Raman scattering in current-carrying molecular junctions.

We present a theory for Raman scattering by current-carrying molecular junctions. The approach combines a nonequilibrium Green's function (NEGF) description of the nonequilibrium junction with a generalized scattering theory formulation for evaluating the light scattering signal. This generalizes our previous study [M. Galperin and A. Nitzan, Phys. Rev. Lett. 95, 206802 (2005); J. Chem. Phys. 124, 234709 (2006)] of junction spectroscopy by including molecular vibrations and developing machinery for calculation of state-to-state (Raman scattering) fluxes within the NEGF formalism. For large enough voltage bias, we find that the light scattering signal contains, in addition to the normal signal associated with the molecular ground electronic state, also a contribution from the inverse process originated from the excited molecular state as well as an interference component. The effects of coupling to the electrodes and of the imposed bias on the total Raman scattering as well as its components are discussed. Our result reduces to the standard expression for Raman scattering in the isolated molecule case, i.e., in the absence of coupling to the electrodes. The theory is used to discuss the charge-transfer contribution to surface enhanced Raman scattering for molecules adsorbed on metal surfaces and its manifestation in the biased junction.