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Abstract

This article is a revue of surface enhanced Raman scattering (SERS) of molecules adsorbed on roughened silver electrodes and on well-characterized, clean Ag-surfaces in ultra high vacuum (UHV). The resonances of the SERS intensities of pyridine, pyrazine and CN^- adsorbed on roughened silver electrodes shift when using different Laser wavelengths. SERS is quenched by pulling the electrodes out of the electrolyte, thus exposing them to oxygen. The resonances are assigned to transient electron transfer within surface complexes, which arise from roughening silver electrodes.  For pyridine and pyrazine an electron is transferred from the Fermi level E_F of the silver electrode to the lowest unoccupied orbital (LUMO). In the case of CN^- the electron is transiently transferred from the highest occupied orbital (HOMO) to E_F. The energy difference between E_F and the LUMO’s of pyridine and pyrazine has been measured by inverse photoemission, in good agreement with the results of SERS. The distance dependence of SERS at well-characterized surface s in UHV shows an enhancement of about 100 restricted to the first adsorbed monolayer, on top of the long range electromagnetic (EM) enhancement. Electron energy loss spectroscopy (EELS) of pyrazine and pyridine on Ag(111) has detected weak structures assigned to electron transfer.  Charge transfer reactions induce surface resistance.