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Vibration and Reaction Dynamics of Single Molecules at Surfaces
Professor Maki Kawai has been and is one of the scientists to represent the spectroscopic field of molecules at surfaces. Major achievements include spatially selected single molecule to observe the electronic, vibrational and spin structure in a quantitative manner. State selective excitation of a molecule and to induce reaction has also been possible. Single molecule spectroscopy as such will be introduced, including a new method to obtain vibrational spectra taking the response in the efficiency of a reaction to the bias voltage applied between the STM tip and the molecule at a surface, i.e. an action spectroscopy.
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Prof. Maki Kawai
The University of Tokyo
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1. Vibrational excitation and reaction of single molecules using inelastically tunneled electrons<
Scanning tunneling microscope (STM) is a tool to map the electronic density of states with the spatial resolution of an atomic scale, simultaneously is used as an electron source that targets a single molecule or atom at surfaces and excites its vibrational states via inelastic electron processes. Vibration spectroscopy and/or chemical reaction of a single molecule is available Prof. Kawai and her group have shown examples for state selective reactions and further more, they pointed out, for the first time, the importance of anharmonic coupling between vibrational states as the mechanism behind.
2. Single molecule vibrational spectroscopy
Prof. Kawai and her group have shown that vibrational spectroscopy of a single molecule is available both by an inelastic tunneling spectroscopy and also by an action spectroscopy using STM. Action spectroscopy is a vibrational spectroscopy acquired utilizing the instability of an adsorbed molecule and is one of her unique achievements. When kinetic energy of tunneling electrons exceeds the energies for elemental excitations, a new tunneling path opens that leads to the change in the conductance. The tunneling current thus reflects the signals of vibrational excitation and is utilized to characterize single molecules. In spite of the superior character that STM-IETS has, there are limited molecular vibrations detected so far, including some of the C-H stretching vibrations. Prof. Kawai has focused her attention to the fact that molecular motion are also induced when vibrationally excited, and thus the probability for the motion (diffusion and reaction) caught in a pseudo potential is expected to enhance when vibrational states of the molecules are excited. Actually, they have shown, for the first time, that the action spectrum as a function of applied bias for the electron tunneling reflects vibrational spectrum of the molecule in quantitative manner. Following their discovery, theoretical study has proven the spectrum quantitatively reflects the phonon density of states of adsorbed molecule. As such, an action spectroscopy has become an effective spectroscopy to characterize the vibration of molecules at heterogeneous solid surfaces.
Single molecule sandwiched between two metal electrodes is studied as a model system for future electronic devices. One of the urgent questions to be solved is how can we confirm the species sandwiched in between. Since the inelastic tunneling process also exists, a small change in the conductivity was observed from the hydrogen molecule sandwiched between Pt electrodes corresponding to the vibrational excitation of the molecule, and such a signal is thought to be an evidence of the existence of hydrogen molecules. Moreover, in the conductivity signals, there exists a small noise like signals at the electron energy corresponding the vibration excitation of the molecular system. this is nowadays thought to be the instability induced by the vibrational excitation, a similar action to the motion of molecules that Prof. Kawai and her group have found.
3. Mechanism of vibrational excitation in inelastic tunneling
Whether selection rule exists or not has been one of the central questions since the discovery of the molecular vibration spectroscopy using STM-IETS. In order to answer the question, it was inevitable to extract some processes that in purely reflecting the inelastic tunneling process of electrons. Conductivity is a quantity of the measure, however, it contains not only the inelastic process but also the elastic processes as well and was not easy to differentiate each other experimentally. On the other hand, action spectrum reflects only the inelastic processes and this could be a candidate to extract the effect quantitative way. With the help of theoretical calculations, Prof. Kawai and her group have succeeded to proof that the vibrational states are excited through the resonant mechanism, which means that the electronic states that couples in the electron tunneling plays an important role in the excitation.
As is described above, Professor Maki Kawai proceeded her research in the field of surface chemistry, especially focusing to single molecule reaction, and has played a strong role in elucidating the vibration and reaction dynamics of molecules at surfaces. Her contribution to the basic understanding of Chemistry is also welcomed in the application fields such as invention of molecular devices as well.
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