Free Clusters Studied by Synchrotron-Based X-ray Spectroscopy: From Rare Gases to Metals

The main topic of this Thesis is the x-ray spectroscopic studies of free nano-clusters with size more than 10^3 atoms. The photoelectron spectroscopy experiments on rare gas and metal clusters were performed at beamline I411 at MAX-lab National Swedish synchrotron radiation facility. Free rare-gas clusters are simplest objects in cluster research. However their electronic structure springs a lot of surprises. In the x-ray absorption spectra of free Kr clusters the positions of the related cluster and atomic states of the higher 3d^{-1}6p and 3d^{-1}7p excited states in bulk and surface atoms are reversed compared to the 3d^{-1}5p state. The energy shifts of the higher core-excited states in cluster atoms grow towards the limiting values of the core-ionized states. The knowledge of the final cluster states reached after Auger decay of the resonantly excited core states threw a new light on this phenomenon. We explain these experimental findings by a spatial spread of the excited orbitals over the cluster lattice. The interplay of the two main effects - confinement and polarization - leads to qualitatively different situations. When the excited orbital radius is less than the interatomic distance in clusters (3d^{-1}5p state in Kr), confinement dominates. If the excited orbital radius exceeds the nearest neighbour distance (3d^{-1}np, n>5 states in Kr), polarization takes over and the energy-level structure becomes ion-like. Interest in studies of more complex than rare gas clusters systems determined the construction of the gas-aggregation metal cluster source. For the vaporisation of low melting point metals (Na) an oven was used. Vapours of higher melting point metals (Ag, Cu, Pb) were produced with a magnetron sputtering source. The design of the gas-aggregation source and description of the experimental setup are presented in this Thesis. Using this source we performed the first core-level photoemission (XPS) experiments on free metal clusters. The recorded spectra have shown that synchrotron-based x-ray core-level photoelectron spectroscopy can be efficiently implemented to free metal clusters. The evolution of the electronic structure with size has been studied by valence ionization, XPS and Auger techniques. These measurements have shown a close similarity between large clusters and solids. The difference between the established ionization potential of metal clusters and the solid has been used for the cluster size estimation for Na, Cu and Ag clusters. The presence of neutral and charged metal clusters in the Pb cluster beam created by magnetron-based source has offered an independent method for estimating cluster dimensions from core-level spectra, in which the charge states are resolved