연세대학교 이과대학 화학과

일시: 2024년 11월 14일 목요일 오후 5시

장소: 과학관 B130호

초록: Zooming Surface- and Junction-Sensitive Electron-Ionic Dynamics in State-Specific Tabletop Transient XUV Absorption/Reflection Spectroscopy

In recent decades, significant progress has been made in developing tabletop time-resolved X-ray
spectroscopy and related studies, offering both precision in probing elements, carriers, and oxidation
states, and flexibility for instrumental modifications. By utilizing gas-phase high-harmonic generation
to produce attosecond pulse trains, extreme ultraviolet (XUV) probes have enabled the exploration of
electronic localization dynamics with millielectronvolt resolution and high surface sensitivity,
leveraging the optical transitions from shallow core levels [1]. Typically, polarons are formed in
transition metal oxides through the interactions with optical phonon baths, which are dominant in the
adiabatic regime. The lattice reorganization energy in this case is so large that the first electron–optical
phonon scattering event creates a small polaron without requiring extensive carrier thermalization [2].
Using the transient XUV absorption/reflection spectroscopy, it was observed for the first time that
disrupting the iron-centered octahedra in the rare-earth orthoferrite ErFeO3 leads to the formation of
nonadiabatic polarons [3] (Fig. 1). Coherent charge hopping between neighboring Fe3+ and Fe2+ sites
persists for several picoseconds before the polaron fully forms. The observed small polaron formation
time is an order of magnitude longer than previous measurements, indicating a shallow potential well,

even in the excited state. These findings highlight the importance of accounting for dynamic electron-
electron correlations, along with electron-phonon–induced lattice changes, in understanding small

polaron behavior for applications in transport, catalysis, and photoexcitation, which can be uniquely
measured with the XUV probe.

References [1] H. Liu, et al. Chem. 7, 2569–2584 (2021), [2] L. M. Carneiro, et al. Nat. Mat. 16, 819–825 (2017),
[3] Y.-J. Kim, et al. Sci. Adv. 10, adk4282 (2024)