Lazemi Masoud, Mohammad Fabian J, Park Sang Han, Katoch Abhishek, Blankesteijn Hans J F A, Botello-Méndez Andrés R, van der Minne Emma, Birkhölzer Yorick A, van den Bosch Iris C G, Kiens Ellen M, Baeumer Christoph, Koster Gertjan, Kwon Soonnam, Bergmann Uwe, de Groot Frank M F
Materials Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, Utrecht, 3584 CG, The Netherlands.
MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, Enschede, 7500 AE, The Netherlands.
Adv Mater. 2025 Jul;37(29):e2502932. doi: 10.1002/adma.202502932. Epub 2025 May 15.
Conducting real-time, element-specific studies of photo-excited systems is a long-standing challenge. The development of X-ray free-electron lasers (XFELs) has paved the way for the emergence of a promising technique: femtosecond X-ray absorption spectroscopy (fs-XAS). This powerful technique reveals electronic and geometric characteristics, providing unprecedented insight into their dynamic interactions under nonequilibrium conditions. Herein, the fs-XAS technique is employed at PAL-XFEL to unravel light-driven ultrafast electronic and structural changes in epitaxial lanthanum iron oxide (LaFeO) thin films. Density functional theory (DFT) and multiplet calculations are utilized to expound on the experimental results. The analyses reveal that photoexcitation initially induces high- and intermediate-spin Fe states through ligand-to-metal charge transfer (LMCT), followed by polaron formation. It is demonstrated that the reduced overlap between the oxygen 2p and iron 3d orbitals accounts for all experimental observations, including 1) the XAS shifts to lower energies, 2) the decrease in the crystal field splitting, and 3) the relatively larger shifts observed in the oxygen 1s XAS.
对光激发系统进行实时、元素特异性研究一直是一项长期挑战。X射线自由电子激光（XFEL）的发展为一种有前景的技术——飞秒X射线吸收光谱（fs-XAS）的出现铺平了道路。这项强大的技术揭示了电子和几何特征，为非平衡条件下它们的动态相互作用提供了前所未有的见解。在此，在PAL-XFEL上采用fs-XAS技术来揭示外延氧化镧铁（LaFeO）薄膜中光驱动的超快电子和结构变化。利用密度泛函理论（DFT）和多重态计算来阐述实验结果。分析表明，光激发最初通过配体到金属的电荷转移（LMCT）诱导高自旋和中自旋铁态，随后形成极化子。结果表明，氧2p和铁3d轨道之间重叠的减少解释了所有实验观察结果，包括1）XAS向较低能量的移动，2）晶体场分裂的减小，以及3）在氧1s XAS中观察到的相对较大的移动。
