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