Si(100)2×1-Cl structure from x-ray-absorption spectroscopy

Surface-extended x-ray-absorption fine-structure (SEXAFS) and near-edge x-ray-absorption fine-structure (NEXAFS) spectroscopy at the Cl K edge have been used to investigate the structure of Si(100)2×1-Cl. A vicinal single-domain Si(100)2×1 substrate was employed to simplify data analysis. This substrate was exposed to ${\mathrm{Cl}}_2$ at about 500 K to yield a coverage of about 0.25 monolayers. The room-temperature SEXAFS data indicates that Cl sits atop the upper Si atom of a buckled dimer at a bond distance of 2.00±0.02 Å. Analysis of the polarization-dependent NEXAFS data, which relies on the pseudointramolecular behavior of a ${\mathrm{\sigma }}^{\mathrm{*}}$ resonance, is consistent with this geometry. However, it is inconsistent with the structural model derived from a recent electron-stimulated-desorption ion-angular-distribution study, in which Cl atoms are bound to both ends of a symmetric dimer. We point out that this discrepancy may arise from a difference in the coverage employed.