On the electronic contribution to single molecule surface enhanced Raman spectroscopy

Among the metallic structures supporting single molecule surface enhanced Raman scattering (SERS) this article considers single molecules in junction sites between metallic nanoparticles in small clusters. Though high electromagnetic fields can exist in these junctions, they do not increase below gap sizes, where electron tunneling sets in. Following the experimental and theoretical work oil DC currents in metal-single molecule-metal systems, a model of resonant optical tunneling via the orbitals of the molecules in the junctions is developed. Since "canonical SERS" is the "many molecules-many hot sites" ensemble of SM-SERS. we discuss cold-deposited silver films in this respect as ensemble of SM-junction-SERS. This is supported by DC resistance measurements during annealing of the cold-deposited films. The individual changes of the DC resistance during exposure of the cold-deposited films to various gases and the simultaneous development of the SERS intensity supports the model of a hybrid field- and electronic SM-SERS in junctions. In cold-deposited films. the electronic contribution to SM-SERS is about 3 orders of magnitude. One may state. that the field part in functions enables that Raman signals can be measured, but the electronic part decides, what is seen. (when no intramolecular resonance Raman effect is involved).