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Photoelectron spectra of Ag n Ϫ clusters with nϭ1-21 recorded at different photon energies ͑hϭ4.025, 4.66, 5.0, and 6.424 eV͒ are presented. Various features in the spectra of Ag 2 Ϫ-Ag 9 Ϫ can be assigned to electronic transitions predicted from quantum chemical ab initio calculations. While this comparison with the quantum chemical calculations yields a detailed and quantitative understanding of the electronic structure of each individual cluster, a discussion in terms of the shell model is able to explain trends and dominant patterns in the entire series of spectra up to Ag 21 Ϫ .

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Photoelectron spectra of mass-separated Nb n Ϫ clusters reveal an even/odd alternation for nϭ6-17, indicating a closed electronic shell of the neutral even-numbered clusters. The HOMO-LUMO gap of Nb 8 , Nb 10 , and Nb 16 is found to be larger than that of the other even-numbered clusters, which correlates with the low H 2 reactivities of these species. The spectrum of Nb 15 Ϫ is different from all other clusters in this size range, which might be an indication for a geometric bcc shell closing. The influen e of the electronic structure of the clusters on the reactivity is discussed.

Photoelectron spectroscopy measurements and density functional theory calculations are combined to determine structures of Nb 2 n (n 3 8) clusters. A detailed comparison between observed and calculated electronic binding energies shows that the clusters have low-symmetry compact 3D structures and the lowest possible total spin, except for the three-and five-ato clusters which are in triplet states. We fin evidence for the coexistence of two isomers of Nb 2 8 under some experimental conditions. This approach shows great promise for structural characterization of small clusters.

Zero kinetic energy (ZEKE) photoelectron spectroscopy of ammonia by nonresonant twophoton ionization from the neutral ground state J. Chem. Phys. 98, 8462 (1993); 10.1063/1.464505 Resonant twophoton ionization spectroscopy of jetcooled Au3 J. Chem. Phys. 95, 8779 (1991); 10.1063/1.461213 Resonant twophoton ionizationphotoelectron spectroscopy of Cu2: Autoionization dynamics and Cu+ 2 vibronic states J. Chem. Phys. 91, 3854 (1989); 10.1063/1.456870Multiphoton ionization and twophoton fluorescence excitation spectroscopy of triethylenediamine Photoelectron spectra of Au; with n=2-4 are reported. Due to the relatively high photon energy used in our experiment (h v= 6.424 e V) and the energy resolution of about 50 me V, various transitions into excited states of the neutral clusters are resolved. It is demonstrated that photoelectron spectra can serve as a map of the electronic states of a cluster, while the high resolution of the resonant two-photon ionization (R2PI) method gains information about the symmetry of the states. The comparison with similar data of Ag; clusters indicates the influence of relativistic effects and the large spin-orbit splitting for Au.

We have produced an endohedrally doped fullerene that shows a metal-like density of states at the Fermi level. Individual La@C 60 clusters deposited onto graphite exhibit a zero band gap as observed by scanning tunneling spectroscopy on single clusters at room temperature. Moreover, we find that an isolated La@C 60 cluster on graphite shows a reversible opening of a band gap at a transition temperature of ϳ28 K. The transition is associated with a freezing of the vibrational motion of the La atom inside the fullerene cage. The metallic behavior of La@C 60 is attributed to the presence of a dynamical dipole in the single cluster.

Using time-resolved photoelectron spectroscopy we show that electron relaxation processes via inelastic electron-electron scattering are efficient energy dissipation channels not only in bulk metals but also in extremely small transition metal clusters. The photoelectron spectra of optically excited Pd 3 Ϫ , Pd 4 Ϫ , and Pd 7 Ϫ reveal effective electron relaxation times of less than 100 fs. Moreover the relaxation times vary with cluster size. In comparison to simple metal clusters the bulklike inelastic scattering rates in open d-shell transition metal clusters are attributed to the larger valence electron level density. An energy transfer to the vibrational degrees of freedom occurs within 10 ps.