THE NATURE OF THE SULFUR INDUCED SURFACE RECONSTRUCTION ON NI(111)

Previous LEED and ion scattering measurements have shown that chemisorbed S on Ni(111) results in the reconstruction of the first Ni layer at coverages above 0.25 ML. Soft X-ray photoelectron spectroscopy (SXPS) and high resolution electron energy loss spectroscopy (HREELS) data are presented that support two possible models for this reconstruction while eliminating several others. SXPS indicates that the S adsorbs in a single state at low coverages and in two different states at high coverages. HREELS shows that the S-Ni coordination increases following reconstruction. These results are consistent with a reconstruction of the first Ni layer into a (100)-like surface with periodic gaps occurring after every four close-packed rows of Ni atoms in the [11 ($) over bar 2] direction. They are also consistent with a recently proposed modification of this model where the (100) terraces are altered by a ''clock'' reconstruction.