Plasma resonance enhancement of Raman scattering by pyridine adsorbed on silver or gold sol particles of size comparable to the excitation wavelength

TLDR

The authors propose a new resonance Raman phenomenon—the Raman component of resonant Mie scattering—where the metal particle polarizability is modulated by vibrations of adsorbed pyridine molecules. Resonant Mie scattering is enabled by the metal dielectric function, allowing the optimum excitation wavelength for plasma resonance‑enhanced Raman studies to be estimated for other metals. Intense, wavelength‑dependent Raman scattering of pyridine on silver or gold sol particles is observed, with a sharp resonance at the Mie extinction maximum that shifts to longer wavelengths as particle size increases, confirming the role of surface plasma oscillations.

Abstract

Intense Raman scattering by pyridine molecules adsorbed on silver or gold aqueous sol particles of dimensions comparable to the wavelength is reported. The degree of intensity enhancement is strongly dependent on the excitation wavelength, with a sharp resonance Raman maximum for excitation at the wavelength of the Mie extinction maximum of the metal particles, and for the silver sols the Raman maximum is shown to follow the extinction maximum to longer wavelengths with increase in particle size. A new resonance Raman phenomenon is thus proposed which is the Raman component of resonant Mie scattering, and in which the polarizability of the metal particles is modulated by the vibrations of the adsorbed molecules. These observations confirm that surface plasma oscillations are involved in the intense Raman scattering already reported for molecules adsorbed at roughened silver surfaces. The metal dielectric function requirements for resonant Mie scattering enable the optimum excitation wavelength for plasma resonance-enhanced Raman studies at the surface of other metals to be estimated.