RESUMO
We present a photoelectron imaging study of the small sodium cluster anions Na3(-), Na5(-), and Na7(-) at photon energies in the visible and near UV range (hv = 1.64-4.28 eV). The resulting angular distributions are remarkably diverse and exhibit a strong dependence on photon energy; only for hv > 3.5 eV do they evolve into more uniform distributions peaked in the direction of the laser polarization. We show that different energy dependencies of the distributions are related to different angular-momentum characters of the bound states. The jellium s-like character of the lowest single-particle states results in photoelectron emission parallel to the laser polarization at all photon energies, whereas the p-like character of the higher states leads to essentially isotropic distributions at threshold and a strong variation with photon energy. Close to the detachment threshold, the asymptotic angular distributions are attributed to the approximate validity of Wigner's law, which states that the spectrum is dominated by the partial wave with the smallest angular momentum. For the planar cluster Na5(-), we observe characteristically different behavior for electrons detached from the two in-plane p-like states, and we show how this correlates with the molecular symmetry. Our results indicate that a simple jellium-like description of the molecular orbitals is appropriate for the three-dimensional cluster Na7(-), despite the energetic splitting of the normally triply degenerate 1p level.
RESUMO
Photoelectron spectra of low temperature silicon doped gold cluster anions Au(n)Si(-) with n = 2-56 and silver cluster anions Ag(n)Si(-) with n = 5-82 have been measured. Comparing the spectra as well as the general size dependence of the electron detachment energies to the results on undoped clusters shows that the silicon atom changes the apparent free electron count in the clusters. In the case of larger gold clusters (with more than about 30 gold atoms) the silicon atom seems to consistently delocalize all of its four valence electrons, while in the case of the silver clusters a less uniform behavior is observed. Here the silicon atoms act partly as electron donors, partly as electron acceptors, without following an obvious simple principle. Additionally some structural information can be obtained from the measured spectra: while Ag(54)Si(-) seems to adopt an icosahedral structural motif, Au(54)Si(-) seems to take on a low symmetry structure, much like the corresponding pure 55 atom clusters. This indicates that for such larger clusters the incorporation of a single silicon atom does not change the ground state geometry significantly.
