X-ray standing waves technique: Fourier imaging active sites

Abstract

An X-ray standing wave (XSW) is created in the overlap region of two coherent X-ray waves, e.g. by diffraction or reflection. The XSW intensity maxima move, when traversing the range of total reflection, causing strong modulation of the photo-excitation of a particular element or atomic species, recorded by electron or X-ray fluorescence spectroscopy. The XSW technique is a Fourier technique, particularly useful for identifying and structurally characterizing diluted, active species. In simple cases, a single XSW measurement allows characterization with pm resolution. Otherwise, employing several XSW measurements, an image can be created by Fourier inversion allowing one to identify individual sites. The principle, strength and limitations of the XSW technique are reviewed briefly and we focus on three examples for identifying active sites: Catalytically active Al in scolecite, magnetically active and counter-active sites of Mn in GaMnAs and sites on the SrTiO3(001) surface active in the splitting of water.