DECAY ENERGETICS OF MOLECULAR CLUSTERS STUDIED BY MULTIPHOTON MASS-SPECTROMETRY AND PULSED-FIELD THRESHOLD IONIZATION

New developments in time-of-flight mass spectrometry combined with multi-photon ionization provide hitherto unachievable information on the decay of weakly bound cluster ions. Dissociation thresholds of cluster ions are important for the understanding of their binding and structure. We present two experimental methods that allow the determination of dissociation energies of weakly bound ionic and neutral clusters. Besides the breakdown technique observing the metastable ion efficiency curves, a novel method is presented and described which is based on delayed pulsed field ionization of long-lived Rydberg states close to the ionization energy. In this way state-selected benzene-noble gas ions are produced and their decay is observed in a reflectron mass spectrometer. Results for dissociation thresholds of various molecular dimers and larger complexes of aromatic molecules are presented. The pronounced differences in dissociation energy between the neutrals and the ions are interpreted in terms of charge-transfer resonance interaction in the ionic complex leading to structural changes after the ionization process.