exafs

**EXAFS** (Extended X-Ray Absorption Fine Structure) is the **oscillatory structure in the X-ray absorption spectrum extending 50-1000 eV above an absorption edge** — caused by interference of the outgoing photoelectron wave with backscattered waves from neighboring atoms, revealing interatomic distances, coordination numbers, and bond disorder. **How Does EXAFS Work?** - **Photoelectron**: Above the edge, a photoelectron is emitted and backscattered by neighbor atoms. - **Interference**: Constructive/destructive interference modulates the absorption coefficient. - **Fourier Transform**: The oscillation frequency encodes interatomic distances. FT of EXAFS gives radial distribution peaks. - **Fitting**: Fit to theoretical scattering paths (FEFF code) to extract $R$ (distance), $N$ (coordination), and $sigma^2$ (disorder). **Why It Matters** - **Local Structure**: Measures bond lengths to ±0.01 Å accuracy without requiring crystallinity. - **Amorphous and Liquid**: Works for any phase — amorphous, nanocrystalline, liquid, gas, solution. - **In-Situ**: Can measure under operating conditions (temperature, pressure, voltage). **EXAFS** is **measuring bond lengths with X-rays** — using photoelectron backscattering interference to determine the exact distances between atoms.