Grazing-angle x-ray standing waves

We have observed fluorescent radiation from surface adatoms of I on Ge(111) using x-ray standing waves created in the grazing-angle geometry. We demonstrate that standing waves which are determined by the real part of the x-ray wave vectors provide accurate atomic positions of adatoms or impurities parallel to the surface. The effect of changing evanescence due to the imaginary part of the wave vectors is directly visible in the standing-wave signal detected from the substrate. When combined with x-ray standing-wave results normal to the surface obtained from diffraction in the Bragg geometry, the measurements reveal that I reacted chemically with the (111) surface of Ge binds to the atop site of the Ge atom.