Transition metal oxides (TMOs) represent promising materials for many photovoltaic and photocatalytic applications, but are often characterized by a complex physics involving the correlated interaction of multiple degrees of freedom. The investigation of TMOs relaxation dynamics calls for experimental approaches able to disentangle electronic, spin and lattice photoresponses with ultrafast time resolution. This talk will present combined femtosecond X-ray and optical studies of a spinel Co3O4, a prototypical case of the intrinsic complexity of TMOs due to its lattice and electronic structure.
Co3O4 contains Co3+ and Co2+ centres respectively occupying octahedral and tetrahedral sites surrounded by O2- anions. The two nonequivalent Cobalt centers are characterized by paired and unpaired d-orbital electronic configurations, determining the presence of Mott-Hubbard and charge transfer gaps close in energy. With ultrafast X-ray emission spectroscopy (XES) at the FXE instrument of the European X-ray free electron laser (EXFEL), we studied the temporal evolution of metal-centred transient electronic configurations, which are degenerate in the optical domain. Specifically, we excited the ligand-to-metal charge transfer (LMCT) and metal-to-metal charge transfer (MMCT) transitions of Co3O4 by pumping a 27 nm thin film at 400 nm and 800 nm, respectively. Our results rule out a stepwise relaxation cascade from the highest LMCT to the lowest d-d gap through the intermediate MMCT state, presenting a radically different picture compared to previous time-resolved optical studies on Co3O4. These results establish correlative time-resolved X-ray emission and optical spectroscopy as a novel strategy for the investigation of condensed matter systems.
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Meeting ID: 926 4861 4749
Sakura Pascarelli / Gabriella Mulá-Mathews