Time-Dependent Single-Molecule Raman Scattering as a Probe of Surface Dynamics

Abstract

Large spectral fluctuations, on a time scale of many seconds, are demonstrated in the surface-enhanced Raman scattering of single rhodamine 6G molecules adsorbed on silver nanocrystals and excited at 532 nm. These fluctuations are especially evident in the case of the vibrational bands at 614 and 774 cm -1 . A definite connection between spectral fluctuations and changes in the shape of the diffuse background emitted with the SERS spectrum is seen, pointing to the possible origin of both in a molecule-surface charge transfer phenomenon. The rate of spectral fluctuations is shown to depend linearly on laser intensity through a nonthermal mechanism. The rate of fluctuations can also be modified by changing the anion concentration in the solution on top of the silver colloids. Analysis of the results suggests that the fluctuations are due to motion of the adsorbed molecule on the surface, triggered by photodesorption events that are mediated by electron tunneling between the molecule and surface. Our results show that single-molecule Raman scattering can serve as a sensitive local probe for the dynamics of adsorbed molecules under ambient conditions.