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 s cattering of mechanical vibrations of surface roughness features and c olloidal particles (inelastic Mie scattering of localized acoustic vib rations) have been carried out at various laser excitation wavelengths . These low frequency bands have been studied either on electrochemica lly roughened silver electrodes or in colloidal silver sols, in the pr esence of polarizable molecules (pyridine, benzoic acid, acridine) or only with various salts. A simple model has been built and allows to a ccount satisfactorily for the experimental band shapes. From the fit b etween experimental and calculated curves, we can approach the size di stribution for resonant particles. These distributions display a shift of their maximum toward large size particles and a broadening of thei r widths when the laser excitation wavelength is turned from violet to red. An unexpected intensity enhancement of these acoustic modes, det ected when excitation takes place in the red, cannot be explained as o riginating solely from electric dipolar plasmon resonance. Likewise, s urface enhanced Raman scattering (SERS) spectra, molecular mechanisms via charge transfer complexes and/or dipolar magnetic scattering are i nvoked in an attempt of explanation. (C) 1996 American Institute of Ph ysics.