Abstract
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.
Published Version
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