Raman scattering and electronic heating in molecular conduction junctions

Surface-enhanced Raman scattering (SERS) was recently used to monitor nonequilibrium properties of molecular conduction junctions. Ward et al. (Nat. Nanotechnol. 2011, 6, 33) have used such measurements to estimate heating of the molecular vibrations (indicated by the ratio between Stokes and anti-Stokes Raman peaks) as well as the electronic metal substrate (inferred from the corresponding components of the Raman continuum). The latter observation suggests, contrary to standard assumptions, significant heating of the metal contacts. Here, we discuss this observation by advancing a theory of the electronic Raman scattering background in biased current carrying molecular junctions and using it to estimate the electronic heating, as seen in the Raman signal. We reach the unexpected conclusion that while heating of the electronic background in Raman scattering from biased molecular junctions is indeed observed, this does not necessarily imply an appreciable deviation from thermal equilibrium in the electronic distributions in the leads.