Direct Experimental Evidence for Substrate Adatom Incorporation into a Molecular Overlayer

While the phenomenon of metal substrate adatom incorporation intomolecular overlayers is generally believed to occur in several systems, the experimentalevidence for this relies on the interpretation of scanning tunneling microscopy (STM)images, which can be ambiguous and provides no quantitative structural information.We show that surface X-ray diffraction (SXRD) uniquely provides unambiguousidentification of these metal adatoms. We present the results of a detailed structuralstudy of the Au(111)-F4TCNQ system, combining surface characterization by STM,low-energy electron diffraction, and soft X-ray photoelectron spectroscopy withquantitative experimental structural information from normal incidence X-ray standing wave (NIXSW) and SXRD, together withdispersion-corrected density functional theory (DFT) calculations. Excellent agreement is found between the NIXSW data and theDFT calculations regarding the height and conformation of the adsorbed molecule, which has a twisted geometry rather than thepreviously supposed inverted bowl shape. SXRD measurements provide unequivocal evidence for the presence and location of Auadatoms, while the DFT calculations show this reconstruction to be strongly energetically favored.