H. Handschuh

Electronic shells or molecular orbitals: Photoelectron spectra of Ag−n clusters

Photoelectron spectra of Ag−n clusters with n=1–21 recorded at different photon energies (hν=4.025, 4.66, 5.0, and 6.424 eV) are presented. Various features in the spectra of Ag−2–Ag−9 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.

VIBRATIONAL SPECTROSCOPY OF CLUSTERS USING A MAGNETIC BOTTLE ELECTRON SPECTROMETER

The design of a high resolution ‘‘magnetic‐bottle’’‐type time‐of‐flight electron spectrometer suitable for the study of mass‐separated metal and semiconductor cluster anions is described. A high collection efficiency is achieved by using magnetic fields to guide the photoelectrons, so that vibrationally resolved photoelectron spectra can be recorded at a low laser pulse energy (<10 μJ focused to 1 mm2) avoiding multiphoton processes. Spectra of clusters with a very low relative abundance, for example the products of chemical reactions involving clusters, can be recorded and an energy resolution of 6 meV (48 cm−1) achieved.

A comparison of photoelectron spectroscopy and two‐photon ionization spectroscopy: Excited states of Au2, Au3, and Au4

Photoelectron spectra of Au−n with n=2–4 are reported. Due to the relatively high photon energy used in our experiment (hν=6.424 eV) and the energy resolution of about 50 meV, 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−n clusters indicates the influence of relativistic effects and the large spin–orbit splitting for Au.

CO chemisorption on Nin, Pdn and Ptn clusters

Abstract Vibrationally resolved photoelectron spectra of mass-selected negatively charged Ni n − ( n = 1–3), Pd n − ( n = 2, 3) and Pt n − ( n = 1–4) clusters are compared with the corresponding spectra of these clusters ligated with m CO molecules ( m = 1–8). The spectra of the Pt n (CO) m − , species reveal part of the valence orbitals, which actually form the chemisorption bond. The data are in good agreement with the Blyholder model for CO chemisorption (σ-donation- π -backdonation scheme) and indicate that saturation corresponds to the formation of a closed electronic shell of the neutral. The strength of the π -backdonation is found to be larger for small particles compared with the corresponding single crystal surfaces, which may be related to the catalytic properties of small particles. The spectra of the unsaturated Ni n (CO) m − show fundamental differences compared with the ones of Pt n (CO) m − clusters owing to the high degree of localization of the Ni 3d orbitals. The spectra of the Pd n (CO) m − species show a rather irregular behaviour.

Delocalized electronic states in small clusters : comparison of Nan, Cun, Agn, and Aun clusters

Photoelectron spectra of Na; , Cu; , Ag; , and Au; clusters reveal the electronic structure of these particles. The experimental results are compared to the predictions of quantum chemical calculations and of the shell model. The spectra of Ag; allow for a stringent test of both approaches, because most of the observed features are assigned to s-derived orbitals and they also display much sharper features than the alkali data. A qualitative equivalence of the electronic shell model with high-level quantum chemical calculations in terms of the symmetries of the involved single particle orbitals is found for the delocalized states derived from the atomic s-electrons.

Electronic and Geometric structure of small mass selected Clusters

Abstract An experimental setup for photoelectron spectroscopy on mass selected cluster anions is described. The photoelectron spectra reveal the electronic structure of the particles as function of their size (mass). For clusters consisting of elements which are counted among the simple metals, i.e., the alkalis as well as Cu, Ag, and Au, the electronic structure is indicative of extended, delocalized states formed by the atomic s-electrons. This is the first indication of metallic behavior even though the particles containing up to 20 atoms clearly exhibit a ‘bandgap’ in the electronic structure. Additionally, in cases where vibrational finestructure is observed in the photoelectron spectra, the measured vibrational quanta allow to deduce the particle geometry. This is especially helpful in studies of chemisorption phenomena on the clusters. Moreover, electron phonon coupling constants can be extracted from a high resolution spectrum of C60, which confirm the alkali doped fullerenes to be conventional BCS Type super-conductors.

VIBRATIONALLY RESOLVED PHOTOELECTRON SPECTRA OF ANNEALED

Photoelectron spectra of annealed carbon-cluster anions n=5–70, are presented which show vibrational fine structure. From the vibrational frequencies, theoretical results, and ion mobility measurements, the geometry of the clusters can be determined: clusters with n=5, 7, and 9 are linear chains; n=10, 12, 14, 16, and 18 build monocyclic rings; for n=20, 24, and 28 the vibrational features correspond best to bicyclic rings; and for even n>30 fullerenes are found. In contrast to carbon, the mass distribution of silicon-cluster anions (up to n≈30) does not change dramatically upon annealing. An enhanced abundance of typical fragmentation products with n=4, 6, 7, and 10 is found. The vibrational fine structure for agrees with that of a pentagonal bipyramid.

AND

{\rm{Pt}}_n^-

{\rm Si}_n^-

clusters produced by a laser-vaporization source are reacted with CO. The various reaction products