Extended x-ray-absorption fine-structure and near-edge-structure studies on evaporated small clusters of Au

The $L_3$ Au edge of small evaporated gold clusters has been studied by x-ray-absorption spectroscopy at the Frascati synchrotron radiation facility. Sample discontinuity and particle size were controlled by optical transmission measurements and electron microscopy analysis. The extended x-ray-absorption fine-structure spectra showed evidence of nearest-neighbor-distance contraction, whose value reached 2.5% for the smallest clusters. The behavior of the nearest-neighbor-distance contraction versus cluster diameter agreed with a macroscopic liquid-drop model. Increases in the nearest-neighbor-distance fluctuations around the equilibrium positions were found due to the higher mobility of the surface atoms with respect to the bulk ones. The slight increase of statical disorder together with the general features of the x-ray-absorption near-edge structure spectra allowed us to exclude structural changes from the fcc bulk metal structure to the icosahedral structure, even for clusters of 50 atoms.