  Ultrafast heating of solids with modern X-ray free electron lasers (XFELs)
leads to a unique set of conditions that is characterized by the simultaneous
presence of heated electrons in a cold ionic lattice. In this work, we analyze
the effect of electronic heating on the dynamic structure factor (DSF) in bulk
Aluminium (Al) with a face-centered cubic lattice and in silicon (Si) with a
crystal diamond structure using first-principles linear-response time-dependent
density functional theory simulations. We find a thermally induced red shift of
the collective plasmon excitation in both materials. In addition, we show that
the heating of the electrons in Al can lead to the formation of a
double-plasmon peak due to the extension of the Landau damping region to
smaller wavenumbers. Finally, we demonstrate that thermal effects generate a
measurable and distinct signature (peak-valley structure) in the DSF of Si at
small frequencies. Our simulations indicate that there is a variety of new
features in the spectrum of X-ray-driven solids, specifically at finite
momentum transfer, which can probed in upcoming X-ray Thomson scattering (XRTS)
experiments at various XFEL facilities.
