Shell-isolated nanoparticle-enhanced femtosecond stimulated Raman spectroscopy reveals ultrafast molecular dynamics...

In-situ tracking of the transient processes of surface/interfacial chemical reactions is of great significance for understanding the essence of catalytic mechanisms but remains highly challenging. Herein, shell-isolated nanoparticle-enhanced femtosecond stimulated Raman spectroscopy (SHINE-FSRS) with high sensitivity and temporal resolution has been developed to explore the ultrafast molecular dynamics of surface reactions. The FSRS signals have been greatly enhanced through the fabrication of plasmonic nanocavities, achieving a maximum enhancement factor of ~10⁶ through optimal matching of the plasmonic band of the nanocavity with the wavelength of the Raman pump laser, using malachite green (MG) as the probe molecule. Using SHINE-FSRS, the ultrafast structural evolution during the photo-induced reaction of malachite green molecules adsorbed on Au or Ag surfaces has been successfully in-situ monitored at the femtosecond timescale. Direct spectroscopic evidence reveals that malachite green undergoes intramolecular twisting with 100 fs, and the strong localized plasmonic field can accelerate the twisting process. This work showcases the capability of SHINE-FSRS to effectively probe the molecular structural evolution of surface species during reactions with high temporal resolution, underscoring its potential for in-situ investigations of surface and interfacial catalytic processes.