Effects of various halide ions and probe molecules on inelastic Mie scattering from surface enhanced Raman scattering active surfaces: Determination of particle size distributions from band shapes simulation

Extensive Raman experiments at very low frequencies arising from the scattering of mechanical vibrations of surface roughness features and colloidal particles (inelastic Mie scattering of localized acoustic vibrations) have been carried out at various laser excitation wavelengths. These low frequency bands have been studied either on electrochemically roughened silver electrodes or in colloidal silver sols, in the presence of polarizable molecules (pyridine, benzoic acid, acridine) or only with various salts. A simple model has been built and allows to account satisfactorily for the experimental band shapes. From the fit between experimental and calculated curves, we can approach the size distribution for resonant particles. These distributions display a shift of their maximum toward large size particles and a broadening of their widths when the laser excitation wavelength is turned from violet to red. An unexpected intensity enhancement of these acoustic modes, detected when excitation takes place in the red, cannot be explained as originating solely from electric dipolar plasmon resonance. Likewise, surface enhanced Raman scattering (SERS) spectra, molecular mechanisms via charge transfer complexes and/or dipolar magnetic scattering are invoked in an attempt of explanation. (C) 1996 American Institute of Physics.