Excitaion mechanism of vibraional modes of single molecue at metal surfaces by inelastic tunneling processes of electrons

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This research aims to clarify (1) the excitaion mechanism of vibrational modes of single molecule on metal surfaces by tunneling electrons, and (2) the electronic properties of nano-structured materials formed by the deposition of transition metals onto Au stepped surfaces. We measured STM-IETS spectra of trans-2-butene, cis-2-butene, and 1,3-butadiene on Pd(110). A strong peak corresponding to CH stretch was observed for trans-2-butene, but no peaks were observed for 1,3-butadiene. Combined with HREELS, EXAFS and theoretical calculations by MOPAC, the wave function of LUMO of trans-2-butene shows higher density of states around CH regions while there exists no density of states for that of 1,3-butadiene. Since tunnling occurs via formation of negative ion states where tunneling electrons are temporarily trapped at the LUMO states and then the vibrations are excited, the exciation mechanism is closely related to the symmetry in the LUMO wavefunctions. Futhremore, to unravel experimental problems that many vibraions of adsorbates are observable in HREELS spectra while these modes are not always observed in IETS spectra. For example, CH strech is observed but other vibraional modes are not detected for IETS spectrum of cis-2-butene on Pd(110). We measured "action spectra" of cis-2-butene on Pd(110) and found that the inter-site hopping was excited at several threshold energies matching substrate-molecule stretch, CH3 deformation, C-C stretch and CH3 stretch. This is a strong evidence that tunneling electrons excite various vibraional modes, and indicates that the action spectrum is comlementary to IETS spectra. We also investigated electronic properties and magnetizm of quasi one-dimensional chains of transition metals by using ARPES and XMCD. For Fe chains, 1D chains show interesting magnetic behavior that the magnetic property varies from ferromagnet, superparamagnet to paramagnet as the increase of temperature.