Circular dichroism in resonant photoelectron diffraction

Peter Krüger

Graduate School of Engineering, Chiba University, Chiba 263-8522 Japan

pkruger@chiba-u.jp

Circular dichroism (CD) is the difference between two spectra, one obtained with right circular polarized light and the other one with left circular polarized light. In absorption spectroscopy, CD is abundantly used to detect structural chirality, usual in the visible or soft UV range, and ferromagnetism in the X-ray range. In angle-resolved photoemission, CD is very common, but the physical interpretation is complicated by the fact that the CD signal depends not only on the ground state of the system, but also on the photoemission final state. Even achiral, non-magnetic systems can show large CD in angular distribution (CDAD). As an example I will show, in the first part of the talk, recent measurement and calculations of strong CDAD in graphene [1]. In x-ray photoelectron diffraction, CD systematically occurs around focussing peaks. This so-called Daimon effect can be used for 3D structural characterization of active sites.
Recently the Daimon effect has been observed in resonant photoelectron diffraction at the resonant 2p3d3d and 2p3p3d-Auger decay of Ni [2] and Cu [3]. Surprisingly, the CD changes sign as a function of binding energy within the same photoelectron band, in this case Ni-3p and Cu-3d. Here I present a theory, based on atomic multiplet calculations and multiple scattering theory, which explains these experimental findings [4,5]. For the resonant Ni-2p3p3d spectra, we reproduce the observed CD sign change between singlet and triplet 3p hole states, which reflects an multiplet dependent angular momentum transfer from the photon to the photoelectron. In the case of the Cu-2p3d3d resonant Auger spectra, we argue that the original interpretation based on the 2p-4s excitation [3] is questionable. Indeed, we show that in metallic Cu, the 2p-d absorption intensity dominates at threshold. Then, a simple, two-step model of the resonant Auger process, allows to reproduce the correct CD and its sign change as a function of binding energy.

[1] P. Krüger and F. Matsui, J. Electron Spectrosc. Related Phenom. 258, 147219 (2022).
[2] F. Matsui, H. Ota, and K. Sugita, Phys. Rev. B 97, 035424 (2018).
[3] F. Matsui, M. Fujita, T. Ohta, N. Maejima, H. Matsui, H. Nishikawa, T. Matsushita, and H. Daimon, Phys. Rev. Lett. 114, 015501 (2015).
[4] R. Sagehashi, G. Park and P. Krüger, Physical Review B 107, 075407 (2023).
[5] P. Krüger, in preparation.