Raman scattering in current carrying molecular junctions. A preliminary account

TL;DR

This paper develops a theoretical framework combining NEGF and scattering theory to analyze Raman scattering in current-carrying molecular junctions, revealing additional scattering contributions under bias and their dependence on electrode coupling.

Contribution

It introduces a generalized NEGF-based approach for calculating Raman signals in molecular junctions, including vibrational effects and excited state contributions, extending previous models.

Findings

Raman signal includes contributions from excited states at high bias.

Coupling to electrodes influences the Raman scattering components.

The theory reduces to standard Raman expression for isolated molecules.

Abstract

This is a preliminary acount of a theory for Raman scattering by current-carrying molecular junctions. The approach combines a non-equilibrium Green's function (NEGF) description of the non-equilibrium junction with a generalized scattering theory formulation for evaluating the light scattering signal. This generalizes our previous study (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 effect 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.