A model for multiphoton ionisation mass spectroscopy with application to benzene

In previous model calculations for multiphoton dissociation (MPD) either no assumption is made about the dynamics of the MPD process or it is assumed that first energy is deposited by the laser solely in parent ions which then, depending on their energy, partition into products. No products are produced during the laser pulse. In the present approach competition between the unimolecular reaction and photon-absorption during the light pulse was taken as a new ansatz in modelling multiphoton dissociation mass spectra. We treat the case that the unimolecuiar reaction rates will become so fast with increasing excitation energy that the competition for further photon-absorption stops at a certain amount of excitation energy thus leading to a cut-off in the excitation ladder and to a switch to a new ladder of product ions for further photon-absorption. The effectiveness of primary to secondary ion conversion and the influence on fragmentation pathways are discussed qualitatively in comparison with other statistical models which involve a separate treatment of excitation and fragmentation. The model is applied to calculate the light intensity dependence of the benzene (MPD) mass spectrum at 2590 Å laser wavelength. RRKM calculations for benzene and its products are performed in order to check the ladder switching assumption. Theoretically computed MPD mass spectra agree well with the experimental ones.