PHOTODISSOCIATION AND PHOTODETACHMENT STUDIES OF TRANSITION METAL CLUSTER IONS

[摘要] A cold cluster ion source has been developed within our lab. This source is capable of generating both positive and negative cluster ions which are cooled in the supersonic expansion and suitable for spectroscopic studies. A tandem time-of-flight apparatus was built for mass selecting a particular cluster ion and then performing a second mass analysis after it has been hit by laser photons. Using these techniques, photodissociation and photodetachment studies have been performed on the transition metal cluster ions with and without chemisorbed species attached. The binding energies and electron affinities of these clusters were measured as a function of cluster size and composition.Laser photodetachment studies have been performed on copper, silver and some other metal cluster negative ions. The electron affinity of copper clusters shows a sharp even/odd alternation superimposed on a steady monotonic increase with cluster size. Of the various netative cluster ions studied, only Ag(,x)('-) displayed photodissociation process as well as photodetachment.The photodissociation process of metal cluster positive ions with and without ligands attached has been studied to determine binding energies. The binding energies of bare cobalt and niobium positive cluster ions are found to be roughly constant with cluster size. By monitoring fragment yield as a function of dissociation laser wavelength, well resolved vibronic bands of some dimer cations have been recorded. For almost all transition metal clusters measured (Fe(,x), Co(,x), Ni(,x), Pd(,x), V(,x), Nb(,x) and Ta(,x) with x between 2 and 20), the primary one-photon fragmentation pathway is always found to be loss of a single metal atom from the cluster. Cluster cations with CO or N(,2) adsorbed have been studied. The binding energies of Nb(,x)CO('+) and Nb(,x)N(,2)('+) display the drastically cluster size dependence. Clusters with multiply CO and N(,2) adsorbed show much lower binding energies. These photodissociation results suggest that d-orbitals play an important role in metal-metal and metal-ligand bondings.

[发布机构] Rice University

[效力级别] chemistry