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

We report on the electronic structure of hydrogen, oxygen, and carbon monox ide on Gd(0001), and the development of adsorption processes studied by mea ns of photoelectron spectroscopy in combination with scanning tunneling mic roscopy 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 crystallogr aphic surface imperfections. From these nucleation centers, a domainlike sp reading is present which is strongly hindered at surface steps. Oxygen on t he 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 a t low temperatures and dissociatively at room temperature. The adsorption p rocess of a hydrogen-covered Gd film being exposed to carbon monoxide consi sts of five steps. At the beginning the whole amount of adsorbed hydrogen a toms is removed from the surface. In the intermediate regime, carbon and ox ygen is adsorbed at or near the surface. The last step demonstrates the oxi dation to Gd2O3, acting as a catalyst for the transformation of CO to CO2 w hich creates stable carbonate species at the surface. This formation also o ccurs for the adsorption of oxygen and carbon monoxide. [S0163-1829(99)0741 1-1].