Structural evolution of sulfur overlayers on Pd ( 1 1 1 )

0039-6028/$ see front matter 2009 Elsevier B.V. A doi:10.1016/j.susc.2009.04.012 * Corresponding author. Tel.: +1 412 2689517; fax: E-mail address: [email protected] (J.B. Mi Low energy ion scattering spectroscopy (LEISS) has been used to characterize the evolution of ordered structures of S on the Pd(1 1 1) surface during annealing. During exposure of the Pd(1 1 1) surface to 0.7 L H2S at 300 K—conditions that produce the S( p 3 p3)R30 overlayer—the intensity of the Pd LEIS signal decreases and a feature assigned to adsorbed S appears as the adsorbed layer forms. When the surface is held at 300 K after exposure to H2S is stopped, the LEIS Pd intensity partially recovers and the S signal weakens, presumably as surface S atoms assume their equilibrium positions in the S( p 3 p3)R30 overlayer. Subsequent annealing of the S( p 3 p3)R30 structure at 700 K causes it to convert into a S( p 7 p7)R19 overlayer, whose LEIS spectrum is identical to that of clean Pd(1 1 1). The absence of LEIS evidence for S atoms at the exposed surface of the S( p 7 p7)R19 overlayer is at odds with published models of a mixed Pd–S top layer. Despite the similarity of the LEIS spectra of Pd(1 1 1) and Pd(1 1 1)– S( p 7 p7)R19, their activities for dissociative hydrogen adsorption are very different—the former readily adsorbs hydrogen at 100 K, while the latter does not—suggesting that S exerts its influence on surface chemistry from subsurface locations. 2009 Elsevier B.V. All rights reserved.