Rotational Spectroscopy at Sub-Angstrom Level: Rotationaland Vibrational Excitations of Molecular Hydrogen measured by the Scanning Tunneling Microscope

The power of rotational spectroscopy has long been demonstrated in the frequency domain by microwave spectroscopy, but its application in real space has been limited. Using a scanning tunneling microscope (STM) and inelastic electron tunneling spectroscopy (IETS), we are able to conduct real-space measurements of rotational transitions of gaseous hydrogen molecules physisorbed on Au(110) surface at 10 K. The j=0 to j=2 rotational transition for para-H$_{2}$ and HD as well as the v=0 to v=1 vibrational transitions for H$_{2}$, D$_{2}$ and HD were observed by STM-IETS. By varying the tip-substrate distance, we could precisely investigate how the environmental coupling modifies the structure, including the bond length, of a single molecule with sub-Angstrom resolution. Rotational spectroscopy at the single molecule level provides a powerful tool for chemical identification as well as bond length measurement in both the frequency and space domains.