How X-Ray Absorption Spectroscopy Reveals Atomic Structure

Introduction

X-ray absorption spectroscopy (XAS) is a workhorse technique for figuring out oxidation states, coordination numbers and bond distances. It complements diffraction by focusing on local structure rather than long-range order.

Background / Prerequisites

• Quantum description of core levels.
• Beer-Lambert law for absorption.
• familiarity with Fourier transforms.

Core Concepts

• XAS measures how strongly a material absorbs X-rays as the energy is scanned across an absorption edge.
• The spectrum has two regions: XANES (near-edge) and EXAFS (extended fine structure).
• Oscillations in EXAFS encode neighbor distances through interference of outgoing and backscattered photoelectrons.

Detailed Explanation

1. Data acquisition – Use a monochromator to scan photon energy. Transmission or fluorescence detectors capture intensity. Normalize by incident beam and subtract background.
2. XANES – Sensitive to oxidation state and site symmetry. Pre-edge features often indicate hybridization or coordination geometry.
3. EXAFS – After background removal, convert to photoelectron wave number k and isolate oscillations chi(k). Fourier transforming chi(k) gives peaks near real-space neighbor shells.
4. Fitting – Use reference standards or theoretical paths (FEFF) to model amplitudes and phases, yielding bond lengths, coordination numbers and disorder parameters.

Examples / Applications

• Tracking valence changes in battery cathode materials during cycling.
• Identifying ligand coordination around catalysts under operando conditions.
• Studying dopant distribution in semiconductor nanocrystals.

Common Mistakes & Tips

• Neglecting self-absorption corrections in fluorescence mode, leading to distorted amplitudes.
• Skipping duplicate scans; having at least two ensures reliability.
• Overfitting spectra with too many shells. Use statistical criteria (R-factor, reduced chi-square).

Summary / Key Takeaways

• XAS links spectral features to local atomic geometry.
• Careful preprocessing (normalization, background subtraction) is essential.
• Combining XANES and EXAFS gives both valence and distance information.

Further Reading / Related Topics

• FEFF and Artemis software tutorials.
• Difference between transmission and fluorescence detection.
• Complementary techniques: X-ray emission, resonant inelastic scattering.