Observation of strain in the Si(111) 7×7 surface

X-ray diffraction intensities for 120 independent superlattice reflections of the clean reconstructed Si(111) 7×{}7 surface have been measured using improved sources and techniques. Starting from the Takayanagi model, we have refined a full set of in-plane structural coordinates and determined the atom positions to within 0.02 A{}. Our structure clearly shows the local strain fields expected around the adatoms as well as a general dilation of the reconstructed layers showing the presence of strain.