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.
