On the nature of “sers active sites”

Active sites of short range enhancement in surface enhanced Raman scattering (SERS) of adsorbates on cold deposited, porous silver films have been envisioned either as “cavity sites with local electromagnetic resonances” or as “sites of atomic scale roughness with photon driven charge transfer between metal and adsorbate”. Whereas SERS of CO and N₂ is observed at low coverages, the Raman signal of physisorbed O₂ is below the noise level, even at monolayer coverage of the “internal surface” or “filling of the pores”. On the other hand, SERS of ethylene and pyridine is quenched by some percent of a monolayer of oxygen. The Raman signal of multilayer condensed O₂ is stronger for a “compact” film than for a porous film. These observations cannot be reconciled with the first hypothesis, but they are consistent with the second.