Probing ultrafast plasmon-molecule interactions using surface-enhanced femtosecond stimulated Raman spectroscopy

Plasmonic nanomaterials can concentrate electromagnetic fields into nanoscale volumes known as hotspots. These highly energetic hotspot regions can impact a nearby molecule by altering its potential energy surface through energy or electron transfer processes. These processes happen at the sub picosecond timescale of plasmon decay making it difficult to probe the underlying mechanism involved in plasmon-molecule interaction. The lack of mechanistic understanding hinders the optimization of plasmonic nanomaterials on an industrially relevant scale. This work focuses on the development of a time-resolved surface-enhanced femtosecond stimulated Raman spectroscopy (SE-FSRS) technique, with the aim of learning about the plasmon decay process through its impact on the nearby adsorbate molecule. SE-FSRS can take molecular snapshots in the form of vibrational signatures revealing structural changes that the molecules undergo during the process of plasmon decay in femtosecond timescale. We have developed a three-pulse SE-FSRS system and acquired vibrational spectra of molecules evolving in plasmonic hotspots on ultrafast timescales. This approach will allow for a greater understanding of how photoexcited plasmons impact molecular potential energy landscapes on femtosecond timescales, guiding insights into the use of plasmonic materials for catalysis, photothermal therapy, and sensing applications.