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The reaction of oxygen with Cu cluster anions consisting of 6-11 atoms was studied by means of Time-of-Flight mass (TOF) spectroscopy and Ultraviolet Photoelectron Spectroscopy (UPS). Using molec- ular oxygen, we found that a Cucluster (n =6 −11) can react only with one single oxygen molecule, which adsorbs molecularly, implying that the clusters studied here are less reactive towards oxygen chemisorption compared to the smaller clusters ( n< 5). This result indicates that chemical properties can alter signifi- cantly with increasing cluster size. Depending on the cluster source conditions, different cluster structures and reactivity patterns were found. These results are used to qualitatively describe the chemisorption energetics of oxygen on Cu cluster anions. PACS. 33.60.Cv Ultraviolet and vacuum ultraviolet photoelectron spectra - 36.40.Jn Reactivity of clusters - 36.40.Mr Spectroscopy and geometrical structure of clusters - 36.40.Wa Charged clusters

Photoelectron spectra of Ag - n clusters with n=1–21 recorded at different photon energies (hnu=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.

Photoelectron spectra of Al - n and Ga - n clusters in the size range n=1–15 are presented. Using 5.0 and 3.68 eV UV light (KrF excimer and nitrogen laser, respectively), electrons from molecular orbitals corresponding to the 3p and 4p orbitals of the atoms are detached. The spectra reveal a rich fine structure not observed in earlier experiments. The data are compared with the results of quantum chemical calculations. A change in the pattern of the spectra near n=6 can be interpreted as a transition from planar to compact 3D structures. The spectrum of Al - 13 agrees with the icosahedral structure predicted for this particle. The data do not agree with jellium model predictions. Differences between Al and Ga data can be correlated with the larger binding energy of the Ga 4s band.

Using a new experimental setup we have measured UV (h ν = 6.4 eV) photoelectron spectra of cold silicon cluster anions Si n - in a very broad size range. For sizes up to n = 46 the spectra exhibit rich structures. For larger sizes only smooth spectra have been obtained. No trace of a bandgap has been found even for clusters with more than 1000 atoms.

Using femtosecond time-resolved two-photon photoemission spectroscopy, relaxation rates of the optically excited states in Al13− , Si4− , and Au6− clusters are estimated to be 250 fs , 1.2 ps , and about 1 ns , respectively. These clusters show closed electronic shell structures and large gaps between the highest occupied molecular orbital and the lowest unoccupied molecular orbital in the anionic (Al13−) or neutral ( Si4 and Au6 ) states. Thus the different behaviors of these clusters for the relaxation dynamics cannot be rationalized within a relaxation mechanism dominated by Auger-like electron-electron coupling. We suggest that for these small clusters, the major decay mechanism might be electron-phonon coupling.

In atomic cluster research, generally both geometric and electronic properties of clusters have to be considered. Until now, however, experiments have yielded only limited information. Usually, either the geometry or the electronic structure, but not both, can be determined. The present work is the first to successfully combine an ion mobility spectrometer (IMS, yielding a cluster's shape) with a photoelectron spectrometer (PES, yielding a cluster's electronic levels). Thus one can measure isomer-separated photoelectron spectra in a systematic way. Both the number of atoms in a cluster and their arrangement are well defined. This does not depend on the conditions inside the cluster source. The proof of principle of the new IMS/PES method was given using carbon cluster anions.The new method would be helpful e. g. for the examination of silicon hydride mixed clusters (Si_m H_n). At silicon bulk surfaces, hydrogen adsorption strongly changes crystal structure and conductivity. It is still unknown what this means with respect to clusters. However, despite the relevance of semiconductor and nanotechnology, only few experimental research has been done considering either the geometric or the electronic structure of Si_m H_n clusters. As it has not yet been possible to apply the new IMS/PES method on these clusters, a comparison of conventional photoelectron spectra and theoretical simulated spectra is presented. Spectra habe been measured for Si_m H_n- anions, where m = 3 10 and n = 0 2. As a result, the observed clusters' structure may be concluded. Moreover it appears that for cluster anions, electron affinity plays an important role in the selection of isomers inside the source.

Copper cluster anions in the size range of 1-18 atoms are studied by photoelectron spectroscopy. Using photons of 5.0 eV energy the electronic level structure built from the delocalized s-orbitals of the atoms is revealed. An attempt is made to assign the observed bands to the electronic shells predicted by the jellium model. Even though the general behavior resembles the predictions of this model, the spectra are much more complicated reflecting the discrete atomic geometry of these particles.

We have obtained photoelectron spectra (PES) for silicon cluster anions with up to 20 atoms. Efficient cooling of species in the source has allowed us to resolve multiple features in the PES for all sizes studied. Spectra for an extensive set of low-energy ${\mathrm{Si}}_{n}^{\ensuremath{-}}$ isomers found by a global search have been simulated using density functional theory and pseudopotentials. Except for $n\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}12$, calculations for ${\mathrm{Si}}_{n}^{\ensuremath{-}}$ ground states agree with the measurements. This does not hold for other plausible geometries. Hence PES data validate the tricapped trigonal prism morphologies for medium-sized Si clusters.

The photoelectron spectra of mass-selected negatively charged Nin- and Cun- clusters show similarities, which indicate an almost total localization of the Ni 3d orbitals corresponding to a maximum magnetic moment of 1μB per Ni atom. The similarity between Nin- and Cun- vanishes for n>7 corresponding to an increase in 3d delocalization. The data of Pdn- clusters suggest a Ni-like ( magnetic ) electronic structure for n = 3 6 and a Pt-like ( nonmagnetic ) one for n>15. There are indications that neutral Pd7 is a closed shell species (spin zero ground state).

The competition between electron emission and unimolecular dissociation following the absorption of a single photon is studied for small clusters of various elements. Electron emission results from either direct photoemission, observed as a series of peaks in the spectrum, or thermionic emission, giving rise to a continuous background. Depending on the energetics involved, thermionic emission is in competition with evaporation of atoms. These results are compared to the corresponding events observed for solids, where the dominant decay products are inelastically scattered photoelectrons.

A systematic study of the size dependence of the vibrational modes of carbon clusters with 5-70 atoms is presented. The vibrational frequencies are measured using photoelectron spectroscopy of mass-separated negatively charged clusters. The clusters are carefully annealed. Only the most stable isomers are present in the beam after the annealing. Observed vibrational modes can be assigned to four dominant isomeric structures: linear chains (C5-, C7-, and C9-), monocyclic rings (C10-, C12-, C14-, C16-, and C18-), bicyclic rings (C20-, C24-, and C28-), and fullerenes (even n>C30-).

This paper reports the observation of autodetaching resonances in photoelectron spectroscopy of Au - 2 according to the process Au - 2 +hv→(Au - 2)*→Au2 +e−. They appear as sharp features superimposed upon a nearly constant signal associated with direct photodetachment process from Au - 2 :Au - 2 +hv→Au2+e−. By altering the cluster expansion conditions, the dimer anion temperature can be varied. This leads to changes in the linewidth and relative intensities of the resonances and allows ground state and hot band transitions to be identified. The resonances correspond to electronic transitions to a long-lived excited state of Au - 2 with a significantly lower vibrational frequency (128±2 cm−1) than that of the neutral ground state (190.9 cm−1), or even the anion ground state (149 cm−1). It decays via electronic autodetachment in a two-electron process similar to autoionization of neutral atoms.

Photoelectrons from mass-identified jet-cooled tin and lead cluster anions (Sn n , Pb n ) are detached by ultraviolet laser light (hngr=3.68 eV). The photoelectron energy spectra give the detachment energies of ground state cluster anions (electron affinities) as well as excitation energies of neutral clusters in the geometry of the anions. The energy spectra for Sn n are dominated by flat thresholds with ann-dependence similar to that of other group IV clusters. In contrast, for Pb n we find pronounced narrow lines close to threshold, generally followed by a 0.3 1.4 eV gap which indicates closed-shell behaviour of Pb n for nearly alln.

Aluminum cluster anions are produced in a laser vaporization source, cooled spectrometer. Pulsed uv light is used for electron detachment; the photoelectrons undergo energy analysis in a magnetic-bottle type spectrometer. The resulting electron spectra reflect the band-structure development of aluminum clusters.

Aluminium cluster anions (Al n ) are produced by laser vaporization without additional ionization and cooled by supersonic expansion. Photoelectrons from mass-identified anion bunches (n=2...25) are detached by laser light (hv=3.68 eV) and undergo energy analysis in a magnetic bottle-type time-of-flight spectrometer. The measurements provide information about the electronic excitation energies from ionic ground states to neutral states of the clusters. In contrast to bulk aluminium these cluster photoelectron spectra partially have well-resolved bands which originate from low-lying excited bands. For small clusters, especially the aluminium dimer and trimer, quantum-chemical calculations will be compared to the measurements. The electron affinity size dependence of larger clusters shows conclusive evidence for "shell" effects.