Surface-enhanced resonance Raman scattering (SERRS) spectra of various rhodamine dyes, of pyronine G and thiopyronine adsorbed on isolated silver clusters were recorded at the ensemble level and at the single-molecule level with a high-resolution confocal laser microscope equipped with a spectrograph and a CCD-detector. Comparing single-molecule spectra with ensemble spectra, various inhomogeneous spectral features, such as line splitting, spectral wandering, spectral diffusion and abrupt spectral jumps between different metastable spectral states, are revealed positions and the relative intensities of the vibronic bands. Resonance enhancement is investigated with respect to single-molecule surface-enhanced Raman scattering (SERS) spectroscopy and is found to be responsible for approximately three orders of magnitude in sensitivity. A significant influence of the substituents on the single-molecule SERRS sensitivity is found, showing that various chemical effects are responsible for surface enhancement in addition to the electromagnetic enhancement effect.

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