BACK-REFLECTION X-RAY STANDING WAVE STUDIES OF ADSORBATES ON METAL SURFACES. By L.E. Berman^*, D. Heskett^**, X. Shi^**, C. Su^**, P. Xu^**, J. Warner^**, C. Kao^*, M.J. Bedzyk^***, ^*NSLS, Brookhaven National Lab., Upton, NY 11973, ^**Dept. of Physics, U. Rhode Island, Kingston, RI 02881, ^***Dept. of Mater. Sci.& Eng., Northwestern U., Evanston, Il. 60208 and Mater. Sci. Div., Argonne National Lab., Argonne, Il. 60439

We have carried out x-ray standing wave studies of the registry of the diatomic molecule CO on Ni, and the alkali atom Rb on Cu, single crystal surfaces. The x-ray standing wave technique is a powerful method for the determination of the positions of atoms and molecules on single crystal surfaces. The dynamical x-ray diffraction conditions that are required to form the standing wave field necessitate the use of perfect crystals, of which there exist relatively few with very small mosaic spread and little static disorder. Near the back-reflection condition (Bragg angle close to 90 deg), however, the intrinsic dynamical diffraction reflectivity width exceeds 1 deg for many crystal Bragg reflections, which greatly expands the range of crystals for which the x-ray standing wave technique is applicable, and has made our measurements on Ni and Cu crystals (each with a mosaic spread of 0.25 deg) possible. In the case of adsorption of a submonolayer of CO on a Ni(111) surface, the positions of the C and O atoms can be probed separately, giving the bond length of the diatomic molecule. Photoemission signals arising from each constituent, as well as from the Ni substrate, were measured separately as two different Bragg reflections were scanned. We determined a C-O bond length of 1.10(.06) Å with the molecular axis assumed to be oriented perpendicular to the surface. A model in which the CO adsorbs in a 50-50 mixture of 3-fold fcc and hcp hollow sites, with the C atom closer to the substrate, compares favorably with the data. We also studied the adsorption of Rb submonolayers on Cu(111) and (110) surfaces. For the Cu(111) case, we determined that the Rb atom preferentially adsorbs in the top site, at a fixed vertical height but with substantial lateral disorder. For the Cu(110) case, which has an alkali-induced (1x2) missing row reconstruction, we found the Rb atom to sit in a fixed vertical position and well localized laterally perpendicular to the missing rows, but highly disordered along the rows.