Visible and infra-red light emitted at a Ag-Ag(111) junction has been investigated from tunneling to single atom contact conditions with a scanning tunneling microscope. The light intensity varies in a highly nonlinear fashion with the conductance of
the junction and exhibits a minimum at conductances close to the conductance quantum. The data are interpreted in terms of current noise at optical frequencies, which is characteristic of partially open transport channels.
The charge flow from a single C60 molecule to another one has been probed. The conformation and electronic states of both molecules on the contacting electrodes have been characterized using a cryogenic scanning tunneling microscope. While the contac
t conductance of a single molecule between two Cu electrodes can vary up to a factor of three depending on electrode geometry, the conductance of the C60-C60 contact is consistently lower by two orders of magnitude. First-principles transport calculations reproduce the experimental results, allow a determination of the actual C60-C60 distances, and identify the essential role of the intermolecular link in bi- and trimolecular chains.
