Characterization of photo-induced charge transfer and hot carrier relaxation pathways in spinel cobalt oxide (Co₃O₄)

The identities of photoexcited states in thin-film Co₃O₄ and the ultrafast carrier relaxation dynamics of Co₃O₄ are investigated with oxidation-state-specific pump-probe femtosecond core level spectroscopy. A thin-film sample is excited near the 2.8 eV optical absorption peak, and the resulting spectral changes at the 58.9 eV M_2,3-edge of cobalt are probed in transient absorption with femtosecond high-order harmonic pulses generated by a Ti/sapphire laser. The initial transient state shows a significant 2 eV redshift in the absorption edge compared to the static ground state, which indicates a reduction of the cobalt valence charge. This is confirmed by a charge transfer multiplet spectral simulation, which finds the experimentally observed extreme ultraviolet (XUV) spectrum matches the specific O^2-(2p) → Co³⁺(e_g) charge-transfer transition, out of six possible excitation pathways involving Co³⁺ and Co²⁺ in the mixed-valence material. The initial transient state has a power-dependent amplitude decay (190 ± 10 fs at 13.2 mJ/cm²) together with a slight redshift in spectral shape (535 ± 33 fs), which are ascribed to hot carrier relaxation to the band edge. The faster amplitude decay is possibly due to a decrease of charge carrier density via an Auger mechanism, as the decay rate increases when more excitation fluence is used. This study takes advantage of the oxidation-state-specificity of time-resolved XUV spectroscopy, further establishing the method as a new approach to measure ultrafast charge carrier dynamics in condensed-phase systems.