Optical properties of highly compressed polystyrene: An ab initio study

Abstract

Using all-electron density functional theory, we have performed an ab initio study on x-ray absorption spectra of highly compressed polystyrene (CH). We found that the $K$-edge shifts in strongly coupled, degenerate polystyrene cannot be explained by existing continuum-lowering models adopted in traditional plasma physics. To gain insights into the $K$-edge shift in warm, dense CH, we have developed a model designated as ``single mixture in a box'' (SMIAB), which incorporates both the lowering of the continuum and the rising of the Fermi surface resulting from high compression. This simple SMIAB model correctly predicts the $K$-edge shift of carbon in highly compressed CH in good agreement with results from quantum molecular dynamics (QMD) calculations. Traditional opacity models failed to give the proper $K$-edge shifts as the CH density increased. Based on QMD calculations, we have established a first-principles opacity table (FPOT) for CH in a wide range of densities and temperatures [$\ensuremath{\rho}=0.1\ensuremath{-}100\phantom{\rule{0.16em}{0ex}}\mathrm{g}/\mathrm{c}{\mathrm{m}}^{3}$ and $T=2000\ensuremath{-}1\phantom{\rule{0.16em}{0ex}}000\phantom{\rule{0.16em}{0ex}}000\phantom{\rule{0.16em}{0ex}}\mathrm{K}$]. The FPOT gives much higher Rosseland mean opacity compared to the cold-opacity--patched astrophysics opacity table for warm, dense CH and favorably compares to the newly improved Los Alamos atomic model for moderately compressed CH (${\ensuremath{\rho}}_{\mathrm{CH}}\ensuremath{\le}10\phantom{\rule{0.16em}{0ex}}\mathrm{g}/\mathrm{c}{\mathrm{m}}^{3}$), but remains a factor of 2 to 3 higher at extremely high densities (${\ensuremath{\rho}}_{\mathrm{CH}}\ensuremath{\ge}50\phantom{\rule{0.16em}{0ex}}\mathrm{g}/\mathrm{c}{\mathrm{m}}^{3}$). We anticipate the established FPOT of CH will find important applications to reliable designs of high-energy-density experiments. Moreover, the understanding of $K$-edge shifting revealed in this study could provide guides for improving the traditional opacity models to properly handle the strongly coupled and degenerate conditions.