Although the electromagnetic mechanism related to localized plasmons of the metal is the most important contribution in SERS, the chemical interaction between metal and molecule plays a crucial role. In SERS electrochemical experiments the adsorbates can undergo photoinduced charge transfer processes and huge energy gains up to 5eV/V have been reported which are in conflict with the classical picture of SERS (G=1). Electronic cluster calculations of metal-molecule nano clusters demonstrate that this "anomalous" gain is mainly due to two different contributions: the sensitivity of the CT electronic states of the metal-molecule system on the effective charge excess of the metal, and, secondly, the enhancement of the electric capacitance of the metal nano cluster. We are able to estimate this capacitance enhancement from SERS spectra and explain it by assuming that the strongest SERS signal comes mainly from those surface nano structures where a higher charge excess is stored.
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