The Mutual Influence of Elemental S and Cs on the Ni(100) Surface at Room and Elevated Temperatures

The behavior of S and Cs during the alternate adsorption of each adsorbate on the Ni(100) surface is studied at room and elevated temperatures by means of low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), thermal desorption spectroscopy (TDS) and work function (WF) measurements. For Cs deposition on the S-covered Ni(100) surface, the presence of sulfur increases the binding energy and the maximum amount of adsorbed cesium, as happens with other alkalis too. The first Cs overlayer is disordered, while the second strongly interacts with S with a tendency toward a CsxSy surface compound formation. This interaction causes the gradual demetallization of the Cs overlayer with the increasing S coverage in the underlayer. When the CsxSy stoicheometry is complete, however, subsequent Cs deposition forms an independent rather metallic overlayer. When the sulfated covers the surface, S(0.5ML)/Ni(100) is preheated to 1100 K, the S-Ni bond strengthens and S-Cs interaction correspondingly weakens to a degree that the S underlayer retains a periodic structure on the Ni substrate. This behavior indicates that the preheated S/Ni(100) surface is passivated to a degree against Cs with reduced mobility of sulfur adatoms. Differently, when S is adsorbed on the Cs-covered Ni(100) surface at room temperature, sulfur adatoms diffuse underneath the Cs overlayer to interact with the nickel substrate and form the same structural phases as on a clean surface. During that process, the sticking coefficient of sulfur remains constant regardless of the amount of pre-deposited cesium. The presence of Cs, however, increases the amount of S that can be deposited on the Ni substrate, probably in favor of the CsxSy compound formation, which demetallizes the surface independent of the sequence of adsorption.