  Conductance measurements of a molecular wire, contacted between an epitaxial
molecule-metal bond and the tip of a scanning tunneling microscope, are
reported. Controlled retraction of the tip gradually de-hybridizes the molecule
from the metal substrate. This tunes the wire into the Kondo regime in which
the renormalized molecular transport orbital serves as spin impurity at half
filling and the Kondo resonance opens up an additional transport channel.
Numerical renormalization group simulations suggest this type of behavior to be
generic for a common class of metal molecule bonds. The results demonstrate a
new approach to single-molecule experiments with atomic-scale contact control
and prepare the way for the ab initio simulation of many-body transport through
single-molecule junctions.
