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

本研究旨在阐明(1)金属表面单分子电子隧穿振动模的激发机制，以及(2)过渡金属沉积在Au台阶表面形成的纳米结构材料的电子性质。我们在Pd(110)面上测量了反式-2-丁烯、顺式-2-丁烯和1，3-丁二烯的STM-IETS谱。反式-2-丁烯有一个与CH伸展相对应的强峰，而1，3-丁二烯则没有。结合HREELS、EXAFS和MOPAC理论计算，反式-2-丁烯的LUMO波函数在CH区附近有较高的态密度，而1，3-丁二烯的LUMO波函数不存在态密度。由于隧穿是通过形成负离子态发生的，其中隧穿电子被暂时俘获在LUMO态，然后激发振动，因此激发机制与LUMO波函数的对称性密切相关。进一步，揭示了在HREELS谱中可以观察到许多吸附物质的振动离子，而在IETS谱中并不总是观察到这些模式的实验问题。例如，对于顺丁烯在Pd(110)上的IETS谱，观察到了CH-strech，但没有检测到其他振动模式。我们测量了顺式-2-丁烯在Pd(110)面上的“作用谱”，发现在与底物-分子伸缩、CH3变形、C-C伸缩和CH3伸展匹配的几个阈值能量处激发了位间跃迁。这是隧穿电子激发各种振动模的有力证据，并表明作用谱与IETS谱是互补的。我们还用ARPES和XMCD研究了过渡金属准一维链的电子性质和磁化强度。对于铁链，一维链表现出有趣的磁性行为，随着温度的升高，其磁性从铁磁、超顺磁体到顺磁体变化。

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.