Conduction in molecular junctions: inelastic effects

The effect of a thermal environment on electron (or hole) transfer through molecular bridges and on the electron conduction properties of such bridges is studied. Our steady state formalism based on an extension of the Redfield theory [J. Phys. Chem. B 104 (2000) 3817; Chem. Phys. 268 (2001) 315] is extended in two ways: First, a better description of the weak-coupling limit, which accounts for the asymmetry of the energy dependence of the quasi-elastic component of the transmission is employed. Secondly, for strong coupling to the thermal bath the small polaron transformation is employed prior to the Redfield expansion. It is shown that the thermal coupling is mainly characterized by two physical parameters: the reorganization energy that measures the coupling strength and the correlation time (or its inverse - the spectral width) of the thermal bath. Implications for the observed dependence of the bridge-length dependence of the transmissions are discussed. It is argued that in the intermediate regime between tunneling behavior and site-to-site thermally induced hopping, the transport properties may depend on the interplay between the local relaxation rate and the transmission dynamics. (C) 2002 Elsevier Science B.V. All rights reserved.