RESUMO
Surface enhanced Raman spectroscopy (SERS) of dyes in solution allows the study of the differences between ensemble averaged spectroscopic signals and single molecular events. We address several outstanding issues in single molecule detection via SERS; in particular, evidence for single molecule vibrational pumping and/or single molecule laser heating, the statistics of hotspots in the liquid, and anti-Stokes/Stokes anomalies. We demonstrate that anti-Stokes/Stokes ratios are a very unreliable measure of temperature, because the two processes are affected differently by the underlying frequency-dependent plasmon resonances. Subtle hints of vibrational pumping and/or heating in single molecules can only obtained via careful cross correlations between the parameters (frequency position, width, and intensity) of the Stokes signals for different excitation lasers. We introduce the use of single-peak parameter cross correlations for the study of these phenomena.
RESUMO
The possibility of achieving anti-Stokes stimulation and/or pumping under surface enhanced Raman scattering conditions has been the source of intense controversies in the literature. With the aim of clarifying some of the aspects of this problem, we study theoretically and experimentally the situation in a model system which highlights some of the difficulties in the interpretation of the data. We show that many of the assumptions often presumed in the literature need to be assessed with care in each case. Through careful experiments we show, in particular, that the anti-Stokes/Stokes ratio for a specific mode in the same sample can depend on the chosen laser wavelength. This latter effect is a manifestation of the influence of the internal plasmon resonances (hot spots) in the result. Different possibilities and further research directions are highlighted and discussed.
