Physics of single molecule fluctuations in surface enhanced Raman spectroscopy active liquids.

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.