报告题目：Momentum space chemistry: from the idea to reality
Over the last half-century an experimental method has been developed for looking at the spatial distributions of each electron orbital in momentum space. The method, called electron momentum spectroscopy (EMS), is a kinematically-complete electron-impact ionization experiment under the Compton scattering conditions, where the collision kinematics most nearly corresponds to a collision of two free electrons with the residual ion acting as a spectator. The value of EMS lies in the exciting possibility of measuring Compton profiles associated with target electrons at different energy levels separately. One is thus capable to measure the momentum density distributions of each bound electron or to realize momentum space chemistry. Note that from consideration of the nature of the Dirac-Fourier transform EMS is expected to be particularly sensitive to the behavior of the outer, loosely bound valence electrons that are of central importance in chemical properties such as bonding, chemical reactivity, and molecular recognition. Indeed, the idea of such momentum space chemistry can be traced back to the early 1940’s. For instance, in 1941 a series of theoretical papers were reported by C. A. Coulson who is a pioneer of molecular orbital theory.
In spite of the unique ability, however, application of EMS had largely been limited to studies on electronic structure of atoms and simple molecules, due mainly to its inherently small cross sections. Under those circumstances, we have joined this research field, in 1992. Firstly, we have improved the collection efficiency of EMS measurements, eventually by a factor of nearly 500,000 through a series of development of a multi-channel coincidence technique. We have then started various kinds of attempts to exploit the unique ability of EMS for momentum space chemistry. For instance, measurements of 3-dimensional electron momentum densities of gaseous, isolated molecules have for the first time been made, as well, such as the first experimental determination of spatial orientations of the constituent atomic orbitals in molecular orbitals and studies on distortion of molecular orbitals due to molecular vibration.
Extension of the applicability of EMS to transient species is also one of the challenges to be tackled, because the change of electron motion is the driving force behind chemical reactions. We have therefore developed time-resolved EMS (TR-EMS) by replacing the continuous incident electron beam with 5-kHz electron pulses, each having 1 ps temporal width, and applied it to the highest occupied molecular orbital (HOMO) of a molecular excited state with a life time of 13.5 ps, opening the door to time-resolved orbital imaging for chemical reactions. Furthermore, we are now also developing a time-resolved version of atomic momentum spectroscopy (AMS), which aims to measure in real time the momentum distribution of each atom with different mass numbers in a decaying system. We believe that the joint use of TR-EMS and TR-AMS would provide a completely new, momentum-space approach to studying chemical reaction dynamics.
Professor Takahashi Masahiko
Principal Investigator of Quantum Electron Science Laboratory,
Institute of Multidisciplinary Research for Advanced Materials,
Tohoku University, Sendai 980-8577, Japan
Tel : +81-(0)22-217-5386, Fax : +81-(0)22-217-5337
E-mail : email@example.com
1981-1984 Faculty of Science, Kyoto University
1985-1986 Department of Chemistry, Graduate School of Science, Kyoto University
1992 Ph.D, Kyoto University
1992 Research Associate, RISM, Tohoku University, Sendai, Japan
2003 Associate Professor, Institute for Molecular Science, Okazaki, Japan
2005 Associate Professor, IMRAM, Tohoku University, Sendai, Japan
2008 Professor, IMRAM, Tohoku University, Sendai, Japan
2018-present Deputy Director, IMRAM, Tohoku University, Sendai, Japan
1. Award of the Association for Scientific Measurements for 2006
2. The Chemical Society of Japan Award for Creative Work for 2007
1. Electronic structure and electron correlation in molecules
2. Electron and nuclear dynamics in molecular systems
3. Stereodynamics of electron-molecule collision