Plasmonic materials are promising materials for converting light into chemical energy, which can be utilized in many different applications. Plasmons are the collective oscillation of conduction band electrons which form upon excitation with light, resulting in nano-focusing of the electromagnetic field and the generation of hot spots. Upon relaxation of the plasmon, hot electrons can be generated by electron thermalization during electron-electron scattering. Plasmon-generated hot electrons play a key role in plasmon-driven photochemistry. However, the mechanism by which these electrons drive photochemistry, as well as the lifetime and quantum yield of plasmon-derived hot electrons is poorly understood. Using ultrafast surface-enhanced Raman spectroscopy (SERS), we can probe the plasmonic-molecular interactions on the picosecond time scale. Within picoseconds, hot electrons can relax through interactions with the metal lattice or adsorbed molecules. Ultrafast SERS is a pump-probe technique where a femtosecond pump beam excites the plasmon and a picosecond probe beam obtains a Raman spectrum of the system before and after the pump beam at set time delays. Here, we investigate the ultrafast SERS response of colloidal gold nanoparticles. We observe transient peak depletions for all Raman modes of the molecular reporter, 4-nitrobenzenethiol, on the picosecond timescale. We attribute this peak depletion to a transient localized surface plasmon resonance red shift due to electron thermalization. Once generated, hot electrons delocalize across the plasmonic nanoparticle aggregates. Due to the relationship between SERS signal magnitude and the localized surface plasmon resonance of the system, we can indirectly estimate the degree of charge delocalization across a gold nanoparticle aggregate to be 0.7%. This charge delocalization corresponds to the displacement of approximately 10<sup>9</sup> electrons per aggregate. By indirectly quantifying the degree of charge delocalization, our measurements will ideally lead to improvements in device design of plasmonic substrates for plasmon-mediated photochemistry.<br/>

Probing charge delocalization in plasmonic gold nanoparticles via a molecular reporter using ultrafast surface-enhanced Raman spectroscopy

254th National Meeting (2017)

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