Control of relative tunneling rates in single molecule bipolar electron transport

The influence of relative electron tunneling rates on electron transport in a double-barrier single-molecule junction is studied. The junction is defined by positioning a scanning tunneling microscope tip above a copper phthalocyanine molecule adsorbed on a thin oxide film grown on the NiAl(110) surface. By tuning the tip-molecule separation, the ratio of tunneling rates through the two barriers, vacuum and oxide, is controlled. This results in dramatic changes in the relative intensities of individual conduction channels, associated with different vibronic states of the molecule.