Adsorbates on Gd(0001): A combined scanning tunneling microscopy and photoemission study

We report on the electronic structure of hydrogen, oxygen, and carbon monoxide on Gd(0001), and the development of adsorption processes studied by means of photoelectron spectroscopy in combination with scanning tunneling microscopy and spectroscopy, respectively. Due to its high spatial resolution, the visualization of the adsorption process is obtained. For hydrogen, it was found that the adsorption occurs in two steps. It starts at crystallographic surface imperfections. From these nucleation centers, a domainlike spreading is present which is strongly hindered at surface steps. Oxygen on the Gd monolayer possesses an ordered structure which could be determined to be (2 root 3 x 2 root 3)R30 degrees. Carbon monoxide adsorbs molecularly at low temperatures and dissociatively at room temperature. The adsorption process of a hydrogen-covered Gd film being exposed to carbon monoxide consists of five steps. At the beginning the whole amount of adsorbed hydrogen atoms is removed from the surface. In the intermediate regime, carbon and oxygen is adsorbed at or near the surface. The last step demonstrates the oxidation to Gd2O3, acting as a catalyst for the transformation of CO to CO2 which creates stable carbonate species at the surface. This formation also occurs for the adsorption of oxygen and carbon monoxide. [S0163-1829(99)07411-1].