CuO is often employed as a photocathode for H2 evolution
and CO2 reduction, but observed efficiency is still far
below the theoretical limit. To bridge the gap requires understanding
the CuO electronic structure; however, computational efforts lack
consensus on the orbital character of the photoexcited electron. In
this study, we measure the femtosecond XANES spectra of CuO at the
Cu M2,3 and O L1 edges to track the element-specific
dynamics of electrons and holes. Results show that photoexcitation
represents an O 2p to Cu 4s charge transfer state indicating the conduction
band electron has primarily Cu 4s character. We also observe ultrafast
mixing of Cu 3d and 4s conduction band states mediated by coherent
phonons, with Cu 3d character of the photoelectron reaching a maximum
of 16%. This is the first observation of the photoexcited redox state
in CuO, and results provide a benchmark for theory where electronic
structure modeling still relies heavily on model-dependent parametrization.
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