Multiphoton mass spectrometry of clusters: Dissociation pathways and energetics of heterogeneous van der Waals cluster ions

Abstract

In this work the metastable decay of heterogeneous benzene/toluene, benzene/cyclohexane, and benzene/para-difluorobenzene van der Waals cluster ions [(MiNk)+ with n=i+k<12] is investigated. Two-photon ionization leads to an internal energy of less than 1.5 eV. On the 100 μs time scale of the linear reflectron time-of-flight mass spectrometer used in this work, the main dissociation process of the cluster ions is the evaporation of a single molecule. A uniform decay behavior is observed for benzene/toluene and benzene/cyclohexane cluster ions; the only dissociation pathway being the evaporation of a single benzene or cyclohexane molecule, respectively. By contrast, the evaporation of either a benzene or a para-difluorobenzene molecule is observed for benzene/para-difluorobenzene cluster ions, depending on the size and the composition of the heterocluster. We apply an energetic model to explain the observed dissociation pathways. This model assumes the total miscibility of the different molecular species and a total dilution of charge. Despite these assumptions, the model predictions are in good agreement with our experimental data.