Temperature Dependence of the Homogeneous Broadening of Resonant Raman Peaks Measured by Single-Molecule Surface-Enhanced Raman Spectroscopy

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 similar 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 similar to 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.