§1 — Why convert units in high-pressure spectroscopy?
In a DAC experiment, the same physical quantity — the energy of a photon — is expressed in different units depending on the technique and community: the CCD spectrometer reads
nanometres (nm), Raman spectroscopists work in
wavenumbers (cm⁻¹), physicists compare in
eV, thermodynamicists think in
Kelvin (via k_BT), chemists use
kJ/mol, and synchrotron sources publish
photon energies in keV. This converter centralises all these units in a single interface.
§2 — Conversion equations (CODATA 2018)
All conversions derive from two fundamental constants that have been exact since the 2019 SI revision — h and c are defined, not measured, values. The product h·c = 1239.84193 eV·nm is therefore
exact [
NIST CODATA] :
λ (nm) = 1239.84193 / E (eV)
E (eV) = 1239.84193 / λ (nm)
ν (cm⁻¹) = 10⁷ / λ (nm)
ν (THz) = 2.99792458×10⁵ / λ (nm)
E (kJ/mol) = E (eV) × 96.4853
T (K) = E (eV) / k_B [k_B = 8.617333×10⁻⁵ eV/K]
Note: h·c = 12398.41984 eV·Å = 1239.841984 eV·nm (both forms are used depending on whether one works in ångströms or nanometres).
§3 — DAC reference wavelengths and what they are used for
| Source |
λ (nm) |
ν (cm⁻¹) |
Use in HP |
| Ruby R1 |
694.25 |
14403 |
Universal pressure gauge 0–150 GPa. Red-shifts under pressure (Ruby2020). Standard gauge in every HP lab and synchrotron. |
| Ruby R2 |
692.86 |
14433 |
Ruby R1/R2 doublet: the R1-R2 splitting = 1.37 nm at P=0 is used to detect non-hydrostaticity [Dewaele & Loubeyre 2007]. |
| Sm³⁺:YAG Y1 |
617.8 |
16187 |
Sm:YAG primary scale (Trots 2013): Y1/Y2 lines near 616–618 nm, nearly temperature-insensitive — ideal for laser-heated DAC. (the distinct SrB₄O₇:Sm²⁺ gauge sits near 685 nm.) |
| BETSA 405 nm |
405 |
24691 |
BETSA standard excitation laser for ruby and Sm:YAG. Low Raman background. Passes well through IIa and CVD anvils. |
| Nd:YAG 532 nm |
532 |
18797 |
Most common Raman laser in HP. Diamond Stokes edge at ~572–576 nm depending on pressure. Used by Akahama & Kawamura for the diamond Raman calibration to 410 GPa. |
| Nd:YAG 1064 nm |
1064 |
9398 |
LH-DAC heating laser (infrared). Absorbed by metals and oxides. Standard double-sided Nd:YAG at ESRF ID27, APS GSECARS, SPring-8 BL10XU. Not used for spectroscopy. |
§4 — Synchrotron energies and what they are used for
HP beamlines use high-energy X-ray photons (keV) to pass through the diamond anvils (~6 mm total thickness). Each beamline has its typical energies, chosen to maximise flux, minimise anvil absorption, and optimise 2θ angular coverage:
These beamlines all use membrane-type DACs — including BETSA cells — with integrated double-sided laser heating systems.
§5 — Raman shifts and absolute wavelengths
Raman spectra are reported as a Stokes shift Δν (cm⁻¹) relative to the laser. The absolute wavelength of the scattered photon is:
λ_scattered (nm) = 1 / [1/λ_laser − Δν/10⁷]
Example: diamond F₂g mode Δν = 1332.5 cm⁻¹ at P=0, excited at 532 nm → λ_scattered = 572.8 nm. At 100 GPa (Δν ≈ 1500 cm⁻¹) → λ = 575.9 nm [Akahama & Kawamura 2010,
doi:10.1088/1742-6596/215/1/012195].
§6 — References and official sources
·
NIST CODATA 2018 — Fundamental constants — exact values of h, c, k_B, N_A
·
NIST Energy Equivalents Calculator — official online converter
·
Shen et al. 2020 — Ruby2020 — AIRAPT-recommended ruby scale, λ₀ = 694.25 nm
·
Trots et al. 2013 — Sm³⁺:YAG primary scale, Y1 ≈ 617.8 nm, T-insensitive
·
Dewaele & Loubeyre 2007 — R1-R2 splitting and hydrostaticity
·
ESRF ID27 — reference European HP beamline, λ = 0.3738 Å