In-situ x-ray diffraction study of underpotential deposition at the gold(111) surface

In situ X-ray diffraction studies of underpotential deposition at the (111) surface of gold are described. By use of a thin film electrode grown on an X-ray transparent substrate, it has been possible to simultaneously vary the potential of the working electrode and collect X-ray diffraction patterns. The distance between the gold surface atoms and the adsorbed atoms is determined by quantitative analysis of the lineshape of the (111) reflection. It is found that the distance between lead and gold depends on the potential applied, while that of thallium on gold remains constant, in the underpotential region. The cause of this behavior is not known but may be due to the difference of valence between the two species.