Dr Thomas Gawne completed his undergraduate degree in Physics in 2019 and his DPhil in Physics in 2023, both at the University of Oxford. His DPhil research examined electron localization mechanisms in high energy density systems, and how this relates to continuum lowering and ionization in these systems. Tom is now a postdoctoral researcher for the Frontiers of Computational Quantum Many-Body Theory group at the Center for Advanced Systems Understanding (CASUS) and the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) in Germany. His current research focus is on the development and application of model-free x-ray Thomson scattering analysis of plasmas produced in laser and free-electron laser facilities.

The treatment of electrons in high energy density (HED) systems as bound or free is convenient: it can allow for the complex many-body equations describing a process to be reduced to tractable analytical forms, and it is computationally very efficient. In particular, the bound-or-free model is often used to quantify the degree of continuum lowering (CL) and ionisation in HED systems. However, such a treatment is also a great simplification of reality in these inherently quantum systems. Indeed, recent studies have consistently shown the bound-or-free treatment of electrons leads to significant discrepancies compared with experimental measurements.

Here, I discuss how first principles calculations can be used to shed light on the ways in which the bound-or-free model fails. By quantifying the localisation of valence states calculated using finite-temperature density functional theory, it is possible to quantify the “boundness” of these states, and thereby directly understand when it is and is not appropriate to consider electrons as bound or free. This method is then applied to examine CL and ionization potential depression in HED systems, where it is shown that the bound-or-free treatment of electrons is generally not applicable. The implications for plasma modelling are discussed.