Raman scattering from biased molecular conduction junctions: The electronic background and its temperature

Abstract

The existence of background in the surface-enhanced Raman scattering from molecules adsorbed on metal surfaces has been known since the early studies about this phenomenon and is usually attributed to transitions between electronic states of the metal substrate. This paper reformulates the theory of this phenomenon in the framework of the nonequilibrium Green function formalism, which makes it possible to extend it to the case of Raman scattering from nonequilibrium (biased and current-carrying) molecular junctions. Following recent experiments, we address, in particular, the Raman-scattering measurement of current-induced electronic heating. The Raman temperature, defined by fitting the ratio between the Stokes and the anti-Stokes Raman signals to a Boltzmann factor is compared to another measure of electronic heating obtained by assuming that, close to the molecule-metal contact, the electronic distribution is dominated by the transmission process. We find that the Raman temperatures considerably exceed this upper bound to the metal-electron heating. In agreement with this observation, we show that the Raman temperature reflects the electronic nonequilibrium in the molecular bridge itself. We also show that the Raman-temperature concept breaks down at large biases.