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-Fe₂O₃). Small polaron formation is evidenced by a sub-100 fs splitting of the Fe 3p core orbitals in the Fe M_2,3 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_2g conduction band. The excitation-wavelength-dependent localization of carriers by small polaron formation is potentially a limiting factor in haematite’s photoconversion efficiency.

中文翻译：

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α-Fe2O3中光激发载流子的激发波长依赖的小极化子俘获

已知小的极化子形成会限制金属氧化物光催化剂中的基态迁移率。然而，尚未确定小极化子形成在光激发态中的作用以及这如何影响光转换效率。在此，瞬态飞秒极端紫外线测量表明，小的极化子定位是导致超快捕集赤铁矿（α -Fe ₂ O ₃）中光激发载流子的原因。Fe M _2,3中 Fe 3 p核轨道的亚100 fs分裂证明了小极化子的形成边缘。小极化子形成动力学再现了通常归因于陷阱态的三重指数弛豫。但是，所测得的光谱特征仅类似于小极化子的光谱预测，而与中间隙陷阱状态所预期的前边缘特征相似。极化子形成的小概率，跳跃半径和寿命随激发波长而变化，随t _2g导带中能量的增加而减小。通过小的极化子形成，载流子与激发波长相关的局部化可能是赤铁矿光转换效率的限制因素。