Resonance Effects in Anomalous Anti-Stokes to Stokes Ratios in Surface-Enhanced Raman Scattering (SERS)

The behavior of the anti-Stokes to Stokes (aS/S) ratio in surface-enhanced Raman scattering (SERS) was investigated for 4-mercaptophenol (4-MPh) adsorbed on a self-assembled film of silver nanoparticles. Spatially resolved Raman mapping and low excitation power were combined to minimize heating effects. The method revealed the contributions of plasmonic resonance asymmetry while enhancing the signal-to-noise ratio (SNR). The SNR increase was required to effectively measure the weak anti-Stokes signal at higher vibrational frequencies. The aS/S SERS ratios demonstrate significant wavelength-dependent deviations from the Boltzmann distribution (thermal equilibrium), confirming the role of plasmon-induced resonance effects. The average near field plasmonic resonance profile was modeled from the plasmonic resonance asymmetry factor. The experimental SERS resonance profile demonstrated a pronounced red shift relative to the average far field scattering spectra obtained by dark-field microscopy. The findings reported here enhance our understanding of resonance conditions at hotspots, providing a framework for optimizing plasmonic nanostructures.