Fragmentation of three isotopic toluene monocations in the 15–100 eV photon energy range Original Research Article

The dissociative photoionization of toluene-h8, toluene-α-d3 and toluene-d8 was studied within the 15–100 eV photon energy range using monochromatized synchrotron radiation as excitation source and a reflectron time-of-flight mass spectrometer in both electrostatic mirror and linear modes. Peak profile analyses of the linear mode spectra provided information on the kinetic energy release and, in some cases, the percentage of mono- to dication dissociations leading to the formation of particular ion fragments. Supporting data from photoelectron–photoion–photoion coincidence measurements on toluene-h8 at 100 eV were used to detail the contributing dication charge separation processes at high excitation energies. Fragment ion formation threshold energies, obtained from photoion mass spectrometry measurements, were used to determine possible fragmentation pathways on the basis of reaction thermochemistry. Where competitive reaction channels are possible, use was made of energy propensity rules, involving product ionization potentials and proton affinities, to propose favored pathways. Previously limited to a maximum of 20 eV photon energy and to studies of only two fragments, C7H7+ and C5H5+, our results extend the data on dissociative photoionization 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 parent ion, its distribution among molecular orbitals and its subsequent effect on fragmentation processes, as a function of photon excitation energy, using ionization partial cross-sections deduced from toluene photoelectron spectra. Particular attention has been paid to isotopic effects revealing isomerization processes and other atom labilities in the parent toluene ion and in C7H7+, C6H5+ and C5H5+ fragment ion formation. The pure Cn+ carbon ions, C+, C3+, C4+, C5+ and C7+, are observed at high excitation energies and it is shown that they may derive from CnH+ precursors as well as from dication charge separation processes. In an Appendix, the photofragment 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 similarities and differences to the CA yields obtained at two different ion acceleration potentials are discussed in terms of differences in energy deposition in the parent ion by the photon excitation and CA techniques.