X-ray-absorption fine structure measurement with parallel-plate capacitor: Observation of surface electronic states of metals

For surface electronic state analyses of metals, we developed a new x-ray-absorption fine structure (XAFS) measurement technique with a parallel-plate capacitor. Since the capacitance is changed by x-ray-induced photoionization on the metal surface, the surface XAFS spectrum of a metal can be obtained from the capacitance dependent on the x-ray photon energy. We adopted this technique to the Cu metals. The XAFS spectrum at the Cu K absorption edge is different from the conventional XAFS spectrum of either Cu or Cu2O. This finding suggests that the XAFS spectrum indicates two-dimensional (2D) electronic states between the Cu bulk and the Cu2O native oxide layer. The 2D electronic states were characterized by degeneration of Cu 4pπ and a blueshift of Cu 4pσ. These characteristics can be explained by hybridization of vertical Cu 4pz with horizontal 4px and 4py in CuO-like structure at the interface