The ‘chemical’ (electronic) contribution to surface‐enhanced Raman scattering

Abstract The model of surface‐enhanced Raman scattering (SERS) by time‐dependent evolution in the intermediate anionic state of the adsorbate is analogous to intramolecular Franck–Condon resonance Raman scattering. For adsorbates with a π* state, the residence time of some femtoseconds (10 −15 s) in the anionic state leads to a separation of electron (e) and hole (h), which quenches SERS at a smooth surface. At so‐called SERS‐active sites, the residence time of the hole is enhanced and therefore there is no final e–h pair and the excitation of only a molecular vibration leads to SERS. In contrast, for molecules with only high‐energy σ* states, the residence time in the anionic state is <1 fs (analogous to the impulse mechanism in electron scattering), and the creation of e–h pairs is less likely. This leads to first‐layer electronic Raman scattering, especially by CH stretch vibrations with an average enhancement of about 30–40‐fold. Copyright © 2005 John Wiley & Sons, Ltd.