Investigating electron transfer across single-molecule junctions

Electron transfer processes are investigated through conductance measurements of single molecules. Measurements are performed on metal-molecule-metal junctions using a modified scanning tunneling microscope technique. Through a series of experimental measurements, and accompanying theoretical models, the influence of the molecule on the measured current is explored. These explorations are presented in five separate chapters. In chapter two, the molecular orbitals of sp-hybridized carbon chains are discussed in detail. It is demonstrated that the molecular orbitals can assume an intriguing helical shape. In chapter three, the length-dependent conductance of sp²-hybridized carbon chains is investigated. Experiment and theory demonstrate that the conductance of odd-numbered chains is nearly uniform with length. In chapter four, a new theoretical scheme to calculate quantum interference is developed. Using this scheme, it is demonstrated that quantum interference yields the decay in conductance with length for molecular wires. In chapter five, current-voltage measurements of redox-active molecular clusters are shown to agree with a hopping transport model. In chapter six, a novel experimental setup is presented that can be used to investigate photoconductivity in single-molecule junctions. This thesis provides a broad, yet rigorous, survey of electron transfer processes in single-molecule junctions.