Molecular Transport Junctions: Propensity Rules for Inelastic Electron Tunneling Spectra

TL;DR

This paper introduces a set of propensity rules derived from a perturbation expansion of the Landauer-Imry formula to interpret inelastic electron tunneling spectra in single-molecule junctions, linking molecular structure to spectral features.

Contribution

It develops new symmetry-based and pathway-based propensity rules for analyzing IETS spectra, enhancing understanding of structure-spectra relationships in molecular electronics.

Findings

Propensity rules accurately predict IETS features for various molecular bridges.

Expansion of the Landauer-Imry formula provides a practical method for spectrum calculation.

Correlation between molecular structure and IETS spectra is improved.

Abstract

We develop a series of propensity rules for interpreting Inelastic Electron Tunneling (IET) spectra of single-molecule transport junctions. IETS has no selection rules, such as those seen in optical, infrared and Raman spectra, because IETS features arise not from the field-dipole interaction characterizing these other spectroscopies, but from vibronic modification of the electronic levels. Expansion of the Landauer-Imry formula in Taylor series in molecular normal coordinates gives a convenient, accurate perturbation-type formula for calculating both frequency and intensity of the IETS spectrum. Expansion in a Dyson-like form permits derivation of propensity rules, both symmetry-based and pathway-deduced, allowing correlation of structure and coupling geometry with the IETS spectrum. These propensity rules work very well for the calculated spectrum of four typical molecular bridges.