The kinetic-energy dependence of the polar-angle photoelectron diffraction of Ga 3d emission was measured on the Si(111)-()ϫ))R30°-Ga surface using synchrotron radiation. The energy-dependent variations in the polar-angle intensity distribution can be reproduced by model calculations essentially assuming one scattering Si atom 2.6 Å below the Ga photoelectron emitter. This fact clearly indicates that the Ga atom adsorbs at the site directly above a second-layer Si atom ͑T 4 site͒. Calculations of the photoelectron diffraction using larger clusters have revealed that the vibrational correlation between the Ga adatom and the Si atom directly below it is very strong. This correlation effect emphasize the scatterings by the Si atom directly below the emitter compared with those by other surrounding atoms.

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