Evidence of 'adatoms' in the Si(111)7x7 reconstructed surface from X-ray diffraction.

Synchrotron radiation has been employed to measure 73 fractional order surface Bragg reflections. Although we have insufficient data for a complete structure determination, we use strong symmetry arguments to determine which sites in the diamond lattice are filled and which are vacant at the surface. We find that both a "stacking fault" under half the unit cell and "adatoms" are required to agree with the observations.