Electronic and Structural Relaxation of Photoexcited WO$_3$ Observed by Femtosecond Resonant X-ray Emission Spectra

Photoexcited states of tungsten trioxide (WO$_3$) 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 L$_3$ edge XAS shifts to lower energy and the energy levels of the t$_{2g}$ and e$_g$ 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 t$_{2g}$ and e$_g$ 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 WO$_3$. This work demonstrates that the detailed dynamics of 5d elements in the femtosecond range can be addressed with HERFD-XAS/RXES.