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 investigat e the fragmentation processes occurring in nitrobenzene as a result of vale nce shell photoionisation. Synchrotron radiation has been used to record sp ectra in the photon energy range 11-31 eV, and appearance energies have bee n determined for 18 fragment ions. These have enabled, previously unknown, heats of formation to be estimated for C5H2+ and C6H3+. The fragment ion ap pearance energies have been compared to similar data for benzene and toluen e in order to highlight the influence of the substituent on the fragmentati on 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 h igh photon energy the peak consists of two components, one of which is narr ow and the other broad. The latter component is due to fragments possessing substantial initial kinetic energy. In contrast, the peak associated with the NO+ fragment always appears broad. This behaviour is discussed in relat ion to the initial formation of a doubly charged ion and a subsequent coulo mb repulsion. In a separate experiment the absolute photoabsorption cross s ection of nitrobenzene has been measured between the ionisation threshold a nd 35 eV using a double ion chamber. Some of the broad features have been a ttributed, tentatively, to valence shell excitations into pi* orbitals. (C) 2001 Elsevier Science B.V.