COVERAGE DEPENDENCE OF SURFACE-STRUCTURES AND VIBRATIONS OF CL NI(100) STUDIED BY CL K-EDGE SEXAFS/

The coverage dependence of surface structures and vibrations of Cl/Ni( 100) have been investigated by means of Cl K-edge SEXAFS (surface exte nded X-ray absorption fine structure) spectroscopy. In the 0.25 ML pha se, Cl was found to locate on the fourfold hollow site with little or no surface reconstructions, this being similar to the c(2 x 2) phase w ith 0.50 ML Cl. The Cl-Ni bond distance was determined to be 2.31 +/- 0.02 Angstrom for 0.25 ML, which is slightly shorter than that of 2.35 +/- 0.02 A for 0.50 ML. The temperature dependence of the SEXAFS spec tra was analyzed to yield the second- and third-order cumulants which provide harmonic and anharmonic force constants. Th Cl-Ni chemical bon d was consequently found to be stiffer and less anharmonic in the 0.25 ML state than in the 0.50 ML state, this corresponding to a slightly shorter bond distance at a lower coverage. Such a trend is discussed i n terms of charge transfer from substrate to adsorbate and the repulsi ve interaction between adsorbates. (C) 1997 Elsevier Science B.V.