§1 — The electromagnetic spectrum and the DAC: overview
In a DAC experiment, five spectral regions are used simultaneously. The Nd:YAG laser heats the sample in the near-infrared, ruby or Sm:YAG fluorescence gives the pressure in the visible, hard X-rays diffract through the anvils to establish the crystal structure and EOS, and soft X-rays probe the electronic state of iron. This versatility — made possible by diamond’s exceptional transparency across a broad spectral window — is what makes the DAC the reference instrument of high-pressure physics. [Fedotenko et al. 2019, Rev. Sci. Instrum. 90:104501]

§2 — Diamond transmission window
Type IIa diamond is transparent from the near UV (∼225 nm) to the far infrared (∼300 μm), with one notable exception: a two-phonon absorption band between 4 and 5 μm (2000–2500 cm⁻¹). This window is exploited both to deliver laser radiation through the anvils without heating them, and to collect optical signals from the sample (fluorescence, Raman, spectroradiometry). Beyond 10 keV, the diamond absorption coefficient is low enough to allow X-ray transmission through several millimetres of anvils — the condition that makes DAC diffraction possible.

§3 — Near-infrared NIR (700 nm – 2.5 μm): Nd:YAG laser heating
The Nd:YAG laser emits at 1064 nm, within diamond’s transparency window. The beam passes through the anvils and is selectively absorbed by the sample — metals, oxides and silicates absorb strongly at 1 μm, unlike pressure-transmitting media (NaCl, MgO, He) which remain transparent. Temperatures of 3000 to 7000 K are generated within a ∼10 μm diameter volume. Temperature is measured by spectroradiometry: the thermal emission spectrum of the sample (600–900 nm) is fitted to Planck’s law [Heinz & Jeanloz 1987, Geophys. Monogr. 39:113]. Double-sided heating — two beams attacking both faces of the sample simultaneously — reduces axial thermal gradients and is now standard on all major beamlines [Campbell 2008, Rev. Sci. Instrum. 79:015108].

§4 — Mid-infrared MIR (2.5–25 μm): CO₂ laser and FTIR
Some materials — MgO, pure silicates, noble gases — do not absorb at 1064 nm. The CO₂ laser at 10.6 μm (943 cm⁻¹) overcomes this limitation: at that wavelength diamond remains transparent (the two-phonon band is at 4–5 μm), while Si–O stretching modes of silicates absorb efficiently. Brillouin scattering measurements on MgO and pyrope at high temperature use this configuration [C. R. Géoscience 2018]. The same MIR window is used for FTIR spectroscopy in the DAC to characterize molecular bonds (H₂O, CO₂, N₂) under pressure.

§5 — Visible (380–700 nm): pressure gauges and Raman
Diamond is fully transparent in the visible — the main optical working window in DAC.
• Ruby fluorescence R1 = 694.25 nm: excited at 405 nm (BETSA standard) or 532 nm. The R1 line shifts to longer wavelengths under pressure. The P(Δλ) relation is defined by the Ruby2020 scale, validated to 150 GPa and endorsed by the AIRAPT [Shen et al. 2020, High Press. Res. 40:299].
• Diamond Raman: F_2g mode at 1332.5 cm⁻¹ (P = 0), calibrated to 410 GPa [Akahama & Kawamura 2010, J. Phys. Conf. Ser. 215:012195].
• Spectroradiometry (600–900 nm): Planck-function fitting for temperature measurement in LH-DAC.

§6 — Ultraviolet (200–380 nm): electronic transitions
Type IIa diamond absorbs below ∼225 nm; it is transparent between 225 and 380 nm. This window is used for UV–vis absorption spectroscopy of electronic transitions: band gaps, insulator–metal transitions, and iron spin crossover characterization in combination with XES [Lin et al. 2013, Rev. Geophys. 51:244].

§7 — Soft X-rays (0.1–10 keV): electronic structure
Strongly absorbed by diamond — Be anvils or nano-polycrystalline diamond (NPD) anvils, or radial-access geometries, are required. Three techniques are central:
• XANES at the iron K-edge (7.1 keV): oxidation state and coordination symmetry of iron in mantle silicates under pressure.
• XES (Kβ line, ∼7058 eV): line shape changes with spin state (high-spin / low-spin) — the direct technique for documenting spin crossover at 40–150 GPa.
• NRIXS (Nuclear Resonant Inelastic X-ray Scattering): partial phonon density of states of iron, sound velocity, Grüneisen parameter. Performed at ESRF ID18 and APS 3-ID-B.

§8 — Hard X-rays (10–100 keV): diffraction and EOS
Low diamond absorption at these energies is the condition that makes DAC diffraction possible. Powder diffraction gives lattice parameters as a function of P and T → equation of state (EOS). Major dedicated beamlines:
• ESRF ID27 (Grenoble): 33 keV, λ = 0.374 Å, beam focused to ≤1 μm
• PETRA-III P02.2 (DESY, Hamburg): 25.6 and 42.7 keV
• APS GSECARS 13-ID-D (Argonne): ∼42 keV, λ = 0.295 Å
• SPring-8 BL10XU (Hyogo): 30–60 keV
In-laboratory single-crystal diffraction uses Mo Kα₁ = 0.70930 Å (17.48 keV).

§9 — Mössbauer and beyond (>100 keV)
⁵⁷Fe Mössbauer spectroscopy exploits the nuclear resonance at 14.4 keV to probe the hyperfine field, spin state and oxidation state of iron under pressure — without ambiguity, unlike indirect methods. With a ⁵⁷Co source, it is usable in the laboratory to ∼100 GPa. At synchrotrons, Nuclear Forward Scattering (NFS) at ESRF ID18 and APS 3-ID-D enables time-domain Mössbauer experiments to 200 GPa and beyond, with BETSA cells used as standard on these beamlines.