UNIMOLECULAR DISSOCIATION OF TRIVALENT METAL CLUSTER IONS (AL-N(-N(+), IN-N(+)) - EVIDENCE FOR A TRANSITION FROM COVALENT TO METALLIC BONDING(), GA)

Unimolecular dissociation of aluminum, gallium and indium clusters is investigated. Small sizes dissociate into two channels: either the eva poration of a neutral or a charged monomer. Above a given size a,, onl y dissociation of a neutral atom subsists. The evaporation of a charge d monomer is characteristic of trivalent metal clusters and is consist ent with the size evolution of the ionization potential towards the at omic value. The experiments are interpreted in the framework of the st atistical R.R.K. model. For smaller sizes (n < n(c)), as two evaporati on processes are in competition, we have evaluated cluster relative di ssociation energies and ionization potentials. The competition between the two evaporation channels is well mirrored by the evolution of the ionization potentials independently measured by near-threshold photoi onization experiments. For gallium, our measurements have revealed tha t the covalent to metal transition occurs for larger sizes (n = 30-50 atoms) than for aluminum clusters.