Theory of First Layer and Single Molecule Surface Enhanced Raman Scattering (SERS)

Abstract

The first layer SERS effect is usually ascribed to dynamical charge transfer (DCT) between metal and absorbate, following a model by Persson. Experiments show that there is no first layer SERS effect at smooth noble metal surfaces, in spite of the surface resistance caused by adsorbates, which is also caused by DCT according to Persson. On the other hand, both SERS and SEIRA (surface enhanced infrared absorption) of ethylene on cold-deposited Cu display at low coverages exactly the same Raman active modes, which can be only understood by DCT. The apparent difference is attributed to special events of double scattering by DCT and atomic scale surface roughness, which prevent dephasing of the optical currents in the electromagnetic resonances. The double scattering model explains the observed change of the SERS background and SERS phonon spectra during annealing of the atomic scale roughness. Single molecule SERS is assigned to molecules within narrow and small contact zones of metal particle. The vibrations are effectively excited by the optical currents flowing through the molecule, without becoming dephased. “Blinking” (sudden changes in intensity and band positions) is assigned to thermally activated atomic scale jumps of the molecule, frozen out at low temperatures in agreement with experimental observations.