The 'chemical' (electronic) contribution to surface-enhanced Raman scattering

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 pi* 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 sigma* 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 (c) 2005 John Wiley & Sons, Ltd.