Small polaron formation is known to limit ground-state mobilities in metal oxide photocatalysts. However, the role of small polaron formation in the photoexcited state and how this affects the photoconversion efficiency has yet to be determined. Here, transient femtosecond extreme-ultraviolet measurements suggest that small polaron localization is responsible for the ultrafast trapping of photoexcited carriers in haematite (α-FeO). Small polaron formation is evidenced by a sub-100 fs splitting of the Fe 3p core orbitals in the Fe M edge. The small polaron formation kinetics reproduces the triple-exponential relaxation frequently attributed to trap states. However, the measured spectral signature resembles only the spectral predictions of a small polaron and not the pre-edge features expected for mid-gap trap states. The small polaron formation probability, hopping radius and lifetime varies with excitation wavelength, decreasing with increasing energy in the t conduction band. The excitation-wavelength-dependent localization of carriers by small polaron formation is potentially a limiting factor in haematite's photoconversion efficiency.

The thermalization of hot carriers and phonons gives direct insight into the scattering processes that mediate electrical and thermal transport. Obtaining the scattering rates for both hot carriers and phonons currently requires multiple measurements with incommensurate timescales. Here, transient extreme-ultraviolet (XUV) spectroscopy on the silicon 2p core level at 100 eV is used to measure hot carrier and phonon thermalization in Si(100) from tens of femtoseconds to 200 ps, following photoexcitation of the indirect transition to the Δ valley at 800 nm. The ground state XUV spectrum is first theoretically predicted using a combination of a single plasmon pole model and the Bethe-Salpeter equation with density functional theory. The excited state spectrum is predicted by incorporating the electronic effects of photo-induced state-filling, broadening, and band-gap renormalization into the ground state XUV spectrum. A time-dependent lattice deformation and expansion is also required to describe the excited state spectrum. The kinetics of these structural components match the kinetics of phonons excited from the electron-phonon and phonon-phonon scattering processes following photoexcitation. Separating the contributions of electronic and structural effects on the transient XUV spectra allows the carrier population, the population of phonons involved in inter- and intra-valley electron-phonon scattering, and the population of phonons involved in phonon-phonon scattering to be quantified as a function of delay time.

Small polaron formation limits the mobility and lifetimes of photoexcited carriers in metal oxides. As the ligand field strength increases, the carrier mobility decreases, but the effect on the photoexcited small polaron formation is still unknown. Extreme ultraviolet transient absorption spectroscopy is employed to measure small polaron formation rates and probabilities in goethite (α-FeOOH) crystalline nanorods at pump photon energies from 2.2 to 3.1 eV. The measured polaron formation time increases with excitation photon energy from 70  10 fs at 2.2 eV to 350  30 fs at 2.6 eV, whereas the polaron formation probability (85  10%) remains constant. By comparison to hematite (α-Fe2O3), an oxide analog, the role of ligand composition and metal center density in small polaron formation time is discussed. This work suggests that incorporating small changes in ligands and crystal structure could enable the control of photoexcited small polaron formation in metal oxides. TOC GRAPHICSKEYWORDS XUV; extreme-ultraviolet; high-harmonic generation; ligand-to-metal charge transfer; light harvesting; photocatalysis.