Monitoring Charge Density Delocalization upon Plasmon Excitation with Ultrafast Surface-Enhanced Raman Spectroscopy

Plasmonic materials hold a great deal of potential for advances in energy and chemical applications by converting light into chemical energy. Many of these processes are aided by plasmon-generated hot electrons. However, the interaction between these hot electrons and adsorbed molecules on a plasmonic substrate is not well understood. Using ultrafast surface-enhanced Raman spectroscopy, we monitor plasmon-molecule interactions in real time using 4-nitrobenzenethiol as a molecular probe. Upon plasmon excitation, we observe transient peak depletions in our ultrafast surface-enhanced Raman spectra on the picosecond time scale. We attribute this peak depletion to a localized surface plasmon resonance red shift as a result of hot electron generation in our aggregated nanoparticles. Once generated, the hot electrons delocalize across the aggregate. By correlating the magnitude of the transient Raman dynamics with the degree of electron delocalization, we estimate charge displacement on the order of 10⁹ electrons per aggregate. This indirect quantification of hot electron delocalization on aggregated nanoparticles will be of use in the rational design of materials for efficient plasmon-driven photochemistry.