The hydrogen molecule in a vice

We conducted experiments on the role of molecular hydrogen physisorbed between two metallic surfaces. Such hydrogen leads to strong signatures in inelastic electron tunneling spectroscopy exceeding the amplitude of typical inelastic transitions by an order of magnitude and is accompanied by a step in the tunneling current leading under certain circumstances to a huge negative differential conductance. We show that the molecular vibration opens an inelastic channel as expected but simultaneously stalls the total elastic channel due to the forces exerted by the vibrating molecule on the electrodes. The required compliance of the stylus is surprisingly large but is possible in the wide spectrum of experimental conditions. Additionally, the inelastic channel experiences a saturation from which the lifetime of the molecular vibration can be deduced to be approximately 1 ns. This experiment allows resolving the forces corresponding to specific vibrational states of a single molecule.