Atomic-scale forces induced by a hydrogen molecule trapped in a tunneling junction

Abstract

Hydrogen molecules can be trapped in the nanocavity formed by the tip of a scanning tunneling microscope and a metal or molecular surface, and produce sharp inelastic non-linearities in the tunneling spectra. Here, we study the interaction effects of hydrogen in a tunneling junction created over Manganese phthalocyanines molecules in two oxidation states. The effect of hydrogen in the tunneling spectra varies strongly depending on the molecular species, but its force spectrum is fairly independent on the molecular state. We find that in mild tunneling conditions hydrogen-induced forces are weakly attractive during a small range of tip sample distance. The van der Waals interaction shows a maximum value of 140 pN, which faintly depends on electrostatic variations along the surface. These results show that AFM can be employed to resolve the complex interaction landscape of a trapped hydrogen molecule and deduce fainter effects such as molecular deformations or dipolar fields.