Establishment of a method for investigating surface characteristics using light

抄録

With the recent development of wave number-resolved photoelectron emission spectroscopy, precise measurements of electronic states in the full wavenumber space (kx,ky,kz) of various functional materials, which were previously unobservable, are rapidly progressing experimentally [1]. Since the late 2010s, Time of flight (ToF) wavenumber-resolved photoelectron spectroscopy measurements have been actively pursued in Germany, and advanced analytical theories, including orbital tomography, continue to be published in Austria. In Japan, a spin-resolving wave number-resolved photoelectron emission spectroscopy has been developed at UVSOR (the Institute for Molecular Science), and will soon be operational. While conventional ARPES measurements can only provide photoelectron states in a limited and narrow direction in a single measurement, wave number resolved photoelectron spectroscopy can simultaneously measure all photoelectron states emitted in all directions of the electron energy analyzer with high efficiency. The revolutionary measurement evolution achieved by using a photoelectron emission electron microscope (PEEM) as the first stage of the analyzer has made it possible to obtain highly reproducible and reliable experimental results that are approximately 10,000 times more efficient than conventional photoelectron spectroscopy measurements and free from surface damage caused by prolonged light exposure.

On the other hand, because many approximations are used in the photoelectron spectral analysis theory, there are reported cases of discrepancies between the latest high-precision experimental data and calculation results. The photoelectron spectral analysis theory requires consideration of the electronic state of the solid surface (initial state), the interaction between electrons and light, and the photoelectron state (final state). In particular, since the photoelectron state has been largely approximated, there is a strong need for calculations that incorporate the "true photoelectron state" including decay effects as well as non-free electron and self-energy effects of the photoelectron state [2,3].

Our group has performed calculations that incorporate the many-body interactions that photoelectrons undergo from surrounding electrons during propagation into the photoelectron state using multiple scattering method [4,5]. These interactions have produced peak shifts and intensity reductions in the photoelectron spectra. In 2019, we developed a theory to replace the previous work on photoelectron emission from the inner shell of atoms to analyze wave number-resolved photoelectron spectra of organic molecular thin films. We developed a theory for photoelectron spectral analysis that formulated the photoelectron emission process from molecular orbitals and created an original program. Furthermore, we calculated the structure of organic molecules adsorbed on the surface based on density functional theory. After incorporating this into the original program as the starting state, the photoelectron spectral analysis described above was performed [6]. The origin of the peak, which could not be reproduced by the previous theory (plane wave approximation), was found to be due to the scattering of photoelectrons. They also succeeded in identifying the positions of adsorbed molecules.

Based on these results, our group analyzed ToF wavenumber-resolved photoelectron spectra measured by a group at the University of Würzburg, Germany, in 2020 [7]. It clarified that changes in the surface charge distribution at the fs level cause molecular deformation and rotation. I have been involved in measuring physical properties using light since my graduate school days [8,9]. These will be combined in an oral presentation.

[1] S. Suga and A. Sekiyama, Springer Series in Optical sciences 176, (2013).

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