Observation of strain in the Si(111) 7x7 surface.

X-ray diffraction intensities for 120 independent superlattice reflections of the clean reconstructed Si(111) 7x7 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 0.02angstroms. 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.