Two-state analysis of single-molecule Raman spectra of crystal violet

Surface-enhanced Raman spectra of individual crystal violet molecules adsorbed on silver colloids were measured and analyzed. It was found that, because of the small number of hot spots on the surface of the colloids, the single-molecule limit is attained already at a molecule to colloid ratio of 103. Series of time-dependent spectra measured from individual molecules showed significant spectral jumps, involving essentially only the low-frequency part of the spectrum. Each spectrum could therefore be analyzed as belonging to one of two groups, depending on the ratio between its two parts. Spectral trajectories were converted into simplified two-state (high/low) trajectories. Probability distributions of the dwell times in the two states were constructed from the trajectories, and statistical analysis showed that the dynamic process underlying the spectral jumps is likely to be Markovian. We suggest that the jumps originate in fluctuations of molecule-surface charge transfer interactions, although our studies did not provide evidence for lateral motion of the molecules, as found previously with rhodamine 6G.