  Single molecule Raman spectroscopy provides information on individual
molecules with vibrational-level resolution. The unique mechanisms leading to
the huge Raman cross-section enhancement necessary for single molecule
sensitivity are under intense investigation in several laboratories. We
recently analyzed large spectral fluctuations in single molecule spectra of
rhodamine 6G on silver surfaces (A. Weiss and G. Haran, J. Phys. Chem. B
(2001), 105, 12348-12354). The appearance of the fluctuations in two particular
vibrational bands, and their dependence on several parameters, suggested that
they originate in a charge transfer interaction of an adsorbed molecule with
the surface. We argued that the fluctuations are due to variations of the local
work function at the position of the molecule. In the current paper the
fluctuations are further analyzed in terms of the intensity ratio between a
fluctuating and a quiescent band, and it is found that the distribution of this
ratio is independent of laser power, unlike the correlation time of the
fluctuations. We show that a simple model, based on the energetics of charge
transfer, can be used to extract the local work function distribution from the
intensity ratio distribution. In a second experiment, single molecule spectra
are collected from colloids immersed in water and in glycerol and a threefold
decrease in fluctuation rate is found in the more viscous fluid. This indicates
that surface dynamics are indeed responsible for the fluctuations, involving
the motion of the adsorbed molecule and possibly also that of surface silver
atoms around it.
