The hydrogen molecule in a vice

TL;DR

This paper investigates how molecular hydrogen between metallic surfaces affects electron tunneling, revealing strong inelastic signatures, negative differential conductance, and insights into vibrational forces and lifetimes at the single-molecule level.

Contribution

It demonstrates the impact of molecular vibrations on tunneling conductance and provides a method to measure forces and vibrational lifetimes of a single molecule.

Findings

Strong inelastic signatures exceeding typical transitions

Observation of negative differential conductance

Vibrational lifetime of approximately 1 nanosecond

Abstract

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.