Ultrafast Dynamics of Electronic and Structural Modifications Induced by Photoexcitation in Cerium Oxide

Pump-probe spectroscopies utilizing X-ray free-electron lasers offer element-specific insights into the processes occurring in photocatalysts following photoexcitation, which are essential for the rational optimization of the efficiency of these materials. This study examines the dynamic evolution of the electronic and atomic structure in stoichiometric cerium oxide films following photoexcitation, employing ultrafast pump-probe X-ray absorption spectroscopy (XAS) at the Ce L3 edge in both the near-edge and extended energy ranges using an X-ray free electron laser. The results reveal a rapid relaxation pathway occurring within the first few hundred femtoseconds, followed by the formation of an excited state with a lifetime on the order of hundreds of picoseconds. The analysis of pump-probe XAS in the extended energy range identifies a structural distortion consistent with the formation of a photoinduced small polaron state. The observed time correlation between the photoinduced electronic and structural changes further reinforces the hypothesis of photoinduced polaron formation. Constrained density functional theory simulations offer insights into the electronic modifications and structural distortions in the photoexcited material. The consequences of the observed processes on material functionality are discussed.