Inelastic rate processes in molecular junctions: Current-induced nuclear excitation and bath-induced vibrational decoherence

The technological potential for constructing electronic devices comprised of active elements on the nano- and subnanoscale has inspired a surge of interest in the conductance of individual molecules. Several demonstrations now exist which show that the tunneling current through a molecular device can interact with the vibrational degrees of freedom to initiate chemical dynamics and mechanical manipulation. Targeting single molecules provides ample motivations for both new fundamental studies in surface chemistry as well as the construction of individually driven molecular machines. The thrust of the research presented here is to understand these large amplitude motions and develop theoretical tools capable of describing nuclear dynamics initiated in a molecular junction.;The focus in the subsequent chapters is on inelastic resonance transport which has been well studied in the contexts of gas phase scattering and surface electron spectroscopies. The rate for current-induced excitation is introduced as the observable for the nuclear dynamics and is formulated within a time independent scattering theory. Application is made to both bound-bound and bound-free transitions in the nuclear subspace and the effect of the coordinate dependence of the electronic coupling on the dynamics is discussed extensively. Having established the rigorous formulation for the excitation process, the qualitative Menzel-Gomer-Redhead theory is used along with established electronic structure methods to investigate the nature of the resonance state and its impact on the inelasticity of the charge transport. Application is made to a candidate molecular machine, a lithium rattle, and a hybrid silicon-organic system previously probed with scanning tunneling microscopy. The former study elucidates the role of charge localization in determining the extent of nuclear dynamics initiated by the current. Focusing on the dynamics induced for cyclopentene on the silicon surface demonstrates the importance of charge localization on the silicon dimer to produce inelastic events leading to desorption and possible failure of an electronic device.;The role of the environment to dissipate energy away from the reaction coordinate is examined within density matrix theory incorporating the role of electrode phonons and generation of electron-hole pairs. Vibrational relaxation is implemented using the well-known Redfield theory and adapted to the Bloch model to describe the relaxation rates to these external degrees of freedom. The model system of CO adsorbed to various transition metals is examined to validate the methods used and qualitative agreement with experiment is shown. Finally, a scattering theory of density matrices is constructed to unite the description of the current-induced excitation with energy relaxation to the dissipative environment. The competition between these rate processes is explored for a variety of bias voltages and temperatures. It is shown that the use of master equations within the secular approximation neglects important components of the quantum dynamics.