Surface enhanced Raman scattering (SERS) by molecules adsorbed at spherical particles

Area of Science:

• Physical Chemistry
• Surface Science
• Spectroscopy

Background:

• Surface-Enhanced Raman Scattering (SERS) is a phenomenon observed at interfaces, often with roughened metal surfaces.
• Previous studies suggested that molecules adsorbed on metal surfaces can exhibit enhanced Raman signals.
• The exact electromagnetic mechanisms responsible for SERS, particularly on specific geometries, require further elucidation.

Purpose of the Study:

• To develop a theoretical model for Raman scattering from a molecule adsorbed on a spherical particle.
• To investigate the potential of this model to explain SERS phenomena observed experimentally.
• To explore the influence of various parameters on SERS intensity and characteristics.

Main Methods:

• A classical electric dipole model for the adsorbed molecule.
• Inclusion of incident and near-scattered fields as the primary stimulation.
• Calculation of dipole and scattered fields at the shifted frequency.
• Analysis of feedback terms between the dipole and the particle.

Main Results:

• The model predicts significant SERS enhancement (~10^6) for pyridine on a silver sphere under specific conditions (radius << wavelength, excitation ~382 nm).
• The relative refractive index of silver near m = sqrt(2)i is crucial for strong enhancement.
• Angular distribution and polarization of Raman emission are sensitive to particle size, molecule distance, excitation wavelength, and location.

Conclusions:

• The proposed model provides a plausible electromagnetic mechanism for SERS on spherical particles.
• This mechanism is consistent with experimental observations at roughened silver electrodes.
• The study predicts similar enhancements for fluorescent scattering, suggesting broader applicability.