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Spectra, Chemistry, Physics, and...

Jun Okabayashi, Associate Professor, Research Centre for Spectrochemistry

Spectroscopy is a necessary technique in any research field to study the response of an object by shining light on it. The visible state of an object varies depending on the wavelength range of the light used, ranging from long wavelength light (infrared rays) to short wavelength light (X-rays and γ-rays). Among these, "synchrotron radiation" has the property of continuously changing the wavelength (energy) of light in the X-ray region. It is important to measure spectra by matching the energy of light to the inner shell level of the element to be studied, and the author has been conducting research by taking advantage of this feature. In this article, the author would like to introduce his research in the fusion of the two fields based on his experience of working in both the Department of Chemistry and the Department of Physics.

In condensed matter science, spectroscopic studies play an important role in understanding the properties of matter, especially electronic states. Chemistry often deals with molecules, and to understand the state of electrons in molecules, it is necessary to investigate the position and energy of electrons in real space based on quantum mechanics. In physics, on the other hand, the energy distribution per momentum is to be studied by assuming the periodicity of a perfect crystal consisting of 10²³ electrons. It is interesting to note that both approaches are based on different counting methods, although they both want to know the state of the electrons. They are also connected by the Fourier transform.

At surfaces and interfaces, periodicity (symmetry) is broken and peculiar properties appear. Chemistry is to design them, and physics is to understand them. It is an area that requires a sense of both. As one can easily imagine, at the interface, the previously isotropic charge distribution becomes anisotropic due to the presence of neighboring heterogeneous atoms. This creates an orbital magnetic moment consisting of the orbital motion of electrons, which allows us to design magnetic materials that are easily oriented in one direction and create new materials unique to the interface. This is necessary for the highly efficient magnetic switching technology called spintronics, and has been the subject of much research. However, spectroscopic methods for measuring orbital magnetic moments are limited, and magnetic spectroscopy using synchrotron radiation is the most suitable. The authors have been measuring new materials at the beamline of the Research Centre for Spectrochemistry at the KEK Photon Factory of the High Energy Accelerator Research Organization (KEK). It is interesting when the results are as expected, but it is even more interesting when the results are unexpected.

It is also interesting to note that the appearance of electronic states changes when the incident energy of light is changed when investigating the optical response of a material. From infrared to visible light, information on vibrational modes and interband absorption can be obtained. The light from vacuum ultraviolet to x-rays reveals the energy band structure, which is widely used in condensed matter physics. Soft to hard x-rays provide information on element-selective outer-shell electrons by excitation from element-specific inner-shell levels. Gamma rays provide information on the state of outer-shell electrons from the interaction between nuclei and electrons. Combining these studies and looking at them from different perspectives provides a multifaceted view of the nature of matter. Thus, from the viewpoint of spectra, the boundary between chemistry and physics does not exist.

X-ray magnetic spectrometer at the beamline of the Research Centre for Spectrochemistry

Recent advances in operando spectroscopy techniques, which allow in-situ measurement of materials in operation, have led to studies on the analysis of the state of catalysts during chemical reactions and their response to external fields using synchrotron radiation spectroscopy. We find that the solutions to problems in different fields are similar. Therefore, when we want to develop a new research field, it may be a good idea to look at the surrounding fields and search for common problems.