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Metal oxides exhibit remarkable catalytic properties that make them promising candidates for efficient photoelectrochemical applications. Carrier lifetime and mobility play a key role in determining their catalytic performance, making it crucial to investigate the ultrafast electronic and structural dynamics induced by photoexcitation. In CeO₂, functionality relies on the reversible exchange of oxygen ions with reactants, and the formation of photoinduced polarons can enhance reducibility and photocatalytic activity. The associated structural distortions may further lower the oxygen vacancy formation energy, improving redox functionality. A previous pump-probe optical spectroscopy study revealed an ultrafast blue shift in photoinduced absorption by 0.4 eV, attributed to the formation of small polarons within 330 fs from the excitation [1]. The present study [2] examines the dynamic evolution of the electronic and crystal structure in stoichiometric CeO₂ films following photoexcitation, employing free-electron laser based pump-probe X-ray absorption spectroscopy at the Ce L$_3$ edge in both the near-edge (XANES) and extended energy (EXAFS) ranges. The analysis of the differential pump probe spectra in the XANES region and of the dynamic evolution of the signal intensity revealed that in the first few tens of fs the electronic structure undergoes modifications that are compatible with a transient occupation of Ce 4f states. Within ~ 500 fs, the electronic structure partially relaxes to a metastable photoexcited state. This partial relaxation is accompanied by a simultaneous structural expansion of about 0.1 Å in the first shell of O atoms around the excited Ce ion as determined by the analysis of pump-probe spectra in the EXAFS region, reinforcing the hypothesis of formation of a photoinduced polaronic state. The electronic and structural modifications persist for more than 300 ps [2].
References:
1. J. S. Pelli Cresi et al., J. Phys. Chem. Lett. 11, 5686 (2020).
2. S.Pelatti, E.Spurio, D.Catone, et al. Adv. Electron. Mater.1 (2025)
| Scientific Topics | Solid State Physics |
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