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 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, PPCF 61, 014015 (2019)
[4] T. Dornheim et al, Nature Commun 13, 7911 (2022)
[5] T. Dornheim et al, POP 30, 042707 (2023)