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Plasmon photocatalysis reactions are thought to occur through vibrationally activated reactants, driven by nonthermal energy transfer from plasmon-induced hot carriers. However, a detailed quantum-state-level understanding and quantification of the activation have been lacking. Using anti-Stokes surface-enhanced Raman scattering (SERS) spectroscopy, we mapped the vibrational population distributions of reactants on plasmon-excited nanostructures. Our results reveal a highly nonthermal distribution with an anomalously enhanced population of multiquantum excited states  (v≥ 2). The shape of the distribution and its dependence on local field intensity and excitation wavelength cannot be explained by photothermal heating or vibronic optical transitions of the metal–molecule complex. Instead, it can be modeled by hot electron-molecule energy transfer mediated by the transient negative ions, establishing direct links among nonthermal reactant activation, plasmon-induced hot electrons, and negative ion resonances. Moreover, the presence of multiquantum excited reactants, which are far more reactive than those in the ground state or first excited state, presents opportunities for vibrationally controlling chemical selectivities.

The local temperatures of a metal nanostructure and its adsorbate carry essential information about the energy dissipation dynamics, calling for nanoscale thermometry techniques. Here we present a surface-enhanced Raman scattering (SERS) thermometry method providing an accurate local temperature of the adsorbates: we use the ratios of anti-Stokes (aS) and Stokes (S) SERS vibrational peaks at the limit of zero (0) probe laser intensity, extrapolated from the spectra acquired with varying laser intensities, as an internal reference of the spectrum–temperature correlation. This self-referencing removes most of the measurement bias and uncertainty created by the different electromagnetic enhancements in aS and S components of SERS spectra and enables reliable thermometry with an accuracy and a precision of <10 K. Using the method, we have quantified the photothermal heating of adsorbates on the surfaces of plasmon catalysts.