Multiphoton ionisation and dissociation of NO2 by 50 fs laser pulses

Multiphoton ionisation and dissociation of NO2 has been studied experimentally at 375 nm for laser pulse widths of 10 ns and 50 fs. The parent NO2 ion peak is not seen in the ns data. In all spectra, the main peak observed is due to the ionisation of the NO molecule which results from the dissociation of excited NO2 formed after absorbing a 375 nm photon. The intensity dependencies of both NO and NO2 ion peaks have also been measured. The data has been analysed within the context of a rate equation model using published cross-sections and dissociation rates except for the two-photon ionisation cross-section for NO2 which was chosen to reproduce the NO2/NO ion signal ratios at 50 fs. The rate equation model provides a good description of the complete set of data. Indirectly, it may be concluded that coherence effects do not play an important role in the multiphoton excitation/ionisation of NO2. The data also rules out the importance of above-ionisation dissociation in NO2 - a conclusion which is consistent with previous data at 496 and 248 nm for laser pulse widths greater than or equal to 300 fs.