Picosecond Stabilization of Transferred Charge Carriers at Plasmonic Metal–Molecule Interfaces

Plasmonic nanoparticles efficiently absorb light across a broad spectral range, enabling energy transfer to adjacent molecules or semiconductors for photocatalytic applications. However, the nature and timescale of charge carrier involvement in these transfer processes remain a subject of ongoing debate. In this study, we employ broad-band femtosecond time-resolved infrared spectroscopy (1100-3000 cm^-1) as a sensitive probe of free charge carriers to investigate charge transfer dynamics in selected molecules adsorbed on silver nanoparticles. Charge transfer is observed exclusively under resonant excitation of the plasmon and in the presence of adsorbed molecules. Notably, the dynamics of the resulting infrared absorption vary significantly with probe frequency and molecular identity. By applying both Drude and Polaron models, we present compelling evidence that the transferred charge carriers undergo stabilization through solvation and polaron formation. As a consequence, the molecule-specific time constants for charge back-transfer extend well beyond the commonly assumed sub-picosecond regime, indicating a more complex relaxation landscape. Furthermore, the temporal evolution of light-induced changes in molecular IR modes closely parallels that of the free carrier signal, reinforcing the presence of strong charge carrier-adsorbate interactions.