XAFS实验方法解读.ppt

XAFS实验方法 谢亚宁 目录 XAFS基础知识 XAFS测量方法介绍 BSRF-XAFS实验系统的使用 基于XAFS的多种实验方法介绍 BEPCⅡ工程完成后BSRF 的状况 XAFS基础知识 What Is XAFS? X-ray Absorption Fine-Structure (XAFS) is the modulation of the x-ray absorption coefficient at energies near and above an x-ray absorption edge. XAFS is also referred to as X-ray Absorption Spectroscopy (XAS) and is broken into 2 regimes: XANES X-ray Absorption Near-Edge Spectroscopy EXAFS Extended X-ray Absorption Fine-Structure which contain related, but slightly different information about an element’s local coordination and chemical state. The information one could be extracted form XAFS: ? local atomic coordination ? chemical / oxidation state The features of XAFS method: ? applies to any element ? works at low concentrations ? minimal sample requirements X-ray Fluorescence When x-rays are absorbed by the photo-electric effect, the excited core-hole will relax back to a “ground state” of the atom. A higher level core electron drops into the core hole, and a fluorescent x-ray or Auger electron is emitted. X-ray fluorescence and Auger emission occur at discrete energies that are characteristic of the absorbing atom, and can be used to identify the absorbing atom. The X-ray Absorption Coefficient: μ The intensity of an x-ray beam passing through a material of thickness t is given by the absorption coefficient μ: Absorption Edge Energies The energies of the K-edge absorption edges go roughly as EK ～ Z2 All elements with Z18 have either a K-, or L-edge between 3 and 35 keV, which can be accessed at many synchrotron sources: X-ray Absorption Measurements XAS measures the energy dependence of the x-ray absorption coefficient μ(E) at and above the absorption edge of a selected element. μ(E) can be measured two ways: Transmission: The absorption is measured directly by measuring what is transmitted through the sample: I = I0 e ? μ(E) t μ(E)t = ?ln ( I / I0 ) Fluorescence: The re-filling the deep core hole is detected.