The XAS experiment

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

Ax-ray source

Bmonochromator

Csample

DI_t · transmission

EI_f · fluorescence

ready·—

6962–7969 eV

transmission · drag · scroll

μ(E) · Fe foil · transmission

detector counts (live)—

I₀0%

I_t0%

I_f0%

k³χ(k)

available when scan completes

|χ(R)|

available when scan completes

sample

mode

noise

speed5×

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.

XAS data lifecycle

1
pre-edge baseline
Sub-threshold absorption from the smooth Victoreen background.
2
edge step / E₀
E₀ is an operational edge energy; Δy₀ is the normalized edge jump.
3
XANES · oxidation fingerprint
White-line position and shape encode oxidation state and site symmetry.
4
EXAFS · χ(k)
Photoelectron wavenumber; interference of outgoing + back-scattered waves.
5
Fourier transform · χ(R)
Phase-shifted peaks in |χ(R)| occur near neighbouring shells; they are not literal bond lengths.

XAS experiment demo

An X-ray absorption spectroscopy (XAS) scan sweeps a monochromator through an element-specific absorption edge while two ion chambers measure the incident and transmitted intensity (and, optionally, an off-axis detector measures the fluorescence yield). At every energy point the absorption coefficient (or for the fluorescence channel) traces a single function of energy that contains every piece of structural and electronic information XAS can deliver.

The same spectrum carries both the XANES region near the edge, where the white-line height, edge position, and pre-edge features fingerprint oxidation state and site symmetry, and the EXAFS region above it, where the slow oscillations come from interference between the outgoing photoelectron wave and waves scattered back from neighbouring atoms. Standard processing isolates from the smooth atomic background and Fourier-transforms into , an R-space magnitude whose phase-shifted peaks occur near neighbouring shells. It is not a literal radial distribution or a direct bond-length readout.

The four reference spectra here come from the public XASDataLibrary (Fe foil, FeO, Fe₂O₃, Fe₃O₄ at room temperature). Post-acquisition background subtraction (Autobk) and Fourier transform (xftf) run offline in xraylarch. The browser animates the scan and propagates simulated detector noise through that fixed preprocessing model into and its Fourier magnitude.

For the theory and analysis methods, see the companion Computational XAS course.