  We study pentanedithiol molecular junctions formed by means of the
break-junction technique with a scanning tunneling microscope at low
temperatures. Using inelastic electron tunneling spectroscopy and
first-principles calculations, the response of the junction to elastic
deformation is examined. We show that this procedure makes a detailed
characterization of the molecular junction possible. In particular, our results
indicate that tunneling takes place through just a single molecule.
