Coverage dependence of surface structure and vibration of Cl/Cu(100) compared to Cl/Ni(100)

Abstract

The coverage dependence of surface structure and vibration was investigated for 0.12- and 0.50-ML Cl adsorbed on Cu(100) by means of temperature-dependent Cl $K$-edge surface-extended x-ray-absorption fine structure. The first-nearest-neighbor Cl-Cu distance was found to be $2.42\ifmmode\pm\else\textpm\fi{}0.02\AA{}$ for 0.12 ML and $2.39\ifmmode\pm\else\textpm\fi{}0.02\AA{}$ for 0.50 ML. Moreover, the Cl-Cu chemical bond was revealed to be softer and more anharmonic in the 0.12-ML case than in the 0.50-ML one. The present findings are completely opposite to the previous results on Cl/Ni(100) in which the Cl-Ni bond was found to be elongated, softer, and more anharmonic with the Cl coverage. For the understanding of the different nature of Cl/Cu from Cl/Ni, density-functional calculations were performed for finite surface clusters. The theoretically estimated coverage dependence for the equilibrium distance was very consistent with the experimental results for both Ni and Cu. The different coverage dependences of the bond length and strength between Cl/Ni and Cl/Cu can consequently be ascribed to the different Cl ionicity at the saturation coverage, which originates from the different roles of metal $3d$ bands.