Journal
JOURNAL OF PHYSICAL CHEMISTRY C
Volume 118, Issue 39, Pages 22774-22784Publisher
AMER CHEMICAL SOC
DOI: 10.1021/jp5071133
Keywords
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Funding
- Materials Science Division of Lawrence Berkeley National Laboratory by the U.S. Department of Energy at Lawrence Berkeley National Lab [DE-AC02-05CH11231]
- NSF Engineering Research Center for Extreme Ultraviolet Science and Technology [EEC-0310717]
- U.S. Department of Defense National Security Science and Engineering Faculty Fellowship program - Office of Assistant Secretary of Defense for Research and Engineering
- Light-Material Interactions in Energy Conversion, an Energy Frontier Research Center - U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences [DE-SC0001293]
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The identities of photoexcited states in thin-film Co3O4 and the ultrafast carrier relaxation dynamics of Co3O4 are investigated with oxidation-state-specific pumpprobe 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,M-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 O2-(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/cm(2)) 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.
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