrostatic mirror and linear modes. Peak profile analyses of the linear mode spectra provided informatio n on the kinetic energy release and, in some cases, the percentage of mono- to dication dissociations leading to the formation of particular ion fragm ents. Supporting data from photoelectron-photoion-photoion coincidence meas urements on toluene-h(8) at 100 eV were used to detail the contributing dic ation charge separation processes at high excitation energies. Fragment ion formation threshold energies, obtained from photoion mass spectrometry mea surements, were used to determine possible fragmentation pathways on the ba sis of reaction thermochemistry. Where competitive reaction channels are po ssible, use was made of energy propensity rules, involving product ionizati on potentials and proton affinities, to propose favored pathways. Previousl y limited to a maximum of 20 eV photon energy and to studies of only two fr agments, C7H7+ and C5H5+ our results extend the data on dissociative photoi onization processes of toluene to 100 eV and to > 40 observed ion fragments . Analysis has been made of the variation in the energy deposited in the pa rent ion, its distribution among molecular orbitals and its subsequent effe ct on fragmentation processes, asa function of photon excitation energy, us ing ionization partial cross-sections deduced from toluene photoelectron sp ectra. Particular attention has been paid to isotopic effects revealing iso merization processes and other atom labilities in the parent toluene ion an d in C7H7+, C6H5+ and C5H5+ fragment ion formation. The pure C-n(+) carbon ions, C+, C-3(+), C-4(+), C-5(+) and C-7(+), are observed at high excitatio n energies and it is shown that they may derive from CnH+ precursors as wel l as from dication charge separation processes. In an Appendix, the photofr agment ion yields are compared with the ion yields obtained in collisional activation (CA) experiments, in particular by Kim and McLafferty, in which the primary toluene ion is formed by electron impact. A number of similarit ies and differences to the CA yields obtained at two different ion accelera tion potentials are discussed in terms of differences in energy deposition in the parent ion by the photon excitation and CA techniques. (C) 1999 Publ ished by Elsevier Science B.V. All rights reserved.