Characterization of Photo-Induced Charge Transfer and Hot Carrier Relaxation Pathways in Spinel Cobalt Oxide (Co₃O₄)

The identities of photoexcited states in thin-film Co<sub>3</sub>O<sub>4</sub> and the ultrafast carrier relaxation dynamics of Co<sub>3</sub>O<sub>4</sub> are studied with oxidation-state-specific pump-probe femtosecond core level spectroscopy. Near the 2.8 eV optical absorption peak, a thin-film sample is excited, and the resulting spectral changes at the 58.9 eV M<sub>2,3</sub>-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<sup>2-</sup>(2p) → Co<sup>3+</sup>(eg) charge-transfer transition, out of six possible excitation pathways involving Co<sup>3+</sup> and Co<sup>2+</sup> in the mixed-valence material. The initial transient state has a power-dependent amplitude decay (190 ± 10 fs at 13.2 mJ/cm<sup>2</sup>) 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. Our work 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.