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.
