Photoemission and high-resolution electron-energy-loss spectroscopy studies of CO chemisorption on thin Pd films on Au(111)

We present experimental observations of CO chemisorption on thin epitaxial Pd films on Au(111) using high-resolution electron-energy-loss spectroscopy (HREELS) and ultraviolet photoemission spectroscopy (UPS). The results confirm earlier UPS studies which show that a supported Pd monolayer exhibits weak CO chemisorption. We associate this behavior with the filling of the Pd d states and a reduced occupation of the molecular 2${\pi }^{\mathrm{*}}$ CO orbital. Consistent with this picture for the Pd monolayer is our HREELS observation of a weak, broadened loss peak for the C–O stretching mode which we attribute to two different adsorption sites: one site on the Pd monolayer with increased stretching frequency due to reduced CO 2${\pi }^{\mathrm{*}}$ occupation, and one site associated with defects in the thin Pd film. For thick Pd overlayers, a coverage-dependent shift in the C–O stretching frequency is observed.