Slow fluctuations in enhanced Raman scattering and surface roughness relaxation

TL;DR

This paper explains slow fluctuations in surface-enhanced Raman scattering by surface roughness relaxation, showing how electromagnetic fields and salt influence the relaxation dynamics, aligning with experimental observations.

Contribution

It introduces a theoretical model linking surface roughness dynamics to Raman fluctuations and predicts effects of laser power and salt on relaxation times.

Findings

Electromagnetic fields accelerate surface smoothing and Raman fluctuation timescales.

Salt retards surface relaxation, increasing fluctuation times.

Model predictions align qualitatively with experimental data.

Abstract

We propose an explanation for the recently measured slow fluctuations and ``blinking'' in the surface enhanced Raman scattering (SERS) spectrum of single molecules adsorbed on a silver colloidal particle. We suggest that these fluctuations may be related to the dynamic relaxation of the surface roughness on the nanometer scale and show that there are two classes of roughness with qualitatively different dynamics. The predictions agree with measurements of surface roughness relaxation. Using a theoretical model for the kinetics of surface roughness relaxation in the presence of charges and optical electrical fields, we predict that the high-frequency electromagnetic field increases both the effective surface tension and the surface diffusion constant and thus accelerates the surface smoothing kinetics and time scale of the Raman fluctuations in manner that is linear with the laser power intensity, while the addition of salt retards the surface relaxation kinetics and increases the time scale of the fluctuations. These predictions are in qualitative agreement with the Raman experiments.