Evidence of free-bound transitions in warm dense matter

Warm dense matter (WDM) is now routinely created and probed in laboratories around the world,
providing unprecedented insights into conditions achieved in stellar atmospheres, planetary
interiors, and inertial confinement fusion experiments. However, the interpretation of these
experiments is often filtered through models with systematic errors that are difficult to quantify.
Due to the simultaneous presence of quantum degeneracy and thermal excitation, processes in
which free electrons are de-excited into thermally unoccupied bound states transferring momentum
and energy to a scattered x-ray photon become viable [1]. Here we show that such free-bound
transitions are a particular feature of WDM and vanish in the limits of cold and hot temperatures.
The inclusion of these processes into the analysis of recent X-ray Thomson Scattering (XRTS)
experiments on WDM at the National Ignition Facility [2] and the Linac Coherent Light Source [3]
significantly improves model fits, indicating that free-bound transitions have been observed without
previously being identified. This interpretation is corroborated by agreement with a recently
developed model-free thermometry technique [4,5] and presents an important step for precisely
characterizing and understanding the complex WDM state of matter.

[1] M. Böhme et al, arXiv:2306.17653
[2] T. Döppner et al, Nature 618, 270-275 (2023)
[3] D. Kraus et al, Plasma Phys. Control. Fusion 61, 014015 (2019)
[4] T. Dornheim et al, Nature Communications 13, 7911 (2022)
[5] T. Dornheim et al, Phys. Plasmas 30, 042707 (2023)