Chemical Sciences: A Manual for CSIR-UGC National Eligibility Test for Lectureship and JRF/X-ray standing waves

The X-ray standing wave technique The X-ray standing wave (XSW) technique can be used to study the structure of surfaces and interfaces with high spatial resolution and chemical selectivity. Pioneered by B.W. Batterman in the 1960s the availability of synchrotron light has stimulated the application of this interferometric technique to a wide range of problems in surface science. Basic Principles right|thumb|250px|Principle of X-ray standing wave measurements An X-ray interference field created by Bragg reflection provides the length scale against which atomic distances can be measured. The spatial modulation of this field described by the dynamical theory of X-ray diffraction undergoes a pronounced change when the sample is scanned through the Bragg condition. Due to a relative phase variation between the incoming and the reflected beam the nodal planes of the XSW field shift by half a lattice constant. Depending on the position of the atoms within this wave field the element specific absorption of X-rays varies in a characteristic way. Therefore, measurement of the photo yield – via X-ray fluorescence or photoelectron spectroscopy – can reveal the position of the atoms relative to the lattice planes. For a quantitative analysis the normalized photo yield is described by , where is the reflectivity and is the relative phase of the interfering beams. The characteristic shape of can be used to derive precise structural information about the surface atoms because the two parameters (coherent fraction) and (coherent position) are directly related to the Fourier representation of the atomic distribution function. none|thumb|650px|X-ray reflectivity (green) and photo yield (red) for different coherent positions Since the emitting atoms are located in the near field, this technique does not suffer from the ubiquitous phase problem of X-ray crystallography. Therefore, and with a sufficiently large number of Fourier components being measured, XSW data can be used to establish the distribution of the different atoms in the unit cell (XSW imaging). Selected Applications which require ultra-high vacuum conditions Physisorption and chemisorption studies Diffusion of dopants in crystals P. Hoenicke et al., Depth profile characterization of ultra shallow junction implants, Anal. Bioanal. Chem., 396 (8), 2825-2832 (2010) Superlattices and Quasi-crystal characterization which do not require ultra-high vacuum conditions Langmuir-Blodgett films Self-assembled monolayers Buried interfaces References J. Als-Nielsen & D. McMorrow, Elements of Modern X-ray Physics, John Wiley & Sons, Ltd (2000) B. W. Batterman & H. Cole, Dynamical Diffraction of X Rays by Perfect Crystals, Rev. Mod. Phys. Vol. 36 681 (1964)