Distinct Plasmonic and Electronically Mediated Pathways for Vibrational Excitation

Understanding how plasmonic energy is transferred into molecular vibrations at metal–molecule interfaces is essential for controlling plasmon-driven chemistry and surface-enhanced spectroscopies. However, distinguishing vibrational excitation mediated by electronic transitions and vibronic coupling (VC) from plasmon-enhanced Raman vibrational pumping (PERVP) remains challenging because both mechanisms generate nonequilibrium vibrational populations. Here, we combine cryogenic surface-enhanced Raman scattering with controlled surface chemistry to investigate these competing excitation pathways using thionine adsorbed on gold nanostructures. Citrate-stabilized gold nanoparticles exhibit two distinct excitation regimes. The first is characterized by unusually intense anti-Stokes scattering despite weak Stokes emission, with laser-power dependences inconsistent with conventional vibrational pumping. This behavior is attributed to VC facilitated by molecular adsorption geometries that favor metal–molecule electronic coupling and transient charge transfer. The second regime exhibits strong Stokes scattering and superlinear anti-Stokes intensity, consistent with PERVP and molecular optomechanics. In contrast, bromide-containing gold nanorods exhibit a single excitation regime with laser-power dependences closely matching theoretical predictions for PERVP, suggesting preferential molecular dipole alignment with the localized plasmonic field. At cryogenic temperatures, pronounced anti-Stokes blinking and the transient activation of weak or Raman-inactive vibrational modes further reveal dynamic interfacial restructuring associated with bromide adsorption. These findings demonstrate how molecular orientation, interfacial electronic coupling, and surface chemistry influence the dominant vibrational excitation pathway, providing a framework for controlling plasmonic energy transfer at metal–molecule interfaces.