We present measurements of photon absorption by free electrons as a solid is transformed to plasma. A femtosecond x-ray free-electron laser is used to heat a solid, which separates the electron and ion heating time scales. The changes in absorption are measured with an independent probe pulse created through high-order-harmonic generation. We find an increase in electron temperature to have a relatively small impact on absorption, contrary to several predictions, whereas ion heating increases absorption. We compare the data to current theoretical and numerical approaches and find that a smoother electronic structure yields a better fit to the data, suggestive of a temperature-dependent electronic structure in warm dense matter.1 MoreReceived 29 May 2017Revised 17 January 2018DOI:https://doi.org/10.1103/PhysRevA.97.023414©2018 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasDensity functional theoryDielectric propertiesElectronic excitation & ionizationElectronic transitionsHigh-energy-density plasmasHigh-order harmonic generationInertial confinement fusionInteratomic & molecular potentialsNonlinear phenomena in plasmasOptical transient phenomenaQuantum description of light-matter interactionStrong field excitationUltrafast phenomenaUltrashort pulsesX-ray beams & opticsPlasma PhysicsAtomic, Molecular & OpticalCondensed Matter, Materials & Applied Physics