Surface-Enhanced Raman Scattering in Small Noble-Metal Nanoparticles

We present a microscopic theory of quantum-size effects in surface-enhanced Raman scattering (SERS) from molecules adsorbed on small metal nanoparticles. In noble-metal nanoparticles, the confining potential has different effect on s-band and d-band electrons. Namely, the spillout of delocalized sp-electrons beyond the classical nanoparticle boundary results in an incomplete embedding of s-electron distribution in the background of localized d-electrons whose density profile follows more closely the classical shape. We demonstrate that a reduction of d-electron screening in the surface layer leads to the enhancement of the surface plasmon local field acting on a molecule located in a close proximity to metal surface. Our numerical calculations of Raman enhancement factor, performed using time-dependent local density approximation, show additional enhancement of the Raman signal which becomes more pronounced for small nanoparticles due to the larger ratio of surface layer to overall nanoparticle size.