On the widths of Stokes lines in Raman scattering from molecules adsorbed at metal surfaces and in molecular conduction junctions

TL;DR

This paper investigates the factors influencing the linewidth of Stokes lines in Raman scattering from molecules in metal junctions, highlighting the role of vibrational relaxation and electronic interactions under various bias conditions.

Contribution

It provides a theoretical analysis of the different contributions to Stokes linewidths in SERS from molecules in junctions, emphasizing the effects of electronic level proximity and bias.

Findings

Linewidth is mainly determined by vibrational relaxation rates.

Proximity of molecular electronic levels to Fermi energy broadens linewidth.

Higher bias introduces additional broadening mechanisms.

Abstract

Within a generic model we analyze the Stokes linewidth in surface enhanced Raman scattering (SERS) from molecules embedded as bridges in molecular junctions. We identify four main contributions to the off-resonant Stokes signal and show that under zero voltage bias (a situation pertaining also to standard SERS experiments) and at low bias junctions only one of these contributions is pronounced. The linewidth of this component is determined by the molecular vibrational relaxation rate, which is dominated by interactions with the essentially bosonic thermal environment when the relevant molecular electronic energy is far from the metal(s) Fermi energy(ies). It increases when the molecular electronic level is close to the metal Fermi level so that an additional vibrational relaxation channel due to electron-hole (eh) excition in the molecule opens. Other contributions to the Raman signal, of considerably broader linewidths, can become important at larger junction bias.