Chemical identification and manipulation of molecules by vibrational excitation via inelastic tunneling process with scanning tunneling microscopy

Recent studies of molecules on surface by the use of tunneling electrons of scanning tunneling microscopy (STM) are reviewed. Characteristic features of tunneling current of STM are used not only for real space imaging with an atomic resolution but also are utilized for chemical analysis of a single molecule and manipulation of a molecule with controlled excitation of their vibration modes. As promising candidates of chemical analysis at a single molecule level, inelastic tunneling spectroscopy (IETS) researches are discussed. The mechanism of vibration excitation in STM-IETS is compared with that in high-resolution electron energy loss spectroscopy (HR-EELS). Successful observations of vibration modes of molecules such as C-H stretching mode are introduced. At the same time, expected increase of the importance of resonant scattering mechanism in STM-IETS compared with the conventional IETS is examined, whose mechanism assumes the trapping of tunneling electrons in the adsorbate resonant state and the formation of temporary negative ions. Next, applications of tunneling electrons to the manipulations of adsorbates are discussed focusing on the phenomena induced by vibrational excitations. STM has a unique character of high current density, which cannot be obtained with conventional sources. The similarity between the vibration excitation with the high current of STM and the phenomenon known as desorption induced by multielectron transfer (DIMET) are discussed. Surface phenomena such as desorption, hopping, rotation and chemical reactions are excited as a consequence of sequential climbing of ladders of a vibrational mode formed in the potential well. The intrinsic high current of STM tunneling current enables such multiple excitations in a more controllable manner than the use of photogenerated hot electrons, and is expected to make a contribution to the understanding of elemental processes of surface phenomena.