Mechanism of resonant multiphoton ionization dissociation of p-xylene

The mechanism of resonant multiphoton ionization dissociation (RMPID) of p-xylene is investigated theoretically based both on the quantum-mechanical MO calculation of the geometrical structures and dissociation energies of the fragments and on the computation of laser power-dependent mass spectra of the fragments produced by RMPID. The geometries and dissociation energies are calculated by using both the MNDO method and the ab initio method with the 6-31G basis set. The computation of the mass spectra is carried out in the absorption multiple fragmentation model. It is shown theoretically that the two-independent reaction sequence mechanism proposed by Takenoshita et al. upon measuring the laser power dependence can explain semi-quantitatively the mass spectra of the RMPID. From comparison of the measured mass spectra with the calculated ones it is suggested that the absorption multiple fragmentation model originally based on the product phase space theory can be applied to the RMPID involving transition states in the course of reactions such as the retro-Diels-Alder reaction C₅H⁺₅ → C₃H⁺₃ + C₂H₂ by taking into account the corresponding activation energy instead of the dissociation energy.