Welcome to the X-Ray Absorption Fine Structure Online Orientation! This orientation will provide you with tips that will help you conduct a successful x-ray absorption fine-structure spectroscopy (XAFS) experiment at the National Synchrotron Light Source (NSLS). We strongly recommend that you explore it whether you are new to XAFS, new to the NSLS, or just want to get the most out of your application for beam time. You may also want to come back to this orientation from time to time to refresh yourself on important points. From this page, you can choose any of the modules below, or just hit the forward arrow to proceed to the Introduction to XAFS. Working through the orientation typically takes about an hour, and does not need to be done at one sitting. Formulating a Problem Proposal Writing Sample Preparation Data Collection Looking Forward to Analysis Overview of an Experiment Introduction to XAFS

What is XAS? If the energy of an x-ray photon exceeds the binding energy of an electron in a material, the x-ray can be absorbed and the electron released from its orbital. This electron then propagates as a wave and scatters off of nearby atoms. The waves can interfere (click the animations for pictorial representations of this process). At the site of the absorbing atom, the interference may be constructive, enhancing the chance that a given x-ray photon is absorbed, destructive, decreasing that chance, or somewhere in between. This animation shows constructive interference at the absorbing atom (in the middle): a wavefront from the absorbing atom is leaving just as a wavefront from the scattering atom is arriving. This animation shows destructive interference at the absorbing atom (in the middle): a wavefront from the absorbing atom is leaving midway between wavefronts from the scattering atom arriving.

Interference depends on: The energy of the incoming photon The orbital of the absorbing electron (for example, the 1s electron in a chlorine atom) The distance between the absorbing and scattering atoms The electrical potential of the absorbing and scattering atoms, which in turn depends primarily upon which elements they are

XAS Spectra Because the absorption depends on the energy of the incoming photon, an x- ray absorption spectrum is given by a plot of absorption versus photon energy. The overall shape consists of a sharp rise, when the energy of the x-ray photon first exceeds the binding energy of an electron in a particular kind of orbital (in the example on the right, the 1s orbital in iron), followed by features caused by interference as described on the previous slide. constructive interference destructive interference

XAS Spectra Because the absorption depends on the energy of the incoming photon, an x-ray absorption spectrum is given by a plot of absorption versus photon energy. To see a description of different features and regions of the spectrum, click on each term below. (All of these terms are often used somewhat loosely; they do not have “official” definitions.) Edge XANES NEXAFS EXAFS White line Pre-edge E oE o

XAS Spectra Edge This word is used in several, related meanings. It is often used to identify the orbital of the absorbing photon: “the spectrum at right is of the iron K edge.” It is sometimes used as a synonym for E o : “the edge is at 7112 eV.”E o It is often used to represent the sharply rising part of the spectrum, although perhaps stopping before the white line: “I managed to collect data over the edge, but then I lost beam.”white line

XAS Spectra XANES: Acronym for x-ray absorption near edge fine structure. The portion of the spectrum from the beginning of the edge to 30 eV or so above the edge. edge While progress is being made in calculating XANES spectra theoretically, these calculations are not yet as accurate as they are for the EXAFS part of the spectrum. EXAFS NEXAFS is a synonym for XANES, although NEXAFS tends to be used at energies below 1000 eV and XANES at higher energies.

XAS Spectra NEXAFS: Acronym for near edge x-ray absorption fine structure. The portion of the spectrum from the beginning of the edge to 30 eV or so above the edge. edge While progress is being made in calculating NEXAFS spectra theoretically, these calculations are not yet as accurate as they are for the EXAFS part of the spectrum. EXAFS XANES is a synonym for NEXAFS, although NEXAFS tends to be used at energies below 1000 eV and XANES at higher energies.

XAS Spectra EXAFS: Acronym for extended x-ray absorption fine structure. The EXAFS region of the spectrum starts about 30 eV above the beginning of the edge.edge EXAFS properly refers to the oscillations and other features visible within that region.

XAS Spectra White line: Some spectra show a sharp feature at the top of the sharply rising part of the spectrum. This feature is called the “white line.”

XAS Spectra Pre-edge: This term is used in at least two ways: To identify the fairly featureless part of the spectrum before the sharp rise associated with the edge. edge To identify small features on the rising portion of the spectrum, such as the small bump highlighted at right. For this purpose, it’s often used as an adjective: “pre-edge feature.” It is perhaps lamentable that the multiple ways in which the terms “edge” and “pre-edge” are used can lead to a feature described as “pre- edge” occuring after a point described as the “edge.” In practice, however, this strange terminology rarely causes confusion.

XAS Spectra E o : The point relative to which energies for this portion of the spectrum are measured. E o is supposed to be the “energy origin” for the electron released when the sample absorbs an x-ray, but there is not a single precise way to define this quantity. Therefore, in practice, E o may be defined in any one of a number of reproducible, but fundamentally arbitrary, ways: At the first peak of the first derivative At the largest peak of the first derivative Half way up the steep rise In such a way as to agree with theoretical models of EXAFSEXAFS From a table of elemental absorptions

Why XAS? Because EXAFS tends to die out within about 1000 eV of the edge and XANES is by definition limited to being near an edge, XAS is element specific; that is, the spectrum of one absorbing element can often be examined without much contamination from other elements in the sample. Because interference is affected by absorber-scatterer distances, XAS, particularly EXAFS, can give information about bond lengths, disorder, and radial distribution functions out to more than five angstroms in some cases. XAS, particularly XANES, yields information about the electronic structure of the absorbing atom, including valence XAS can also yield information about the type, number, and arrangement of scattering atoms XAS works for a wide variety of samples: amorphous and crystalline; solid, liquid, and gas; magnetic and nonmagnetic, etc..