Negative Ion Laser Photoelectron Spectroscopy of Mass Selected Small Metal Clusters.

Negative ion photoelectron spectroscopy is used to study various small metal clusters. In these experiments, a mass-selected beam of metal cluster anions is probed by continuous monochromatic laser radiation, and the kinetic energy distribution of photoelectrons detached from the negative ions is measured. The photoelectron spectra provide important information on electronic and vibrational structure, as well as metal bonding properties. The photoelectron spectra of Cu_sp {n}{-}, Ag_sp {n}{-} (n = 1-10), and Au _sp{n}{-} n = 1-5) are presented. Franck-Condon analyses of the ground state transitions (X ^1Sigma_sp {g}{+} >=ts X ^2Sigma_sp{u}{+}) of the three dimeric spectra yield the adiabatic electron affinities and molecular constants. The major feature in the Cu_sp{3}{ -} and Ag_sp{3}{ -}^ectra is assigned to the linear-linear transition from the anion ground state to a neutral excited state. The cluster detachment energies as a function of size are determined, and the size dependence upon the electron affinities and excited state energy levels are discussed. The photoelectron spectra of Ni_sp {2}{-}, Pd_sp {2}{-}, and Pt_sp {2}{-} exhibit high densities of low-lying electronic states. Electron affinities and spectroscopic parameters for the anion and neutral electronic ground states are determined. The bond strengths of Ni _sp{2}{-} and Pt_sp{2}{-} are slightly smaller than those of the neutral dimers, while the Pd_sp{2}{-} bond strength is more than twice that of Pd _2. The ground state configurations of Pd _2 and Pd_sp{2 }{-} are found to be different than those of nickel and palladium dimers. The anomalous behavior of Pd_2 is attributed to the unique stable ground state of Pd. Comparison between the nickel group and coinage metal dimers suggests little or no 3d electron contribution to the bonding in Ni_2 , and a substantial 5d contribution to bonding in Pt_2. From the photoelectron spectra of Sn_sp {2}{-}, Pb_sp {2}{-}, and SnPb^ {-}, adiabatic electron affinities and spectroscopic parameters for the anion ground states and numerous neutral electronic states are determined. The dissociation energies of the three anions are significantly larger than those of the neutrals. Angular distribution measurements show differences in psigma_ {rm g} and ppi_ {rm u} electron detachment. The ppi_{rm u} orbital is found to dominate the bonding for tin and lead dimers. The observed electronic bands have been assigned to low-lying electronic states.