White beam (many wavelengths, before the DCM)

Monochromatic X-ray (after the DCM, I₀)

Transmitted X-ray (I_t, dimmed by e^−μd)

Fluorescent X-ray (Kα, to SDD A and SDD B)

Scattered photoelectron (expanding)

Back-scattered return path

Current energy / μ(E) point

Double-crystal monochromator (DCM): two Si(220) crystals set the Bragg angle θ_B so that only one wavelength, λ = 2d sin θ_B, leaves the monochromator. Before the DCM the beam is white; after it, monochromatic.

Ion chambers (transmission): I₀ measures the beam before the sample; I₁ and I₂ measure it after the sample and after a reference foil. The signal falls as I_t = I₀e^−μ(E)d.

Silicon drift detectors / SDD A and SDD B (fluorescence): both sit at 90° to the beam in the horizontal plane, on opposite sides of the sample. They count the characteristic Kα X-rays emitted after the core hole is filled. The fluorescence signal grows with the absorbed fraction, 1−e^−μ(E)d.

White beam → monochromatic: the source emits many wavelengths. The double-crystal monochromator selects one, λ = 2d sin θ_B, so turning the Bragg angle scans the photon energy E = hc/λ through the absorption edge.

Pre-edge: photon energy below the 1s binding energy E₀ — the core electron stays bound, so absorption is small.

XANES: at the edge the photon ejects a 1s core electron; the 1s→4p transition (hybridized with 3d) gives the white line. Its energy and shape track oxidation state and coordination.

EXAFS: the ejected photoelectron wave scatters from neighbours and returns to interfere with the outgoing wave. The round-trip path 2R sets the oscillation χ(k) ∝ sin(2kR+φ); the amplitude decays with k (mean-free-path + thermal damping), and a Fourier transform of kχ(k) gives the neighbour distances R.

Photon wavelength: λ = hc/E. At the Cu K-edge (≈8.99 keV) the X-ray wavelength is ≈1.38 Å; the animation uses this real wavelength, so the drawn wave becomes tighter at higher energy.

Transmission vs fluorescence: an ion chamber after the sample measures I_t/I₀ = e^−μd. A fluorescence detector (SDD) measures photons emitted in all directions; its signal rises as 1−e^−μd and saturates for thick samples. The two detectors are complementary: transmission is strongest below the edge, fluorescence above it.