Adsorption of oxygen on Cu(100). I. Local structure and dynamics for two atomic chemisorption states.

Abstract

By means of temperature-dependent surface-extended x-ray-absorption fine structure (SEXAFS) measurements we characterize two different chemisorption states of atomic O/Cu(100), one corresponding to a reconstructed surface and the other to an unreconstructed surface, respectively. The unreconstructed state is characterized by an O-Cu nearest-neighbor (NN) bond length of 1.88(3) \AA{}, the O atoms being located 0.8(3) \AA{} above the first Cu layer. Upon reconstruction, the O-Cu NN bond is shortened by 0.020(6) \AA{}, the local geometry as determined by SEXAFS supporting a missing-row model for the reconstruction, with the adsorbate atoms being located 0.2(1) \AA{} above the first Cu layer. Static and dynamical contributions to the local disorder (Debye-Waller factor) as well as anharmonic contributions could be separated. Evidence for the existence of surface strain upon occurrence of a surface reconstruction is found as the static part of the local disorder for the NN O-Cu bond increases by 7.8(4) ${10}^{\mathrm{\ensuremath{-}}3}$ \AA{} for the reconstructed state. The static part of the anharmonic contributions to the disorder varies as well upon reconstruction. Through the systematic comparison of the effective pair potentials, obtained using the dynamic part of the disorder, of the two O/Cu chemisorption states with the ones of other reconstructed (N/Ni) and unreconstructed (O/Ni) surface systems, we find evidence that the bond is predominantly ionic for the unreconstructed case and more covalent for the reconstructed case. For the case of the ionic systems the charge transfer to the adatom is found to be 0.92(3) electrons based on a Madelung form of the pair potential, in agreement with theoretical calculations.