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 ΔJ=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.
@article{arxiv.1403.1312,
title = {Rotational Excitation Spectroscopy with the STM through Molecular Resonances},
author = {Fabian Donat Natterer and François Patthey and Harald Brune},
journal= {arXiv preprint arXiv:1403.1312},
year = {2014}
}