Surface Diffraction

Introduction
Surface diffraction refers to diffraction applied to two dimensional, adsorbed layers, surface reconstruction and relaxation, and buried interfaces; this area includes a number of methods, including GIXS and reflectivity. Surface diffraction is frequently used for surface crystallography - the determination of atomic structure of adsorbed layers and at surfaces and buried interfaces. This involves intensity measurements of Bragg diffraction rods (from two dimensional layers) and crystal truncation rods (from surfaces and interfaces). Surface diffraction is also used in studies of surface and adsorbed-layer phase transitions.

Instrumentation
Beamline 7-2 is frequently used for surface diffraction measurements and indeed some of the pioneering surface diffraction measurements were done on this beamline. Soller slits or fixed slits are commonly used to analyze the scattered beam (depending on resolution), but for high resolution a crystal analyzer can be used. It is advantageous to have a flat surface with an area of about one cm².

Analysis
For surface crystallography, structural models are used to analyze the data. Some programs have been written for this (ROD) [1], but often the user develops the analysis methods. Recently, some progress has been made in 'direct methods' for surface crystallography [2]. Some details on data reduction can be found in the references below.

Applications of surface diffraction
Surface diffraction measurements of solid-liquid interfaces at SSRL have shed insight into electrocatalysis [3,4]. These experiments have focused on the interface structure and phase transitions of CO molecules adsorbed on Pt surfaces in a number of electrolytes. More details can be found here.
Recently, surface diffraction has been used to determine surface structure at oxide-aqueous interfaces, such as alumina, barite (BaSO4), and calcite [5-7]. Such interfaces are relevant to geochemistry.

1. E. Vlieg, "ROD: a program for surface X-ray crystallography", J. Appl. Cryst. 33, 401 (2000).
   
2. "Surface electrochemistry of CO on Pt(1 1 1): anion effects" N.M. Markovic, C.A. Lucas, A. Rodes, V. Stamenkovic, P.N. Ross, Surf. Sci., 499 (2002), L149-L158.
   
3. "The adsorption and oxidation of carbon monoxide at the Pt(111)/electrolyte interface: atomic structure and surface relaxation" C.A. Lucas, N.M. Markovic, P.N. Ross, Surf. Sci., 425 (1999) L381-L386.
   
4. "Crystal Truncation Rod Diffraction Study of the a-Al2O3 (1 -1 0 2) Surface", T.P. Trainor, P.J. Eng, G.E. Brown, Jr., I.K. Robinson and M. De Santis Surf. Sci. 496, 238 (2002).
   
5. "Structure of the Hydrated alpha-Al2O3 (0001) Surface", P.J. Eng, T P. Trainor, G.E. Brown, Jr., G.A. Waychunas, M. Newville, S.R. Sutton and M.L. Rivers, Science 288, 1029 (2000).
   
6. "Structure of Barite (001) - and (210) - Water Interfaces", P. Fenter, M.T. McBride, N.C. Sturchio and D. Bosbach, J. Phys. Chem. B 105, 8112 (2001).

Further General Reading:

• "Oxide Surfaces and Metal/Oxide Interfaces Studied by Grazing Incidence X-ray Scattering", G. Renaud, Surf. Sci. Repts. 32, 1 (1998).
  
• "The Structure of the Surface of Pure Liquids", J. Penfold, Repts Prog. Phys. 64, 777 (2001).
  
• "X-ray Crystallography of Structures and Interfaces", Acta Cryst. A 54, 772 (1998).