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

Previous LEED and ion scattering measurements have shown that chemisor bed S on Ni(111) results in the reconstruction of the first Ni layer a t 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 i n 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 occur ring after every four close-packed rows of Ni atoms in the [11 ($) ove r 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.