ORDERED STRUCTURES AT THE METAL ELECTRODE-SOLUTION INTERFACE. C. A. Lucas, N. M. Markovic and P. N. Ross. Materials Sciences Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720

Unraveling the atomic structure at the metal-electrode/solution interface presents a great challenge to the experimentalist due to its inherently complex nature. X-ray diffraction is an ideal tool for studying this structure and the related surface electrochemical phenomena, as the penetrating nature of x-ray radiation allows in-situ study of the metal surface. In contrast to studies of adsorbate systems in ultra-high-vacuum, adsorbate structures at the electrode surface are complicated by the range of possible adsorbing species in solution. We have performed a series of experiments with Pt(hkl) electrodes to determine the role of anion adsorption in surface reconstruction, surface relaxation and during the underpotential deposition (UPD) of metals. Information is obtained via measurement of the in-plane diffraction satellites due to ordered 2D adlayers and by measurement of the the 'crystal truncation rods' (CTR's) to relate the positions of the surface atoms with respect to the bulk Pt lattice. Monitoring the scattered intensity at selected reciprocal lattice points as a function of the electrode potential is key to understanding the sequence of adsorbed structures. Interpretation of the x-ray results is aided by the use of anomalous scattering methods to obtain chemical sensitivity in deriving structural models. This work was supported by the Director, Office of Energy Research, Office of Basic Energy Science, Materials Sciences Division (MSD) of the U.S. Department of Energy (DOE) under Contract No. DE-AC03- 76SF00098.