Citation: Bailie, D.; White, S.; Irwin, R.; Hyland, C.; Warwick, R.; Kettle, B.; Breslin, N.; Bland, S.N.; Chapman, D.J.; Mangles, S.P.D.; et al. K-Edge Structure in Shock-Compressed Chlorinated Parylene. Atoms 2023, 11, 135. https://doi.org/10.3390/ atoms11100135 Academic Editor: Frank B. Rosmej Received: 17 August 2023 Revised: 22 September 2023 Accepted: 13 October 2023 Published: 18 October 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). atoms Article K-Edge Structure in Shock-Compressed Chlorinated Parylene David Bailie 1 , Steven White 1 , Rachael Irwin 1 , Cormac Hyland 1 , Richard Warwick 1 , Brendan Kettle 1,2 , Nicole Breslin 1 , Simon N. Bland 2 , David J. Chapman 2 , Stuart P. D. Mangles 2 , Rory A. Baggot 2 , Eleanor R. Tubman 2 and David Riley 1, * 1 Centre for Light-Matter Interaction, School of Mathematics and Physics, Queen’s University Belfast, University Road, Belfast BT7 1NN, UK; swhite06@qub.ac.uk (S.W.); rirwin11@qub.ac.uk (R.I.) 2 Plasma Physics Group, Blackett Laboratory, Imperial College, London SW7 2BZ, UK; david.chapman@eng.ox.ac.uk (D.J.C.) * Correspondence: d.riley@qub.ac.uk Abstract: We have carried out a series of experiments to measure the Cl K-absorption edge for shock- compressed samples of chlorinated parylene. Colliding shocks allowed us to compress samples up to four times the initial density with temperatures up to 10 eV. Red shifts in the edge of about 10 eV have been measured. We have compared the measured shifts to analytical modelling using the Stewart–Pyatt model and adaptions of it, combined with estimates of density and temperature based on hydrodynamic modelling. Modelling of the edge position using density functional theory molecular dynamics (DFT-MD) was also used and it was found that good agreement was only achieved when the DFT simulations assumed conditions of lower temperature and slightly higher density than indicated by hydrodynamic simulations using a tabular equation of state. Keywords: ionization potential depression; continuum lowering; X-ray spectroscopy PACS: 52.38.Ph; 52.38.Dx; 52.70.La 1. Introduction It is commonly acknowledged in the literature relating to warm dense matter, e.g., [1–3], that it is not only a challenging state of matter to describe theoretically, but that it is also challenging to diagnose experimentally. The theoretical challenge arises from the presence of strong inter-particle correlation, partial ionisation and partial degeneracy. The diagnostic challenges include the fact that the extreme conditions of warm dense matter generally require dynamic methods of sample generation, often on timescales of nanoseconds or shorter. In addition, the density is too high for optical emission to escape, except from the surface of the sample, where the plasma temperature and density may not reflect the interior values. Further, the sample is usually too cold for X-ray emission. For this reason, there has been a concerted effort over the last couple of decades to develop X-ray probe diagnostics, such as X-ray Thomson scattering and K-edge spectroscopy, including techniques such as X-ray absorption near edge structure (XANES), which looks in the region near to the K-edge, and extended X-ray absorption fine structure (EXAFS), looking at oscillations in absorption further from the edge. Both are influenced by the environment of the species whose edge is explored. At higher temperatures, the detailed edge structure present in XANES and EXAFS tends to be washed out and the position of the edge and the slope become the features of interest. For this, the ionisation potential depression (IPD) is a key factor. Ionisation potential depression and pressure ionisation are important processes in shock- compressed warm dense matter (WDM) where the electronic structure of the ions plays a key role in the bulk properties of the matter. In the past, K-edge shift experiments [4–6] have been modelled, typically by using an ion-sphere approach to IPD. An important Atoms 2023, 11, 135. https://doi.org/10.3390/atoms11100135 https://www.mdpi.com/journal/atoms