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Electronic and geometric structures of MxSy (M = W, Mo; x=1,2,4;y=1–12) clusters have been studied using density functional theory calculations, and compared to experimental photoelectron spectra. For the metal atoms, an uptake of up to six sulfur atoms has been observed, which can be explained by the bonding of S3 - chains. A structural difference to the corresponding oxides is the preference of bridging sites for S, which might be the origin of the differences between the structures of bulk MO3 and MS2. For x=1,2 the HOMO–LUMO gaps vary irregularly. For x=4, a large HOMO–LUMO gap has been found for y=6,7, and 8 and the W4S6 and Mo4S6 clusters have been found to be magic with an extraordinarily high stability.

In order to study the finite temperature behavior of magnetic nanoparticles a novel Monte Carlo method has been developed. The energy of a new trial configuration during the simulation is calculated directly from the expansion of the band energy avoiding a set up of an a priori Heisenberg-type model. The electronic structure of the cluster is determined by means of the embedded-cluster Green's function technique based on the Korringa–Kohn–Rostoker method within the local spin-density approximation of the density functional theory. As a benchmark the ground state of anti-ferromagnetic clusters and the temperature dependence of the magnetization of a flat square cluster of 16 Co atoms on a Cu(001) surface have been studied.

Die grundlegende Motivation der Dissertation bildet die Suche und die Erforschung und Herstellung von anwendungsbezogenen Nanomaterialien.Atomgenaue Nanostrukturen bilden die Grundlage für potenzielle Werkstoffe, die Anwendung in der Katalyse, Energiegewinnung oder -umwandlung finden können. Das systematische Studium von Clustern und Clusternanomaterialien in der Gasphase oder auf Oberflächen kann einen Beitrag leisten, die Klima-, Energie- und Umweltprobleme unserer Gesellschaft zu lösen.Die Idee der vorliegenden Arbeit ist es, neue experimentelle Untersuchungsmethoden und -apparaturen für Cluster und Nanopartikel zu realisieren. Ziel des Forschungsvorhabens war es dabei, die bestehende und gut verstandene kontinuierliche Magnetronclusterquelle mit einem Sektormagneten zur Massenseparation zu nutzen und mit neuen Spektroskopietechniken zu koppeln. Im Rahmen des Forschungsvorhabens wurden hierfür zwei Apparaturen entwickelt:Zum einen die Depositionsapparatur: Die Idee hinter der Appartur ist, neue Materialien, sogenannte Cluster-Assembled-Materialien, zu synthetisieren. Hierzu wurden in-situ Mo-Cluster auf eine HOPG-Oberfläche deponiert und mit einem Rastertunnelmikroskop (STM) spektroskopiert.Zum anderen die Gasphasen-Spektroskopieapparatur: Mit diesem Aufbau wird das XPS-Festkörperanalyseverfahren auf Gasphasenteilchen erweitert. Dies ist durch den Einsatz eines Velocity Map Imaging Spektrometers (VMI) und Synchrotronstrahlung erfolgt. Mit Hilfe eines Labor-Testlasers wurde die Funktionsweise der Apparatur getestet und unter Beweis gestellt.

Metal clusters serve as model systems to study basic problems of electronic correlation. Vacuum ultraviolet light from the free-electron laser FLASH ionizes 5d electrons from mass-separated negatively charged clusters, thus transiently leading to core-ionized neutral systems. Shielding of the core hole affects the electron binding energy. From the strong deviation from expectations of the metallic droplet and jellium models we conclude on reduced electronic shielding once the cluster size falls below about 20 atoms. This suggests a metal-to-nonmetal transition, in agreement with previous local density approximation calculations.

In der vorliegenden Arbeit wurde in einer PACIS Clusterquelle Aluminium unter Zugabe von gasförmigem Wasserstoff verdampft und somit Aluminiumhydridcluster hergestellt. Diese Cluster wurden massensepariert und mittels Anionenphotoelektronenspektroskopie untersucht, um ein Abbild ihrer elektronischen Struktur zu erhalten. Die geometrischen und elektronischen Strukturen der Cluster wurden außerdem in einer Kooperation mit der Virginia Commonwealth University von Puru Jena et al. mittels Dichtefunktionaltheorie berechnet.Eine Gruppe von Clustern, namentlich Al4H4 , Al4H6 und die Serie AlnHn+2 mit 5 ≤ n ≤ 8 zeigten große HOMO-LUMO-Gaps und relativ geringe Adiabatische Elektronenaffiniäten, was auf deutlich erhöhte Stabiliät dieser Cluster hinweist. Die dabei festgestellten Ähnlichkeiten zu den Strukturen bekannter stabiler Borane BnHm führten zu Untersuchungen, ob die Regeln von Wade auch auf die gefundenen Alane AlnHm angewendet werden können. Ein Vergleich der experimentell gemessenen Werte für HOMO-LUMO-Gap, Adiabatische Elektronenaffinität und Vertical Detachment Energie mit den berechneten Werten ergab geometrische Strukturen der Grundzustände, die für die Cluster AlnHn+2 (5 ≤ n ≤ 8) Wades (n + 1)-Regel folgen: Sie nehmen geschlossene, käfigartige closo -Strukturen mit einem endständigen Wasserstoff- pro Aluminiumatom und jeweils zwei weiteren, auf Brückenpositionen befindlichen Wasserstoffatomen an.Die beiden Cluster Al4H4 und Al4H6 liegen in tetraederförmigen Strukturen vor. Dabei nimmt Al4H4 die Geometrie eines closo-tetrahedralen Käfigs an, während Al4H6 eine leicht verzerrte, D2d -symmetrische Tetraederstruktur mit zwei Wasserstoffatomen auf Brückenplätzen zeigt. Al4H6 wies zudem mit 1,9 ± 0,1 eV das größte HOMO-LUMO-Gap von allen untersuchten Clustern auf. Dieser scheinbare Widerspruch zu Wades (n + 1)-Regel kann mit der Polyhedral Skeletal Electron Pair Theory (PSEPT) verstanden werden. Die aus dieser Theorie hervorgehenden Molekülorbitale ergeben für D2d Symmetrie ein Termschema, das im Vergleich zum Schema für perfekte Tetraedersymmetrie eine Aufspaltung der obersten beiden, in Td Symmetrie entarteten, Orbitale zeigt. Dadurch ergibt sich ein HOMO-LUMO-Gap zwischen den fünften und sechsten Skelettelektronenpaaren. Al4H6 , ein Cluster mit insgesamt 10 Skelettelektronen, also fünf Elektronenpaaren, nimmt daher eine Struktur mit D2d Symmetrie an und erreicht aufgrund der symmetriebedingten Absenkung des fünften Orbitals eine stabile elektronische Konfiguration.Die Anwendbarkeit der ursprünglich anhand der Borane entwickelten Regeln von Wade und der PSEPT auf die Aluminiumhydridcluster zeigt an, dass eine enge Verwandtschaft zwischen den beiden Gruppen besteht und dass die AlnHm Cluster als Analoga zu den Boranen angesehen werden können.Die ermittelten Werte für HOMO-LUMO-Gap und Elektronenaffinität lassen die Möglichkeit plausibel erscheinen, dass sich aus den untersuchten oder aus ähnlichen Clustern ein stabiles Clustermaterial in makroskopischen Mengen synthetisieren lässt. Dieses besäße zum einen Potential für einen Einsatz in der Wasserstoffspeicherung, könnte aber auch als Primärenergieträger, z. B. in Feststoffraketen, dienen, da die Aluminiumwasserstoffsysteme eine außergewöhnlich hohe Verbrennungsenthalpie bei der Reaktion mit Sauerstoff zeigen. Für Al4H6 wurde die bei der Verbrennung zu Al2O3 und Wasser frei werdende Wärme zu 438 kcal/mol berechnet, was mehr als dem 2,5-fachen der bei der Verbrennung von Methan freiwerdenden Wärmemenge entspricht. Sollte sich ein solches Material erzeugen lassen, so könnte es ein wichtiger Energieträger der Zukunft werden.

Ag cluster anions consisting of 3–16 atoms were deposited on sputter-damaged HOPG surfaces using a soft-landing technique (mean deposition energy less than 0.2 eV/atom) at room temperature. For investigations of the structures of deposited clusters, X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) were used. In addition, the chemical properties of deposited clusters were studied using atomic oxygen and CO. Comparison of the properties of deposited Ag clusters and Ag islands with similar sizes grown by evaporating Ag atoms on the same substrate shows different results, implying that two different preparation methods give either different shapes of Ag clusters and islands, or dissimilar metal–support interactions.

Using the electronic shell closure criteria, we propose a new electron counting rule that enables us to predict the size, composition, and structure of many hitherto unknown magic clusters consisting of hydrogen and aluminum atoms. This rule, whose validity is established through a synergy between first-principles calculations and anion-photoelectron spectroscopy experiments, provides a powerful basis for searching magic clusters consisting of hydrogen and simple metal atoms.

The decay mechanism of excited electronic states in small Ag nanoclusters is studied using time-resolved photoelectron spectroscopy. The low density of states in these clusters inhibits Auger-like decay channels that are responsible for ultrafast relaxation of optically excited states in bulk Ag. Thus, much longer relaxation times are expected for small clusters showing large gaps between electronic states. In contrast to this expectation, lifetimes below 1 ps were observed for most of the Ag n n 22 studied here. The only exception is the magic Ag7 with a relaxation time of 3.8 ps. The observed fast relaxations are discussed in view of their ability to undergo fast shape deformations. This approach can also explain the slow relaxation of the rigid Au n nanoclusters and the extremely fast relaxation of the flexible magic Al13 . PACS numbers: 61.46.Bc, 73.22.f, 78.47.p, 79.60.i I. INTRODUCTION Behaviors of nanoclusters in many aspects differ from those of the corresponding bulk materials. One major issue about the unique properties of clusters is the electronic relaxation after photoexcitation. Experimentally, this can be explored using time-resolved photoelectron spectroscopy: a pump photon excites a single electron from an occupied level to an unoccupied state, and the subsequent development of the electronic structure is monitored with a second photon

The electric conductance of a strip of undoped graphene increases in the presence of a disorder potential, which is smooth on atomic scales. The phenomenon is attributed to impurity-assisted resonant tunneling of massless Dirac fermions. Employing the transfer matrix approach we demonstrate the resonant character of the conductivity enhancement in the presence of a single impurity. We also calculate the two-terminal conductivity for the model with one-dimensional fluctuations of disorder potential by a mapping onto a problem of Anderson localization., Comment: 6 pages, 3 figures, final version, typos corrected, references added

For many catalytic processes the adsorption and activation of oxygen is of major importance. Nano-particles (cluster) essentially differ in there properties from the well-known macroscopic environment. For example: metal cluster can act as semiconductors or even insulators.In the context of this work the chemisorption of oxygen on metal (Cu, Pd) and semiconductor clustern (Si) is studied. By means of anion photoelectron spectroscopy (PES) it could be shown that oxygen dissociates on small copper cluster anions. However, molecular oxygen forms an activated hyperoxide species on clusters consisting of 6 to 10 copper atoms.On small (1-3 atoms) palladium cluster anions oxygen adsorbs dissociatively. With increasing cluster size the influence of oxygen bonding on the forming conduction band of the palladium cluster is fading. However, a definite answer on the chemisorption of oxygen on the larger palladium clusters is not possible up to this point.Stable clusters can be used as building blocks for new cluster materials. An example is the Fullerit, a solid state formed of C60 clusters. In the present work, "magic" Si7-clusters on surfaces were examined, in particularly the suitability of these cluster as components of a new solid body. The clusters are produced in a magnetron sputter gun, separated and soft landed onto different surfaces.By means of X-ray electron spectroscopy (XPS) first indications are found, which point to the fact that Si7 on surfaces is very stable in relation to chemical reactions and not likely to merge with neighboring clusters. In contrast, deposited silicon clusters with 8 or 9 silicon atoms exhibit a high reactivity towards oxygen. Results which point in the same direction are obtained by Auger electron spectroscopy (AES) and high-resolution electron energy loss spectroscopy (HREELS). In addition theoretical simulation are carried out.

We compare experimentally measured and ab initio computed photoelectron spectra of nega- tively charged deuterated silicon clusters (SimD � ,4 ≤ m ≤ 10, 0 ≤ n ≤ 2) produced in a plasma environ- ment. Based on this comparison, we discuss the kinetics and thermodynamics of the cluster formation and the effect of deuterium on the geometrical and electronic structure of the clusters. PACS. 36.40.-c Atomic and molecular clusters - 61.46.+w Nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals - 73.22.-f Electronic structure of nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals

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.

Enhanced stability, low electron affinity, and high ionization potential are the hallmarks of a magic cluster. With an electron affinity of 0.28 eV, ionization potential of 11.43 eV, and a large binding energy, AlH3 satisfies these criteria. However, unlike other magic clusters that interact only weakly with each other, two AlH3 clusters bind to each other with an energy of 1.54 eV. The resulting Al2H6, while also a magic cluster in its own right, possesses the most unusual property that the difference between its adiabatic and vertical detachment energy is about 2 eV the largest of any known cluster. These results, based on density functional theory, are verified experimentally through photodetachment spectroscopy.