Transient X-ray techniques at synchrotrons, free-electron lasers, and table-top setups are pushing the boundaries of both experiment and theory. Electron and hole energies, structural distortions like phonons and polarons, and element- or layer-specific charge transfer have all been measured. However, most photocatalytic and photovoltaic devices operate under constant illumination, and access to transient X-ray measurements is still limited. In this work, we show that steady-state, illuminated X-ray measurements from a standard synchrotron beamline can distinguish fundamental photoexcited effects in materials. We use an ab-initio excited-state X-ray method, developed in our lab, to simulate a set of plasmonic core-shell nanoparticles systematically designed to isolate hot electron, photoexcitation, and thermal effects. Specifically, changes in the Ti L2,3 edge of metal-(SiO2)-TiO2 plasmonic systems are collected with and without broadband (&gt;400 nm) continuous wave photoexcitation. The presence of the SiO2 and the composition of the plasmonic metal are varied to isolate pure heating, hot electron transfer, and dipole-dipole coupling in the TiO2. Photoexcited changes in the Ti L2,3 edge of the TiO2 are confirmed using an adiabatic excited state approximation to an underlying Bethe-Salpeter equation approach (OCEAN).

Ab initio x-ray theory shows that photoexcited, hot electron, and thermal effects are distinguishable using illuminated, non-time-resolved synchrotron measurements

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