Single-molecule vibrational pumping in SERS

Single-molecule vibrational pumping in surface-enhanced Raman scattering (SERS) is demonstrated rigorously using the bi-analyte SERS method at low temperatures. These experiments reveal a systematic difference between the radiative SERS cross section estimated from the Stokes intensity and that obtained by pumping itself (from the anti-Stokes-to-Stokes ratio), the latter being always larger. This difference can only be reliably demonstrated in the single-molecule SERS regime, for it is otherwise affected by complications of the averaging (over the enhancement distribution) of the signals of several molecules. The findings in this paper highlight the limitations of the pumping cross-section, which cannot (in general) be taken as a reliable measure of the SERS cross-section itself. We provide a discussion of the main possible explanations for the systematic difference of the two estimates.