We have measured the polarization and incident angle dependence of the Surface-Enhanced Raman Scattering (SERS) signal of a nile blue monolayer adsorbed on a flat gold surface. Comparisons with predictions of electromagnetic (EM) theory indicate that the molecules are predominantly adsorbed flat on the surface. These results provide the most direct demonstration of the concept of surface selection rules in SERS, and further confirm the validity of the SERS-EM model beyond the |E|(4)-approximation.

Photobleaching is a widespread problem in any optical study or application involving fluorophores, and is particularly acute in the context of single-molecule detection. Fluorophores adsorbed on metallic nanostructures are not immune to this problem, but little effort has been devoted to understanding how photobleaching is modified on metallic surface and if any additional mechanisms are involved. Here we tackle this issue both theoretically and experimentally. We present a model accounting for the modification of the standard tripletstate-mediated photobleaching mechanism on metal nanostructures supporting plasmon resonances. The wide distribution of enhancement factors on the surface is shown to result in an extremely non-exponential decay with a range of decay rates covering several orders of magnitude. This model is verified experimentally by performing time-dependent and powerdependent surface-enhanced Raman and fluorescence measurements on uniform arrays of gold nanodiscs with tunable plasmon resonances. We moreover discuss the possibility of extracting the enhancement factor distribution from such experiments. Finally, additional mechanisms of photobleaching associated with photo-induced heating of the metallic nanostructures are shown to play a role in some conditions. This study of photobleaching on nanoantennas paves the way for developing new techniques to either mitigate the problems or further exploit them.

The temperature dependence of the homogeneous and inhomogeneous broadenings of the 590 cm–1 Raman peak of the dye Nile Blue (excited at 633 nm) was measured by means of single-molecule surface-enhanced Raman spectroscopy (SM-SERS). In the investigated temperature range (298–77 K), frequency variations among single molecules are the dominant contribution to the inhomogeneous broadening, with a negligible temperature dependence. By contrast, the homogeneous broadening of SM spectra shows a sharp decrease at low temperatures, down to ∼0.9 cm–1, a remarkably low figure. The temperature dependence of the homogeneous broadening follows the predictions of the vibrational energy exchange model (VEE), with exchange modes at ∼160 cm–1, close to the Debye frequency of Ag, suggesting anharmonic coupling of the molecule with the substrate. SM-SERS provides a unique tool for observing a yet inaccessible fundamental aspect of vibrational spectroscopy of molecules, that is, anharmonic interactions of a single adsorbed molecule on a surface.