Determination of the (3 x 3)-Sn/Ge(111) structure by photoelectron diffraction

The bonding geometry of Sn on Ge(111) has been quantitatively determined for the (3 X 3) phase at 150 K. Energy scan photoelectron diffraction of the Sn 4d core levels has been used to independently measure the bond length between Sn and its nearest-neighbor Ge atoms and the vertical distance between Sn and the Ge atom beneath. This latter distance is found to be similar to0.3 Angstrom larger for one Sn atom out of the three contained in the lattice unit cell. The bond lengths and the bond directions, obtained by the angular scans, are found to be practically the same for the three Sn atoms within +/-0.03 Angstrom and +/-3 degrees, respectively. The three nearest-neighbor Ge atoms thus partially follow the Sn atom in its vertical ripple.