LASER-INDUCED DISSOCIATION, IONIZATION AND FRAGMENTATION PROCESSES IN NITROAROMATIC MOLECULES

Laser multiphoton ionisation and fragmentation processes have been studied in many molecular species, including a large number of substituted aromatic compounds. Recent studies in this laboratory on nitrobenzene have yielded significant data on the wavelength dependence of laser-induced ionisation and fragmentation processes: in particular, the verv striking behaviour of the NO fragment. In this paper we extend the wavelength range to study nitrobenzene at shorter wavelengths and also investigate another nitroaromatic compound, o-nitrotoluene, where similar behaviour is observed. We also report laser ionisation of nitric oxide (NO) and nitrogen dioxide (NO2) over the same wavelength region. Comparison of the various spectra suggests that the nitroaromatic parent molecules predissociate releasing NO2, which in turn predissociates to form NO before going through a multiphoton ionisation process. The hydrocarbon components of the o-nitrotoluene molecule also ionise, but there is no evidence of distinctive resonance enhancement of any of these fragment ions in the wavelength range studied.