High-Resolution Inelastic X-ray Scattering for Temperature and Dynamics in High-Energy-Density Matter

Measuring ion temperature in high-energy-density matter remains one of the field’s central challenges, particularly in solid-density plasmas at electronvolt-scale temperatures, where standard diagnostics fail. We demonstrate a major advance using high-resolution inelastic X-ray scattering (IXS), which enables direct, model-independent measurement of ion temperature via Doppler broadening of the Rayleigh feature in backscattering geometry. In forward-scattering geometry, the technique resolves collective ion modes, where the sound speed is obtained from the Brillouin shift, and the temperature can be independently inferred from detailed balance between the Stokes and anti-Stokes peaks.

We report results from three experiments conducted at the Matter in Extreme Conditions instrument at LCLS, each demonstrating the capabilities of high-resolution inelastic X-ray scattering (IXS) across distinct physical regimes. First, we tracked ultrafast ion heating in thin gold foils irradiated by femtosecond lasers, revealing sub-picosecond electron-ion equilibration and crystalline lattice temperatures exceeding 14 times the melting point, well beyond the predicted entropy catastrophe threshold [1]. This marks the first direct, model-free measurement of ion temperature in such an extreme regime. In shock-compressed iron, Doppler shifts and broadening enabled simultaneous measurement of bulk flow velocity and post-shock ion temperature, providing benchmark Hugoniot data to validate VISAR-based diagnostics. Finally, by extending the technique to forward-scattering geometry, we resolved Brillouin peaks from acoustic modes in warm dense methane; the dispersion across multiple scattering angles yielded a sound speed 5.9 km/s, confirming the applicability of Birch’s law in this regime [2].