M.Sc Thesis
M.Sc Student Brisker-Klaiman Daria Anharmonicity Effects in Electron Transfer through Vibrating Molecular Bridges Department of Chemistry Professor Uri Peskin

Abstract

Effects of anharmonic bridge vibrations on electronic tunneling in donor-bridge-acceptor complexes are studied using a model of anharmonic bridge vibration coupled non-linearly to an electronic degree of freedom. Electronic-nuclear coupling is assumed to displace the nuclear potential and consequently change the electronic on-site energy. Increasing electronic-nuclear coupling strength is found to accelerate electronic tunneling between the donor and the acceptor by increasing nuclear reorganization energy and thus decreasing the electronic barrier energy. An anharmonicity parameter is introduced, enabling to reproduce the standard harmonic model with linear coupling as a limiting case. The frequency of electronic tunneling oscillations between the donor and acceptor depends on the nature of the nuclear mode, where stretching and compression modes have an opposite effect on the electronic tunneling frequency. While anharmonic mode stretching increases the electronic tunneling frequency with respect to the harmonic case mode compression decreases it. Perturbative analysis in the "deep-tunneling" regime attributes this qualitative difference to the accessibility of energetically favorable pathways controlled by nuclear Franck-Condon factors. A scenario for electron-induced nuclear dissociation is considered. These phenomena, that cannot be accounted for within the harmonic approximation are analyzed and explained.