Extended x-ray absorption fine structure (EXAFS) spectroscopy is used to study the size-dependent structural evolution of free clusters. K-edge EXAFS data are obtained for Ar clusters in beams having mean cluster size ranging from 12 to 2900 atoms per cluster. EXAFS analysis yields size-dependent features which indicate the presence of an fcc structure at ͗N͘ $ 200. It has been proposed in the literature that small rare-gas clusters have an icosahedral structure. The transition of this structure to the experimentally observed fcc lattice of rare-gas solids has been of long-standing interest and debate. The implications of our results are discussed in this light.

Site-specific excitation in Kr clusters is investigated by high resolution inner-shell excitation in combination with model calculations, which are based on a core exciton model. Partial cation and total electron yield spectra of variable size Kr clusters are reported for the Kr 3d excitation regime (90 -96 eV) using synchrotron radiation. A cluster size-dependent spectral evolution is observed, corresponding to the transition from low lying cluster Rydberg states into surface and bulk exciton states. The results indicate that individual sites in perfectly and imperfectly shaped clusters are clearly distinguished.[S0031-9007 (98)05596-3] PACS numbers: 36.40.Mr, 61.10.HtClusters play a central role in linking properties of isolated atoms to those of the condensed phase of matter [1,2]. The size-dependent evolution of the electronic and geometric structure is therefore of primary interest. Van der Waals clusters, such as rare gas clusters, are ideal systems to study size effects in a systematic way. Rare gas clusters are known to consist of geometrical structures which are not well defined since perfect icosahedral structures [3] and imperfect structures [4] may be generated in jet expansions, a typical cluster production technique. Resonant photon excitation of clusters is a sensitive method to probe changes in electronic structure as a function of cluster size. Recent instrumental developments in the field high resolution x-ray spectroscopy using synchrotron radiation make high spectral resolution experiments on free clusters in the gas phase possible [4,5]. The inherent advantage in exciting highly localized core levels is due to element specific, and often site specific, processes which cannot be obtained by valence shell excitations.In this Letter we present experimental results supported by model calculations. High spectral resolution in combination with results from model calculations gives, to our knowledge for the first time, site-specific information on absorption processes of homogeneous clusters occurring in the inner-shell excitation regime. We have chosen the Kr 3d regime (90-100 eV), since it is well investigated for the atom [6] and the solid [7]. The preedge regime is dominated by two series of Rydberg states (Kr3d 5͞2 ! np and Kr3d 3͞2 ! np) in the atom, whereas in the solid the broad features of low lying core excitons are observed.The experimental setup consists of a jet expansion [8]. The skimmed jet is crossed with the beam of monochromatic synchrotron radiation (BESSY-I, beam line: U2-FSGM [9]). Cations are accelerated into a time-of-flight mass spectrometer. Total electron yields (TEY) are extracted into the opposite side using a channeltron adjacent to the cluster beam. Yields of mass-selected cluster cations are used for experiments on electronic properties of small clusters (typical size range: Kr n , with n , 70). Fragmentation is initiated in the Kr 3d regime by resonant MNNAuger decays of the core hole leading predominantly to doubly charged clusters [10], which are known to decay via ...

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