Coverage dependence of surface structures and vibrations of studied by Cl K-edge SEXAFS

The coverage dependence of surface structures and vibrations of ClNi(100) have been investigated by means of Cl K-edge SEXAFS (surface extended X-ray absorption fine structure) spectroscopy. In the 0.25 ML phase, Cl was found to locate on the fourfold hollow site with little or no surface reconstructions, this being similar to the c(2 × 2) phase with 0.50 ML Cl. The ClNi bond distance was determined to be 2.31 ± 0.02 A 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 spectra was analyzed to yield the second- and third-order cumulants which provide harmonic and anharmonic force constants. The ClNi chemical bond 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 in terms of charge transfer from substrate to adsorbate and the repulsive interaction between adsorbates.