Electronic and Structural Relaxation of Photoexcited WO₃ Observed by Femtosecond Resonant X-ray Emission Spectra

Photoexcited states of tungsten trioxide (WO3) were observed using femtosecond high-energy-resolution fluorescence detection X-ray Absorption Spectra (HERFD-XAS) and resonant X-ray emission spectra (RXES). In the initial state of the photoexcitation (∼100 fs), the W L3 edge XAS shifts to lower energy and the energy levels of the t2g and eg peaks are modulated due to the photoexcited electrons in the conduction band. The electronic state of the photoexcited W atoms is modified by 500 fs. The crystal field splitting between the t2g and eg peaks shrinks by 500 fs, which indicates local structural changes around the W atoms due to the formation of polarons. HERFD-XAS and RXES provide more details about the early state of the photoexcited states of WO3. This work demonstrates that the detailed dynamics of 5d elements in the femtosecond range can be addressed with HERFD-XAS/RXES.