Rotational Excitation Spectroscopy with the STM through Molecular Resonances
Abstract
We 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), using rotational excitation spectroscopy (RES) with the scanning tunneling microscope. The rotational thresholds are in good agreement with J=2 transitions of freely spinning para-H2 and ortho-D2 molecules. The line shape variations in RES for H2 among the different surfaces can be traced back and naturally explained by a resonance mediated tunneling mechanism. RES data for H2/h-BN/Rh(111) suggests a local intrinsic gating on this surface due to lateral variations in the surface potential. An RES inspection of H2, HD, and D2 mixtures finally points to a multi molecule excitation, since either of the three J=0→2 rotational transitions are simultaneously present, irrespective of where the spectra were recorded in the mixed monolayer.
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