Dr. N. Kosugi has accomplished leading research in the chemical application of X-ray absorption spectroscopy by developing several new experimental and theoretical methods. His major achievements are summarized below.
1. Methodological developments in the chemical application of molecular inner-shell excitations
The photoabsorption is a most fundamental process in several interactions of light with matter and the transmission measurement is required to observe photoabsorption spectra. In the chemical application, in situ and atmospheric conditions are not avoidable. Soft X-rays excite inner-shells of chemically important light elements, such as carbon, nitrogen and oxygen, and therefore in situ soft X-ray absorption spectroscopy is difficult to realize due to strong photoabsorption by chemically environments such as atmosphere, water and solvents. Dr. Kosugi successfully solved several technical difficulties in the development of in situ soft X-ray transmission system making it possible to optimize the sample thickness below 1 micron under the vacuum condition. His pioneering work of in situ and nano-scale microscopic measurements of soft X-ray absorption spectra in transmission mode has initiated new chemical applications of the molecular inner-shell excitation.
Dr. Kosugi developed theoretical approaches for prediction and analysis of inner-shell excited states by using his original ab initio quantum chemical calculation code. It is difficult to calculate highly excited states such as inner-shell excitations as stationary stable solutions by using widely-used general-purpose ab initio codes but is easy by using his code. He proved that his code is so efficient to analyze molecular soft X-ray absorption spectra and distributed his code to several experimentalists in Japan and in the world. Thus, he is recognized as a leading scientist in advanced experimental and theoretical studies on molecular inner-shell excitation.
2. Local intermolecular interactions revealed by the chemical shift in inner-shell excitations
Dr. Kosugi carried out local structure analysis of interacting molecular systems by observing small chemical shifts in soft X-ray absorption spectra arising from the change in chemical environments. In most cases in previous studies, the chemical shift in inner-shell excitations was rather simply discussed, similarly to the chemical shift in inner-shell ionizations, which arises from the valency of a core hole atom. On the other hand, Dr. Kosugi revealed theoretically that the small chemical shift in soft X-ray absorption spectra arises from the exchange and polarization interactions between an inner-shell excited molecule and its surrounding molecules and is dependent on the diffuseness and directionality of excited orbitals. Thus, he succeeded in getting new information on local electronic and geometric structures of molecular clusters and liquid solutions.
Typical examples are as follows. The chemical shift of single-component clusters is dominantly dependent on the number of nearest neighbor atoms of a core excited atom and is getting smaller as the excited orbital becomes more diffuse. Based on this theoretical background, binary clusters are found to show phase-transition-like behaviors as depending on the ratio of two components. In addition, binary liquid solutions are found to show three major phases from the local structural point of view: one-component dominant structural phases with diluted distributions of the other components and complete mixed phases around an even ratio of two components.
3. Many electron processes revealed by the polarization dependence in inner-shell excitations
Inner-shell excited molecules show complicated decay channels. Dr. Kosugi focused on the decay channel followed by axial ionic dissociations and established a unique polarization-dependent ion-yield spectroscopy to reveal not only the symmetry of inner-shell excited states but also the vibronic coupling and multi-electron excitation feature of closely-located electronic states. In addition to gas-phase molecular spectroscopy, he succeeded in understanding new features in the coordinate bond by carrying out polarization dependent measurements of single crystals of planar transition metal complexes.
Typical examples are as follows. In polarized metal 2p excitation spectra of cyano complexes, several MLCT (Metal to Ligand Charge Transfer) bands are strongly observed as depending on the back-bonding and hybridization to ligand π* orbitals by metal 3d electrons with the same symmetry. This is a new finding through polarized spectroscopy, because these bands were incorrectly interpreted as strong LMCT (ligand to metal CT) bands arising from the metal 2p core hole screening by ligand σ electrons. Furthermore, from the analysis of polarized spectra of planar molecules, the exchange interaction between core and excited electrons is quite small in the perpendicular transition of the 2p-π* excitations and therefore the jj spin-orbit coupling scheme is dominant in perpendicularly polarized X-ray absorption spectra. On the other hand, in the parallel transition, the exchange and spin-orbit interactions are comparable and the spectra are observed in accordance with the intermediate coupling scheme. These characteristics have been generalized and widely applied to inner-shell excitations of molecular systems.
In symmetry-resolved spectroscopy using polarization-dependent ion yields in gas-phase molecular systems, complicated many-electron processes, which were unknown before, are newly found; for example, the finding of double and triple excitations in the inner-shell excitation of nitrogen molecule, the finding of bending mode couplings through the conical intersection of two potential energy surfaces of the 1s-σ* excited state with a dissociative character and the 1s-π* excited state with the Renner-Teller effect in acetylene molecule, and the finding of complicated non-adiabatic transitions among core-to-Rydberg bound excited states and core-to-σ* dissociative excited states in triplet oxygen molecule.
As summarized above, Dr. Kosugi succeeded in finding various new aspects in molecular systems by his original chemical applications of soft X-ray absorption spectroscopy. His pioneering studies on the molecular inner-shell excitation as a unique probe are well known internationally and are affecting soft X-ray photoelectron spectroscopy and soft X-ray emission spectroscopy resonating with precisely selected inner-shell excited states. Furthermore, Dr. Kosugi leaded the upgrade projects UVSOR-II and UVSOR-III of the UVSOR Synchrotron and expanded chemical studies using soft X-rays in complementary collaboration with foreign synchrotron radiation facilities. His achievements in advanced experimental and theoretical studies on molecular inner-shell excitation have been recognized with the Chemical Society of Japan (CSJ) Award.