UC Irvine

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 were able to conduct real-space measurements of rotational transitions of gaseous hydrogen molecules physisorbed on surfaces at 10 K. The j=0 to j=2 rotational transition for para-H2 and HD were observed by STM-IETS. It is also found that the rotational energy is very sensitive to its local environment, we could precisely investigate how the environmental coupling modifies the structure, including the bond length, of a single molecule with sub-Angstrom resolution. Due to this high sensitivity, the spatial variation in the potential energy surface can be quantified by the rotational and vibrational energies of the trapped H2. The ability of the tip to drag along a hydrogen molecule as it scans over another adsorbed molecule combined with the sensitivity of the hydrogen rotational excitation recorded by IETS to its immediate environment lead to the implementation of rotational spectromicroscopy. Hydrogen rotational spectroscopy and microscopy provides novels approach toward visualizing and quantifying the intermolecular interaction as well as the intermediate processes of chemical reactions.