THE EU/PD(111) INTERFACE - SPECTROSCOPIC AND STRUCTURAL STUDIES

Growth of Eu ultra thin films on a Pd(111) single crystal, kept at room temperature, has been studied by spectroscopic (XPS, UPS, AES) and structural (RHEED) methods. Eu atoms of the first two deposited monolayers are divalent and form a p(2 X 2) arrangement on the Pd(111) surface. During the completion of the third Eu monolayer, this ordered structure smears out in connection with the appearance of trivalent Eu atoms at the interface. The fine structure modifications of Pd and valence band XPS spectra indicate the 4d band filling, via a charge transfer mechanism, of the Pd atoms involved in the interface. For thicker Eu coverages a diffusion process leads to the formation of disordered divalent Eu-Pd alloys. After annealing the sample at 820 K, RHEED diffraction patterns reappear (p(2 X 2)) and, simultaneously, an increase in the Eu mean valence is observed. XPS results show that segregation of divalent Eu occurs at the surface of the new ordered bulk-trivalent Eu-Pd compound which forms at the interface, This behaviour occurs whatever the thickness of the Eu deposit is.