Theory of resonant angle-resolved photoemission and Auger emission from surfaces

抄録

Angle-resolved photoemission spectroscopy (ARPES) is one of the main experimental techniques for probing the electronic structure of crystal surfaces and oriented molecules. In recent years ARPES has become very popular because of great instrumental advances, e.g. with photoelectron momentum microscope. While normal ARPES lacks elemental and local information, such information may partially be gained by performing ARPES at a core-level resonance. Although this is well known for long, generally no qualitative analysis of the data is done, because of a lack of computational methods for resonant ARPES (RARPES) in solids. The theoretical challenge is that a realistic description of RARPES must include both longe-range band structure [1] and short-range electron correlation effects, where the latter give rise to pronounced multiplet structures in the resonant spectra. Here, I present our recent advances in the theory of RARPES and resonant Auger electron spectroscopy and diffraction from valence bands. We have developed a theoretical method for RARPES which combines a ligand field multiplet calculation to describe the resonant photoemission process at the core-hole site [2]. The emitted photoelectron wave is propagated using multiple scattering theory which accounts for the photoelectron diffraction effects [3]. We have applied the theory to resonant photoemission with circular polarized light from a Ni surface either magnetized or non-magnetized. For the magnetized surface [2], we have reproduced the complex angular dependence of the circular dichroism (CD) signal, which is dominated by a dipole-type angular distribution due to XMCD effect in the core-excitation step. Moreover the fast angular variations of the CD are also qualititively reproduced and are explained to be due to photoelectron scattering, as in the Daimon effect. For the non-magnetized Ni surface, we could explain the strong Daimon effect which was recently reported for the Ni-2p3d3d and Ni-2d3p3d resonant spectra and the CD sign reversal observed at certain binding energies [4]. These phenomena are well reproduced by our theory and explained it in terms the angular momentum transfer from the photon to the excited electron or the ionized surface [5]. Finally we discuss the CD in normal Auger emission as well as in resonant spectator Auger emission and show that the main effect can be explained in a two-step model where the CD is determined by the core-hole alignement with the adsorbed photon field. The figures shows the calculated Cu 2p3d3d normal Auger spectra for different 2p-core-hole initial states. The black lines are the emission intensities and the red lines are the intensity multiplied by the magnetic quantum number of the emitted electron.

[1] F. Da Pieve and P. Krüger, Phys. Rev. Lett. 110, 127401 (2013). [2] R. Sagehashi, G. Park and P. Krüger, Phys. Rev. B 107, 075407 (2023). [3] P. Krüger et al. Phys. Rev. Lett. 108, 126803 (2012). [4] F. Matsui et al., Phys. Rev. B 97, 035424 (2018). [5] P. Krüger, in preparation.