Theoretical studies on dichroism in X-ray absorption and photoemission spectroscopy of nanomaterials
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https://hasegawa.issp.u-tokyo.ac.jp/nanosci-seminar-231114
I present some recent studies on the electronic and geometrical structure of nanomaterials using polarization-dependent X-ray absorption (XAS) and photoemission spectroscopy and first-principles calculations.
First, the crystal phase transformations observed in titanium dioxide nanoribbons, going from NaHTi3O7 titanate over the TiO2–B to the anatase phase is analyzed using polarization-dependent XAS in scaning transmission X-ray microscopy [1,2]. The oxygen K-edge spectra of the three phases display marked differences which are well reproduced with DFT calculations. Strong linear dichroism is observed in single nanoribbons, reflecting preferential O-2p to Ti-3d bond orientation in the low symmetry crystal structures. A simple bond counting model is developed which semi-quantitatively accounts for the major dichroic effect. It is shown how the crystal orientation in the nanoparticles can be inferred from the polarization-dependent XAS spectra.
Next, I present theoretical advances in the theory of angle resolved photoemission spectroscopy (ARPES). For ARPES of graphite, we reproduce and explain the strong circular dichroism (CD) in angular distribution and show that final state photoelectron scattering plays a substantial role in the formation of the dichroic signal [3].
Finally I discuss the CD at the photoelectron diffraction peaks (the so-called Daimon effect) that was recently observed in ARPES of Ni and Cu at the 2p-3d resonance. The intensity of the CD is strongly binding-energy-dependent and indicates reversed angular momentum transfer from the photon to the electron in some cases. To explain these findings we have developed a theory of resonant ARPES, combining atomic multiplet and multiple scattering theory [4] and we obtained excellent agreement with the data.
[2] Chemical Bond Modification upon Phase Transformation of TiO2 Nanoribbons Revealed by Nanoscale X-ray Linear Dichroism, P. Krüger, M. Sluban, P. Umek, P. Guttmann and C. Bittencourt, J. Phys. Chem. C 121, 17038-17042 (2017)
[3] Observation and theory of of strong circular dichroism in angle-resolved photoemission from graphite, Peter Krüger and Fumihiko Matsui, J. Electron Spectrosc. Related Phenom. 258, 147219 (2022)
[4] Theory of circular dichroism in angle-resolved resonant photoemission from magnetic surfaces,, Ryunosuke Sagehashi, Godeung Park and Peter Krüger, Physical Review B 107, 075407 (2023)