Single Molecule Raman Spectroscopy at the Junctions of Large Ag Nanocrystals

Abstract

Molecular surface enhanced Raman scattering (SERS) in compact clusters of 30−70 nm Ag nanocrystals has shown single molecule Raman scattering cross sections that are orders of magnitude larger than free space single molecule luminescence cross sections. We analyze certain aspects of this phenomenon with new numerical electromagnetic calculations, and we also present new spectral depolarization data for single molecule rhodamine 6G scattering. We stress the central role of the Ag femtosecond radiative lifetime, and the spatial distribution of the excited Ag electrons, in the near field and far field optical properties. The fundamental nature of the Ag plasmon excited-electronic-state changes from a volume excitation to a surface junction excitation as two particles approach each other within 1 nm. Adsorbed molecules in the junction interact directly with the metallic excited-state wave function, showing electron-transfer-initiated photochemistry as well as enhanced Raman scattering. Depolarization studies show an uniaxial local electromagnetic symmetry at the junction site. Simultaneous intensity fluctuations in both the R6G molecular lines and the accompanying Ag electronic Raman continuum appear to reflect R6G adsorption−desorption kinetics. We outline the wavelength-dependent properties of a hybrid molecular-metallic wave function as the Raman resonant state.