Resonant-Enhanced Spectroscopy of Molecular Rotations with a Scanning Tunneling Microscope

摘要

We use rotational excitation spectroscopy with a scanning tunneling microscope to investigate the rotational properties of molecular hydrogen and its isotopes physisorbed on the surfaces of graphene and hexagonal boron nitride (h-BN), grown on Ni(111), Ru(0001), and Rh(111). The rotational excitation energies are in good agreement withΔJ = 2 transitions of freely spinning p-H_2 and o-D_2 molecules. The variations of the spectral line shapes for H_2 among the different surfaces can be traced back to a molecular resonance-mediated tunneling mechanism. Our data for H_2/h-BN/Rh(111) suggest a local intrinsic gating on this surface due to lateral static dipoles. Spectra on a mixed monolayer of H_2, HD, and D_2 display all three J = 0 → 2 rotational transitions, irrespective of tip position, thus pointing to a multimolecule excitation, or molecular mobility in the physisorbed close-packed layer.