  We demonstrate rotational excitation spectroscopy with the scanning tunneling
microscope for physisorbed hydrogen and its isotopes hydrogen-deuterid and
deuterium. The observed excitation energies are very close to the gas phase
values and show the expected scaling with moment of inertia. Since these
energies are characteristic for the molecular nuclear spin states we are able
to identify the para and ortho species of hydrogen and deuterium, respectively.
We thereby demonstrate nuclear spin sensitivity with unprecedented spatial
resolution.
