Unimolecular dissociation of trivalent metal cluster ions: The size evolution of metallic bonding

The unimolecular decomposition of size selected cluster cations of trivalent metals (Aln+, Gan+, and Inn+), induced by high fluence laser ionization, has been investigated in the n=7 to n=85, 55, and 75 size ranges, respectively. This method is applied for the first time to photoexcited trivalent clusters generated in an evaporative ensemble and the experimental data cover a size range that was not explored in previous pioneering experiments on their dynamics. Small clusters dissociate through the loss of a neutral or a charged atom whereas clusters larger than a well defined critical size merely dissociate through the first channel. In the framework of the RRK statistical theory, the measured evaporation rates provide some information about the size evolution of the cluster dissociation energies and their ionization potentials in the low size range. The competition between the ion and the atom evaporation is found to be consistent with the size evolution of the ionization potentials independently measured by direct photoionization. The agreement between theory and experiment is discussed in relation to cluster structure, especially in the case of gallium.