Real Space Rotational Spectroscopy: Measurement of the Rotational Excitation of a Single Molecule by the Scanning Tunneling Microscope

The power of rotational transition spectra has long been demonstrated in the frequency domain by microwave spectroscopy, but its application in real space is 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. By varying the tip-substrate distance, we could precisely investigate how the environmental coupling modifies the structure 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 frequency and space domains.