x-ray absorption spectroscopy

**XAS** (X-Ray Absorption Spectroscopy) is a **synchrotron technique that measures the absorption of X-rays as a function of energy near an elemental absorption edge** — revealing the oxidation state, coordination chemistry, and local atomic structure of a specific element. **How Does XAS Work?** - **Absorption Edge**: Tune the X-ray energy through the absorption edge of the element of interest. - **XANES**: Near-edge structure (±50 eV of edge) — fingerprint of oxidation state and coordination geometry. - **EXAFS**: Extended fine structure (50-1000 eV above edge) — oscillations from backscattering by neighboring atoms. - **Detection**: Transmission, fluorescence, or electron yield detection modes. **Why It Matters** - **Element-Specific**: Only probes the selected element — works in complex, multi-component materials. - **Chemical State**: Identifies oxidation state (e.g., Cu⁰ vs. Cu$^{2+}$, Hf$^{4+}$ bonding environment). - **Amorphous Materials**: Works equally well for crystalline and amorphous materials (unlike XRD). **XAS** is **element-specific X-ray fingerprinting** — revealing the chemical state and local atomic neighborhood of a specific element in any material.