OBSERVATION OF SMALL DOUBLY CHARGED NIOBIUM CLUSTERS

Small doubly charged niobium clusters are observed in a molecular beam emerging from a laser vaporization/supersonic expansion source if electron impact ionization is utilized in the expansion region. Doubly charged clusters from n = 2 to n = 15 are readily detected. The doubly charged dimer cation has the same charge to mass ratio as the very abundant singly charged monomer. However, the doubly charged dimer can be detected with collision-induced charge exchange reactions and collision-induced dissociation. The kinetic energy released into the fragment ions of the cluster decay process allows us to estimate the charge separation in the parent (4.3 angstrom for the dimer and 5.7 angstrom for the trimer). The results strongly imply that predictions of the stability of doubly charged clusters on the basis of binding energy of the neutral and Coulomb repulsion energy alone do not suffice and that particular bonding conditions can yield metastable states with substantial lifetimes.