Alkanethiol self-assembled monolayer (SAM) phases on Au(111) have been assumed to involve direct S head group bonding to the substrate. Using x-ray standing wave experiments, we show the thiolate actually bonds to gold adatoms; self-organization in these archetypal SAM systems must therefore be governed by the movement of these $\mathrm{Au}\mathrm{\text{−}}\mathrm{S}\mathrm{\text{−}}R$ moieties on the surface between two distinct local hollow sites on the surface. The results of recent ab initio total energy calculations provide strong support for this description, and a rationale for the implied significant molecular mobility in these systems.

• Received 6 July 2006

DOI:https://doi.org/10.1103/PhysRevLett.97.166102