Bound-bound features in x-ray Thomson scattering signals*

Warm dense matter (WDM) experiments rely on diagnostic techniques such as x-ray Thomson scattering, where plasmonic features in the inelastic scattering spectrum are sensitiveto electronic temperature. Typical approaches to thermometry are based on the applicationof detailed balance to red- and blue-shifted plasmons, but various sources of uncertaintiespose challenges for this method. We propose peaks arising from bound-bound transitions, i.e., scattering into thermally depleted core orbitals, as alternative diagnostic features whichwe predict to become increasingly prominent at high temperatures. For the cases of iron and aluminum isochorically heated to 1 eV and 20 eV, our first-principles, real-time time-dependent density functional theory calculations validate an average atom model modifiedto treatd-band electrons as quasibound states. This work lays the foundation for accurate diagnostic tools as higher temperatures become accessible on XFEL platforms.