Time-of-flight mass spectrometry study of the fragmentation of valence shell ionised nitrobenzene

A time-of-flight mass spectrometry study has been carried out to investigate the fragmentation processes occurring in nitrobenzene as a result of valence shell photoionisation. Synchrotron radiation has been used to record spectra in the photon energy range 11-31 eV, and appearance energies have been determined for 18 fragment ions. These have enabled, previously unknown, heats of formation to be estimated for C₅H₂⁺ and C₆H₃⁺. The fragment ion appearance energies have been compared to similar data for benzene and toluene in order to highlight the influence of the substituent on the fragmentation patterns. The time-of-flight spectra show that the peak associated with the NO⁺ fragment changes shape as a function of excitation energy, and at high photon energy the peak consists of two components, one of which is narrow and the other broad. The latter component is due to fragments possessing substantial initial kinetic energy. In contrast, the peak associated with the NO₂⁺ fragment always appears broad. This behaviour is discussed in relation to the initial formation of a doubly charged ion and a subsequent coulomb repulsion. In a separate experiment the absolute photoabsorption cross section of nitrobenzene has been measured between the ionisation threshold and 35 eV using a double ion chamber. Some of the broad features have been attributed, tentatively, to valence shell excitations into p* orbitals. © 2001 Elsevier Science B.V.