Quantum‐mechanical model for surface (Un‐)enhanced resonance raman scattering of adsorbates at metals. Common effects in SERRS and SUERS

A recently developed quantum‐mechanical model for surface enhanced (resonance) Raman scattering (SERRS) has some implications for surface resonance Raman scattering (SURRS) in general: The molecule‐metal interaction for SURRS at any smooth metal surface is similar to that of SERRS except for the electromagnetic surface enhancement Hence, the total SURRS process consists of (i) a lossy coupling of an induced dipole to electron hole pairs, (ii) an increase of the resonance Raman contribution over the RRS gain of the unbonded species due to a suppression of molecular dephasing and relaxation caused by energy transfer molecule → metal and (iii) the usual resonance Raman contribution; the latter two mechanisms require some molecular resonance with the exciting photon energy (or CT resonance between adatom and adsorbate). In the case of molecular (pre‐) resonance, the loss (i) and the gain (ii) cancel each other. Thus, the overall photon scattering shows either a loss (process i) or an apparently unperturbed, (pre)‐resonance Raman enhancement (e.g. process iii).