Multiphoton ionization and dissociation of mixed van der Waals clusters in a linear reflectron time-of-flight mass spectrometer

Slow metastable fragmentation of benzene/toluene and benzene/para-difluorobenzene clusters is observed in a newly developed linear reflectron time-of-flight mass spectrometer after two-photon ionization. The breakdown of the metastable intensity with decreasing two-photon energy is measured and yields the appearance potential for the main dissociation channels of the homo- and the heterodimers. Based on these values, the dissociation energies of the neutral dimers are obtained and shown to be consistent with the changes of the polarizability and dipole moment of the components. In addition, from the appearance potentials and the measured ionization potentials, the dissociation energies of the charged dimer clusters are found. The binding energies of the heterocluster ions and the para-difluorobenzene homodimer ion are smaller than the respective binding energies in the benzene and toluene homodimer ions. This is explained by a larger contribution of charge transfer resonance interaction to the binding energy of the latter homodimer ions. On the basis of these results we present an energetic scheme for prediction of the dissociation pathways of the trimer ions in agreement with the measured results.