Dresden 2014 – wissenschaftliches Programm
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O: Fachverband Oberflächenphysik
O 100: Surface Dynamics II
O 100.10: Vortrag
Freitag, 4. April 2014, 12:45–13:00, PHY C 213
Rotational Excitation Spectroscopy with the Scanning Tunneling Microscope — •Fabian Donat Natterer, François Patthey, and Harald Brune — Institute of Condensed Matter Physics, École Polytechnique Fédérale de Lausanne (EPFL), Switzerland
The great potential of inelastic electron tunneling spectroscopy with the scanning tunneling microscope (STM) is based on its unmatched spatial resolution that allows for a study of the magnetic, electronic and vibrational properties of individual atoms and molecules. Complementary information about surface processes could be harvested from rotational excitations of molecules, but hitherto remained elusive. Here we show rotational excitation spectroscopy (RES) with the STM for hydrogen, its isotopes, and mixtures thereof, physisorbed on graphene and on hexagonal boron nitride. In consequence of strict symmetry requirements for molecules with identical nuclei (Pauli principle), a certain alignment of nuclear spins demands a specific set of rotational levels J. For this reason, the molecular nuclear spin states can be derived from the measured rotational levels. We observed excitation energies at 44 and 21 meV, corresponding to rotational transitions J = 0→2 for hydrogen and deuterium. Notably, these values identify the nuclear spin isomers para-H2 and ortho-D2. For H2:HD:D2 mixtures, we observe RES transitions of all three isotopes, irrespective of where the spectra were recorded; a sign of a collective excitation. Our results mark the potential of STM-RES in the study of nuclear spin states of homonuclear molecules with unprecedented spatial resolution.