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The XAS experiment

A live energy scan across the Fe K-edge, from synchrotron beam to χ(R).

X-ray sourceA broadband, divergent source (here, a synchrotron bend-magnet) emits photons across a wide energy range. The downstream mono picks one energy at a time.
MonochromatorA double-crystal mono (typically Si(111)) selects a single energy by Bragg diffraction. Rotating the crystals scans the energy across the absorption edge.
I₀, incident fluxIon chamber before the sample, measuring the flux arriving at it. Same hardware as the I_t chamber downstream. It is the denominator of both channels — ln(I₀/I_t) and I_f/I₀ — so it normalises away drift in the source and in the mono's throughput as the scan sweeps.
SampleAtoms here absorb photons matching their core-level binding energy. The sharp edge in μ(E) is the threshold for core-electron ionisation; the fine structure above the edge is the EXAFS signal: interference between the outgoing photoelectron and waves scattered back from neighbouring atoms.
I_t, transmissionIon chamber after the sample. μ(E)x = ln(I₀/I_t), Beer–Lambert. Best for concentrated samples (μx ≈ 1).
I_f, fluorescenceSolid-state detector at 90° to the beam, away from elastically scattered photons. Records Kα fluorescence after core-hole decay. Best for dilute samples; μ(E) ∝ I_f / I₀.
Ready
6962–7969 eV
drag to orbit, scroll to zoom
μ(E), Fe foil, transmission
700072007400760078000.01.02.03.0E₀energy (eV)μ(E)
Detector counts, live—
I₀0%
I_t0%
I_f0%
k³χ(k)
24681012k (Å⁻¹)
Available when the scan completes
|χ(R)|
123456R (Å)
Available when the scan completes
Sample
Mode
Noise
5×
Readypoint 0 / 348
—
E₀ (edge)inflection of μ(E)
7111.0 eV
Edge step Δμ₀post-edge − pre-edge
2.957
Compare FeO vs Fe₂O₃. The white-line peak shifts up by several eV: XANES fingerprints the oxidation state.
Switch to fluorescence. The transmitted-channel meter dims; the off-axis detector lights up.
Crank noise to high. The edge stays put, but the EXAFS oscillations get harder to read past k ≈ 10.
The XAS data lifecycle
  1. 1
    Pre-edge baseline
    y(E)=ln⁡ ⁣(I0/It)=μ(E)x≈CE−3y(E)=\ln\!\bigl(\textcolor{#4a9c4d}{I_0}/\textcolor{#c97e46}{I_t}\bigr)=\mu(E)x\approx C E^{-3}
    Sub-threshold absorption from the smooth Victoreen background.
  2. 2
    Edge step / E₀
    E0=arg⁡max⁡(dy/dE),Δy0=ypost(E0)−ypre(E0)E_0 = \arg\max\bigl(dy/dE\bigr),\quad \Delta y_0 = y_{\text{post}}(E_0) - y_{\text{pre}}(E_0)
    E₀ is an operational edge energy; Δy₀ is the normalized edge jump.
  3. 3
    XANES, oxidation fingerprint
    y(E)=y0(E)+Δy0 χ(E)y(E)=y_0(E)+\Delta y_0\,\chi(E)
    White-line position and shape encode oxidation state and site symmetry.
  4. 4
    EXAFS, χ(k)
    k=2meℏ2(E−E0),χ(k)=y(E)−y0(E)Δy0k = \sqrt{\tfrac{2m_e}{\hbar^2}(E - E_0)},\quad \chi(k) = \dfrac{y(E) - y_0(E)}{\Delta y_0}
    Photoelectron wavenumber; interference of outgoing + back-scattered waves.
  5. 5
    Fourier transform, χ(R)
    χ~(R)=∫kmin⁡kmax⁡k3 χ(k) W(k) e2ikR dk\tilde\chi(R) = \int_{k_{\min}}^{k_{\max}} k^{3}\,\chi(k)\,W(k)\,e^{2ikR}\,dk
    Phase-shifted peaks in |χ(R)| occur near neighbouring shells; they are not literal bond lengths.