There is significant interest in photochemical processes that occur under low intensity CW illumination at the interfaces of heterogenous photocatalysts. This is especially true in energy science where a variety of chemical reactions are envisioned to be powered by sunlight, e.g. solar fuel production, water splitting, or CO2 reduction. The dynamics of electrons in this regime of low intensity CW excitation is poorly understood because of a lack of suitable measurement strategies, in comparison with more established ultrafast, high intensity, pump-probe, time-resolved spectroscopies. Our laboratory has recently developed a spectroscopic technique that provides an unprecedented picture of non-equilibrium electronic processes that occur during relatively low power CW optical excitation. Our strategy takes advantage of an anti-Stokes Raman signal that is due to a direct interaction between photons and electrons. Unlike more familiar Raman signals which result from the coherent scattering of photons by specific vibrational modes, this electronic Raman signal gives rise to a broadband anti-Stokes spectrum that is characteristic of the entire energetic distribution of electrons during steady state illumination. To date, our experiments have measured the electronic behavior of plasmonic nanostructures under different environmental conditions and with differing chemical adsorbates. We have established that it is possible to quantitatively determine the characteristic non-equilibrium temperature of a sub-population of photo-excited electrons that have yet to thermalize with the environment. The technique also measures the size of this sub-population, and therefore the lifetime of electrons in this sub-population, as well as the magnitude of electron coupling to different relaxation pathways through coupling to molecules. Simultaneously, the technique provides quantitative measure of the phononic (or vibrational) temperature during photo-thermalization. When direct comparisons to established time-resolved can be made, for example by determining the electron-phonon coupling constant of the metal, there is excellent agreement. However, because these spectral features are characteristic of the steady state behavior of the sample, many of the insights are quite different from what is provided by ultrafast time-resolved decay signals, informing the nanoscale and atomic-scale design of metal nanostructures for use in solar photochemistry.

Steady state hot electron dynamics in nanoscale plasmonic photocatalysts

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.