- Vibrational Excitations in Single-Molecule Junctions

Vibrational excitations have an important effect on the transportproperties of molecular junctions. Which modes are importantdepends strongly on the coupling of the molecule to the leads. Inthis chapter we analyze these effects for both strongly and weaklycoupled molecules and give approximate selection rules for thevibrational modes. For weakly coupledmolecules the low-frequencymodes tend to be dominant. Their relative intensities are deter-mined by the so-called Franck-Condon factors. For strongly coupledconjugated molecules, on the other hand, the modes involvingstretching of the π -bonds determine the transport properties. Thischapter shows how to calculate vibrational effects in both regimesfor the full vibrational spectrum of a molecule.