Core level spectroscopy of free titanium clusters in supersonic beams

Synchrotron radiation x-ray absorption spectroscopy (XAS) is one of the most powerful techniques to interrogate the local electronic structure and chemical status of bulk and nanostructured systems. The application of this technique to the study of size effects in free clusters of transition metal atoms would advance substantially fundamental knowledge of nano-objects and the tailoring of their magnetic and catalytic properties. To date core level spectroscopy of free transition metal clusters has been out of reach due to the lack of a cluster source able to produce clusters in the gas phase with a density suitable for synchrotron radiation sources. Here we demonstrate the XAS characterization of free titanium clusters in a supersonic molecular beam. We use a high-intensity cluster beam source coupled to a synchrotron beamline to investigate the size dependence of core level excitation of Tin clusters in the mass range 15 < n < 1000. The x-ray absorption of Tin evolves from a multi-peaked complex structure similar to that of Ti atoms towards spectra characterized by two main absorption features as in bulk titanium. The intensities and the fine structure of the spectra are size dependent showing regularities compatible with geometric shell closings and the presence of a structural transition at about 540 atoms/cluster.