Coverage dependent adsorption site for sulfur on Ni(111)

Angular-resolved photoemission spectra have been observed from two ordered sulfur overlayers, p(2×2) and (√3 ×√3)R30°, representing different coverages on the Ni(111) surface. Utilization of selection rules applicable to photoemission along the sample normal permits an assignment of placement and composition of the adsorbate induced bands. The spectra show dramatic changes in peak positions between these two overlayers similar in magnitude to those observed on Ni(100). The shifts in peak positions are attributed to symmetry prescribed changes in the surface unit cell relating to the character of the bond. Arguments that these changes provide evidence that the bonding site of the adsorbate atom may change with coverage are presented.