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

电子非弹性隧道过程在金属表面单分子振动模式的激发机制

基本信息

批准号：
16201023
负责人：
KAWAI Maki
金额：
$ 30.04万
依托单位：
The University of Tokyo
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (A)
财政年份：
2004
资助国家：
日本
起止时间：
2004 至 2005
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/en/grant/KAKENHI-PROJECT-16201023/
关键词：
Scanning Tunnling Microscopy Scanning Tunnling Spectroscopy Inelastic Electron Tunneling Spectroscopy Surface Vibrational Spectroscopy Single crystal surface Ultra-High Vacuum Adsorption Surface magnetism アセチレン Pd Au Cu 単一分子 STM

项目摘要

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.

本研究旨在阐明（1）金属表面单分子振动模式的隧穿电子激发机制，（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。我们测量了反式-2-丁烯、顺式-2-丁烯和1，3-丁二烯在Pd（110）上的STM-IETS光谱。对于反式-2-丁烯观察到对应于CH伸缩的强峰，但对于1，3-丁二烯没有观察到峰。结合HREELS、EXAFS和MOPAC的理论计算，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波函数在CH区附近没有态密度。由于隧穿是通过形成负离子态而发生的，其中隧穿电子被暂时捕获在LUMO态，然后振动被激发，因此激发机制与LUMO波函数中的对称性密切相关。此外，为了解决在HREELS谱中可以观察到吸附物的许多振动模式，而在IETS谱中并不总是观察到这些振动模式的实验问题。例如，顺式-2-丁烯在Pd（110）上的IETS谱中观察到CH伸缩振动，但未检测到其它振动模式。我们测量了顺式-2-丁烯在Pd（110）上的作用光谱，发现在与底物-分子伸缩、CH_3形变、C-C伸缩和CH_3伸缩相匹配的几个阈值能量处，激发了位间跳跃。这是隧道电子激发各种振动模式的有力证据，并表明作用谱与IETS谱是互补的。我们还利用ARPES和XMCD研究了过渡金属准一维链的电子性质和磁性。对于Fe链，一维链表现出有趣的磁性行为，随着温度的升高，磁性从铁磁体、超顺磁体到顺磁体变化。