Experimental Measurements and Density Functional Theory Calculations of Continuum Lowering in Strongly Coupled Plasmas

An accurate description of the ionization potential depression (IPD) of ions in plasmas due to their interaction with the environment is a fundamental problem in plasma physics, playing a key role in determining the ionization balance, charge state distribution, opacity and plasma equation of state. Here I present the first experimental investigation of the IPD as a function of ionic charge state in a range of dense Mg, Al and Si plasmas, using the Linac Coherent Light Source X-ray free-electron laser. The measurements show significantly larger IPDs than are predicted by the most commonly used models, such as that of Stewart-Pyatt, or the ion-sphere model of Zimmerman-More. Instead, plasma simulations using finite-temperature density functional theory with excited-state projector augmented-wave potentials show excellent agreement with the experimental results and explain the stronger-than-expected continuum lowering through the electronic structure of the valence states in these strong-coupling conditions, which retain much of their atomic characteristics close to the ion core regions. These results have a profound impact on the understanding and modelling of plasmas over a wide range of warm- and hot-dense matter conditions. \\[4pt] [1] S.M. Vinko {\it et al.}, Nature {\bf 482}, 59 (2012).\\[0pt] [2] O. Ciricosta {\it et al.}, PRL {\bf 109}, 065002 (2012).\\[0pt] [3] S.M. Vinko {\it et al.}, Nat. Comm. {\bf 5}, 3553 (2014).\\[0pt] [4] J.C. Stewart and K.D. Pyatt, Astrophys. J. {\bf 144}, 1203-1211 (1966).\\[0pt] [5] G.B. Zimmerman and R.M. More, J. Quant. Spectrosc. Radiat. Transfer {\bf 23}, 517-522 (1980).