The Mutual Influence of Elemental S and Cs on 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 it happens with other alkalis too. The first Cs overlayer is disordered, while the second strongly interacts with S in a tendency toward a CsxSy surface compound formation. This strong interaction causes the gradual demetallization of the Cs overlayer with increasing S coverage in the underlayer. When the CsxSy stoicheometry is complete, however, subsequent Cs deposition forms an independent rather metallic overlayer. When the sulfur covered surface, S(0.5ML)/Ni(100) is preheated to 1100 K, the S-Ni bond strengthens and S-Cs interaction correspondingly weaken 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. 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 in the same manner as on clean surface. During that process the sticking coefficient of sulfur remains constant regardless the amount of predeposited 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 surface compound formation, which demetallizes the surface independently of the sequence of adsorption.