UNIMOLECULAR FRAGMENTATION KINETICS AND BINDING-ENERGIES OF MASS-SELECTED BENZENE CLUSTER IONS

In this work the unimolecular decay of cluster ions is investigated in order to deduce energetic and structural information on clusters as a function of size. The dissociation behavior of benzene cluster ions (C6H6)n+ with n = 4 to n = 200 is observed after multiphoton ionization of a neutral benzene cluster beam within a reflectron mass spectrometer. With a pulsed deflection technique of undesired ions the metastable decay of mass-selected cluster sizes within the drift region was observed. The metastable decay efficiency is monotonically increasing in this mass range but pronounced anomalies are found at n = 14, 20, 24, and 27. Using an evaporative ensemble ansatz and an RRKM model with a phase space restricted to the intermolecular cluster modes we were able to determine the dissociation energies for clusters up to n = 23 from a fit of the calculated metastable decay ratios to the measured ones. For this fit the size dependent sequential evaporation of two benzene molecules is important. The dissociation energy is largest for the dimer cation and found to decrease for clusters up to n = 5. For larger clusters it begins to increase,with a step-like behavior after n = 14 and n = 20. This is interpreted in terms of shell filling achieved for icosahedral structures about a center dimer cation.