A quantitative determination of 2s vacancy lifetimes in surface and bulk atoms of free Ne clusters has been made. While for free atoms the 2s inner-valence hole has a ps lifetime, it reduces to 6+/-1 fs for cluster bulk atoms. For surface atoms, the lifetime is on average longer than 30 fs. The lifetime estimate was obtained from fits of high-resolution photoelectron spectra of Ne clusters. The shortening of the lifetime is attributed to the coordination dependent interatomic Coulombic decay, which is extremely sensitive to internuclear distances.

(H2O)(N) clusters generated in a supersonic expansion source with N approximately 1000 were core ionized by synchrotron radiation, giving rise to core-level photoelectron and Auger electron spectra (AES), free from charging effects. The AES is interpreted as being intermediate between the molecular and solid water spectra showing broadened bands as well as a significant shoulder at high kinetic energy. Qualitative considerations as well as ab initio calculations explain this shoulder to be due to delocalized final states in which the two valence holes are mostly located at different water molecules. The ab initio calculations show that valence hole configurations with both valence holes at the core-ionized water molecule are admixed to these final states and give rise to their intensity in the AES. Density-functional investigations of model systems for the doubly ionized final states--the water dimer and a 20-molecule water cluster--were performed to analyze the localization of the two valence holes in the electronic ground states. Whereas these holes are preferentially located at the same water molecule in the dimer, they are delocalized in the cluster showing a preference of the holes for surface molecules. The calculated double-ionization potential of the cluster (22.1 eV) is in reasonable agreement with the low-energy limit of the delocalized hole shoulder in the AES.

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A new approach for obtaining an estimate of the effective size of the free neutral clusters is proposed. The approach relies on an experimental measure of the surface and interior or "bulk" cluster atoms provided by the x-ray photoelectron spectroscopy and on a model for the attenuation of photoelectrons ejected from the bulk of the cluster as the result of the ionizing irradiation. The experimental part gives the ratio of the electron signal from the bulk cluster atoms to that from the cluster surface atoms for a wide range of cluster sizes and electron kinetic energies. The attenuated response of the bulk atoms is modeled using an exponential law with the cluster size and kinetic-energy-dependent electron escape depth as parameters. For the experimental size range, model-based calculations for Ar, Kr, and Xe clusters are presented. The cluster size estimates obtained from comparison of the model calculations and experimental results agree well with those determined from the parameters of the cluster creation process. The combination of experiment and modeling also makes it possible to estimate the effective escape depth for electron propagation in free clusters. For Ar, Kr, and Xe clusters of varying mean size, absolute determination of the surface and bulk electron binding energies of the core levels used in the experiments has also been made.

Clusters formed by a coexpansion process of argon and neon have been studied using synchrotron radiation. Electrons from interatomic Coulombic decay as well as ultraviolet and x-ray photoelectron spectroscopy were used to determine the heterogeneous nature of the clusters and the cluster structure. Binary clusters of argon and neon produced by coexpansion are shown to exhibit a core-shell structure placing argon in the core and neon in the outer shells. Furthermore, the authors show that 2 ML of neon on the argon core is sufficient for neon valence band formation resembling the neon solid. For 1 ML of neon the authors observe a bandwidth narrowing to about half of the bulk value.