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Delayed pulsed field threshold ionization of clusters excited to high long-lived Rydberg states is used to study their dissociation behavior. Benzene–Ar and benzene–Kr dimers are excited by resonance enhanced two-photon ionization to Rydberg levels close to various ionization thresholds. The field ionized threshold ions are monitored and separated from the non-energy-selected ions in a reflecting field mass spectrometer with high mass resolution. The appearance of threshold ions at the daughter ion mass indicates the onset of a dissociation process. Daughter ions are first observed for the 16161(3/2) level of the two investigated dimers. This leads to an upper limit of the dissociation energy of benzene–Ar of 340 cm−1 which is probably higher than the true dissociation energy. For the first time threshold ions are observed for large internal energies of some 5 eV in the core indicating that high Rydberg states maintain their long lifetime even if the core is electronically or vibrationally excited by several eV.

High-resolution state-selected ion-molecule reaction studies using pulsed field ionization photoelectronsecondary ion coincidence method Rev. Sci. Instrum. 74, 4096 (2003); Absorption spectroscopy of massselected ions in neon matrices

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.