Professor Kaoru Yamanouchi has promoted innovative research with high originality on the ultrafast variation of the geometrical structure of molecules and established a new research field in molecular science using intense laser fields.

1. Development of laser assisted electron diffraction methods

The gas electron diffraction method has been known as an experimental method by which geometrical structures of molecules can be determined with high precision. However, it has been a consensus among researchers in these decades that the gas electron diffraction method could not be applied for probing dynamical processes of molecules in real time with high temporal resolution.

Professor Yamanouchi developed a unique and highly original gas electron method called laser assisted electron diffraction (LAED) by using laser assisted electron scattering process as an ultrafast optical shatter, in which electrons scattered by atoms and molecules gain or lose their energy with multiples of photon energy through the interaction with a laser field existing at the moment of the scattering.

By this method, Prof. Yamanouchi recorded for the first time the LAED patterns of CCl₄ at the moment of the femtosecond laser (520 fs) irradiation onto CCl₄. The temporal resolution of this method can in principle become as high as 10 fs, and Professor Yamanouchi's achievement is the important and promising first step for probing in real time the temporal variation of geometrical structures of molecules, which has been the long lasting dream of chemists.

2. Development of ultrafast molecular imaging methods and discovery of intramolecular chemical bond rearrangement

When polyatomic molecules are exposed to an ultrashort pulsed intense laser field, they are multiply ionized and decomposed into fragment ions. Professor Yamanouchi found that the multiple ionization proceeds prior to a large-scale change in geometrical structures of molecules, and demonstrated for the first time that geometrical structures of multiply charged parent molecules just before the fragmentation can be determined by applying the coincidence momentum imaging (CMI) method to CS₂ and N₂O, in which all the fragment ions generated from a single multiply-charged parent ion are detected in coincidence.

Furthermore, Professor Yamanouchi discovered, using the CMI method that, when hydrocarbon molecules such as methanol, allene, methylacetylene, and butadiene are prepared in the highly excited states upon their photoionization, hydrogen atoms and protons move along a molecule frame in the femtosecond time domain, which is called "ultrafast hydrogen atom migration."

Professor Yamanouchi showed visually the spatial distribution of rapidly moving protons within a multiply charged parent ion as a proton map, and discovered, using such maps of partially deuterated species, the existence of "ultrafast hydrogen atom scrambling," in which more than one protons move simultaneously with in a ultrashort period of time.

Through the pioneering studies of Professor Yamanouchi, it has become widely known that "ultrafast hydrogen atom migration" and "ultrafast hydrogen atom scrambling" are universal phenomena origination from the characteristic and prominent dynamical behavior of protons within hydrocarbon molecules prepared by the irradiation of an intense laser field.

Professor Yamanouchi systematically investigated the multiple ionization processes of hydrocarbon molecules by the irradiation of an intense laser pulse, and discovered also the existence of reaction routes through which H₃⁺, a triatomic hydrogen molecular ion, is generated, and identified that the generation proceeds extremely slowly in the picosecond time domain.

By employing quantum chemical calculations, Professor Yamanouchi showed that a neutral hydrogen molecule, H₂, is formed within a parent dication, and it rotates and vibrates for a certain period of time before it is combined with a proton within the parent dication to form H₃⁺.

This discovery of neutral H₂ acting as the precursor species for the formation of H₃⁺ showed that "hydrogen atom migration" within a hydrocarbon molecule proceeds not only by the motion of hydrogen atoms or protons but also by the motion of a unit of H₂.

3. Probing and control of chemical reactions in intense laser fields

    For chemists, it has been one of the most attractive themes whether dynamical behavior of molecules can be controlled by laser light. However, as long as we employ conventional weak laser light, it is impossible to concentrate energy at a specific chemical bond within a molecule because the intramolecular vibrational energy redistribution proceeds very rapidly.

Professor Yamanouchi realized the importance of the fact that the shapes of potential energy surfaces within a molecule can be varied when it interacts with an intense laser field, and demonstrated using ethanol that dynamical processes of a molecule governed by the shape of the potential energy surfaces can be controlled by varying the duration of the light-molecule interaction period. Indeed, Professor Yamanouchi succeeded in showing that the relative yields of the breakings of two different chemical bonds can be controlled by varying systematically the pulse duration of the laser pulses.

This pioneering study of Professor Yamanouchi showed clearly that not only the laser intensity but also the duration of the laser pulses need to be optimized in controlling chemical processes in intense laser field, and provided an important guideline for the subsequent research on photochemical reaction control.

As described above, Professor Yamanouchi developed the light source technologies and measurement methods with high originalities, and explored the new frontiers in the studies of dynamical processes of polyatomic molecule on the basis of his profound knowledge and experience on gas electron diffraction and high-resolution molecular spectroscopy.

 The leading role played by Professor Yamanouchi in exploring the new research frontiers in chemistry has been widely recognized and highly evaluated by international research communities. His significant achievements have given a significant impact on a wide range of research fields not only in physical chemistry and organic chemistry but also in other related research areas. Professor Yamanouchi's achievements are therefore deemed worthy of the Chemical Society of Japan Award