11.2 X-ray absorption and fluorescence spectroscopy

X-ray absorption and fluorescence spectroscopy are powerful techniques for probing atomic structure and composition. These methods use X-rays to excite electrons in atoms, revealing information about oxidation states, coordination environments, and elemental makeup.

XANES and EXAFS analyze absorption spectra, while XRF detects emitted X-rays. These techniques find applications in materials science, environmental studies, and archaeology. They provide valuable insights into local atomic structures and elemental compositions of various samples.

X-ray Absorption Spectroscopy

XANES and EXAFS Techniques

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• X-ray absorption near-edge structure (XANES) analyzes the absorption spectrum near the absorption edge
• XANES provides information about the oxidation state and coordination environment of the absorbing atom
• Extended X-ray absorption fine structure (EXAFS) examines the oscillations in the absorption spectrum beyond the absorption edge
• EXAFS yields data on the local atomic structure, including bond distances and coordination numbers

Absorption Edge and Synchrotron Radiation

• Absorption edge occurs when incident X-ray energy matches the binding energy of a core electron
• Edge energy depends on the element and its oxidation state
• K-edge involves 1s electrons, while L-edge involves 2s and 2p electrons
• Synchrotron radiation produces high-intensity, tunable X-rays for absorption experiments
• Synchrotron facilities generate radiation by accelerating electrons in a circular path

Applications and Data Analysis

• XANES applications include studying catalysts, environmental samples, and materials science
• EXAFS used in determining local structure of amorphous materials and dilute systems
• Data analysis involves background subtraction, normalization, and Fourier transformation
• Theoretical modeling compares experimental data with simulated spectra for structural determination

X-ray Fluorescence Spectroscopy

Principles and Instrumentation

• X-ray fluorescence (XRF) detects characteristic X-rays emitted by atoms after excitation
• Primary X-rays excite inner-shell electrons, creating vacancies filled by outer-shell electrons
• Energy difference between shells released as fluorescent X-rays
• XRF spectrometers consist of X-ray source, sample holder, and detector

Energy-dispersive and Wavelength-dispersive XRF

• Energy-dispersive XRF (EDXRF) uses a solid-state detector to measure X-ray energies directly
• EDXRF offers rapid, simultaneous multi-element analysis
• Wavelength-dispersive XRF (WDXRF) employs a crystal to diffract X-rays before detection
• WDXRF provides higher spectral resolution and lower detection limits than EDXRF

Applications and Sample Preparation

• XRF used for elemental analysis in geology, archaeology, and materials science
• Non-destructive technique suitable for analyzing solids, liquids, and powders
• Sample preparation methods include pressed pellets, fused beads, and thin films
• Quantitative analysis requires matrix correction and calibration with standards

Electron Spectroscopy

X-ray Photoelectron Spectroscopy (XPS)

• XPS analyzes the kinetic energy of photoelectrons ejected by X-ray irradiation
• Binding energy calculated from the difference between X-ray energy and photoelectron kinetic energy
• Chemical shifts in binding energies reveal information about chemical environment and oxidation state
• XPS provides surface-sensitive analysis (top 1-10 nm of the sample)

Auger Electron Spectroscopy and Instrumentation

• Auger electron spectroscopy detects electrons emitted during the Auger process
• Auger process involves filling an inner-shell vacancy and ejecting a second electron
• Auger electrons have element-specific kinetic energies independent of the excitation source
• Instrumentation includes ultra-high vacuum chamber, electron gun, and electron energy analyzer

Applications and Data Interpretation

• XPS applications include surface analysis, thin film characterization, and corrosion studies
• Auger electron spectroscopy used for elemental mapping and depth profiling
• Data interpretation involves peak identification, background subtraction, and quantification
• Spectral features include main peaks, shake-up satellites, and multiplet splitting