title: On the widths of Stokes lines in Raman scattering from molecules adsorbed at metal surfaces and in molecular conduction junctions.
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) exciton in the molecule opens. Other contributions to the Raman signal, of considerably broader linewidths, can become important at larger junction bias.
authors.affiliation: Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, California 92093, USA.
authors.forename: Yi
authors.initials: Y
authors.lastname: Gao
authors.affiliation: Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, California 92093, USA.
authors.forename: Michael
authors.initials: M
authors.lastname: Galperin
authors.affiliation: Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA and School of Chemistry, Tel Aviv University, Tel Aviv 69978, Israel.
authors.forename: Abraham
authors.initials: A
authors.lastname: Nitzan