Observation and analysis of Fano-like lineshapes in the Raman spectra of molecules adsorbed at metal interfaces

Surface enhanced Raman spectra from molecules (bipyridyl ethylene) adsorbed on gold dumbells are observed to become increasingly asymmetric (Fano-like) at higher incident light intensity. The electronic temperature (inferred from the anti-Stokes (AS) electronic Raman signal increases at the same time while no vibrational AS scattering is seen. These observations are analyzed by assuming that the molecule-metal coupling contains an intensity dependent contribution (resulting from light-induced charge transfer transitions as well as renormalization of the molecule metal tunneling barrier). We find that interference between vibrational and electronic inelastic scattering routes is possible in the presence of strong enough electron-vibrational coupling and can in principle lead to the observed Fano-like feature in the Raman scattering profile. However the best fit to the observed results, including the dependence on incident light intensity and the associated thermal response is obtained from a model that disregards this coupling and accounts for the structure of the continuous electronic component of the Raman scattering signal. The temperatures inferred from the Raman signal are argued to be only of qualitative value.
20 pages, 12 figures