Resonant Auger spectroscopy at the L2,3 shake-up thresholds as a probe of electron correlation effects in nickel

📝 Abstract

The excitation energy dependence of the three-hole satellites in the L3-M4,5M4,5 and L2-M4,5M4,5 Auger spectra of nickel metal has been measured using synchrotron radiation. The satellite behavior in the non-radiative emission spectra at the L3 and L2 thresholds is compared and the influence of the Coster-Kronig channel explored. The three-hole satellite intensity at the L3 Auger emission line reveals a peak structure at 5 eV above the L3 threshold attributed to resonant processes at the 2p53d9 shake-up threshold. This is discussed in connection with the 6-eV feature in the x-ray absorption spectrum.

💡 Analysis

The excitation energy dependence of the three-hole satellites in the L3-M4,5M4,5 and L2-M4,5M4,5 Auger spectra of nickel metal has been measured using synchrotron radiation. The satellite behavior in the non-radiative emission spectra at the L3 and L2 thresholds is compared and the influence of the Coster-Kronig channel explored. The three-hole satellite intensity at the L3 Auger emission line reveals a peak structure at 5 eV above the L3 threshold attributed to resonant processes at the 2p53d9 shake-up threshold. This is discussed in connection with the 6-eV feature in the x-ray absorption spectrum.

📄 Content

PHYSICAL REVIEW B 58, 3677 (1998) 1 Resonant Auger spectroscopy at the L2,3 shake-up thresholds as a probe of electron correlation effects in nickel M. Magnuson, N. Wassdahl, A. Nilsson, A. Föhlisch, J. Nordgren and N. Mårtensson Department of Physics, Uppsala University, P. O. Box 530, S-751 21 Uppsala, Sweden

Abstract The excitation energy dependence of the three-hole satellites in the L3-M4,5M4,5 and L2- M4,5M4,5 Auger spectra of nickel metal has been measured using synchrotron radiation. The satellite behavior in the non-radiative emission spectra at the L3 and L2 thresholds is compared and the influence of the Coster-Kronig channel explored. The three-hole satellite intensity at the L3 Auger emission line reveals a peak structure at 5 eV above the L3 threshold attributed to resonant processes at the 2p53d9 shake-up threshold. This is discussed in connection with the 6-eV feature in the x-ray absorption spectrum. 1 Introduction The 3d transition metal systems have been extensively studied because these systems show influence of many-body effects which are interesting both from an experimental and theoretical point of view [1,2,3,4,5,6,7]. The occurence of distinguishable satellite structures in the spectra is a sign of localization tendencies of the 3d valence electrons which retain some of their atomic-like properties in the metal. In this respect, Ni is often considered a prototype system in the 3d transition metal series regarding strong correlations and configuration interaction (CI) in the ground, core- and valence-excited states. Ni has recieved much attention especially in connection with the well-known 6-eV photoemission satellite of both the valence-band and core-levels [8,9,10,11]. The strong satellite structures and the small bandwidth in the Ni metal are directly connected to and give important information on the correlation effects which have been described in terms of a complex, energy dependent self-energy [12,13,14,15,16]. The 6-eV satellite in valence band spectra has been observed to display Fano-like intensity variations both at the 3p [17] and 2p [18] core-level thresholds due to interference effects. A good understanding of the interesting physical properties of Ni requires detailed spectroscopic studies of the electronic structure performed at sufficiently high resolution. The interpretation of the 6-eV feature in the Ni L2,3 x-ray absorption spectrum is still controversial. It has been observed that the photon energies of the main line and the 6 eV feature in the x-ray absorption spectrum coincide with the corresponding core level binding energies in the x-ray photoelectron spectrum [19]. The satellite in the photoemission spectrum has been found to be due to a 2p53d9 double hole state and it has therefore been natural to identify the x-ray absorption feature with the same type of final state. In this contribution, we address the controversial issue of the interpretation of the 6-eV feature at the L2,3 edges in the x-ray absorption spectrum in Ni based on a quantitative study of the three-hole satellites (3d7 final state configuration) at the L2,3 shake-up PHYSICAL REVIEW B 58, 3677 (1998) 2 thresholds. The large L2,3 spin- orbit splitting makes it possible to compare the behavior at the two thresholds separately, thereby exploring the consequences of the competing Coster-Kronig (CK) channel at the L2 threshold. The relative weight of the 3d7 final state satellite intensity of the non- radiative decay spectra provide a quantitative probe of the population of the localized 2p53d9 core-level shake-up states in the Ni L2,3
absorption spectrum. This gives direct insight into the origin of the 6-eV spectral feature in the absorption spectrum which has been discussed both in terms of delocalized one-electron band states [20] and localized 2p53d9 multiplet states [21]. 2 Experimental Details The Auger measurements were performed at beamline 8.0 at the Advanced Light Source (ALS), Lawrence Berkeley National Laboratory (LBNL). The beamline comprises a 5 cm period undulator and a spherical-grating monochromator. The experimental station built at Uppsala University includes a rotatable Scienta SES200 electron spectrometer [22]. The base pressure was lower than 2×10−10 Torr during preparations and measurements. The Ni(100) single crystal sample was of high purity and crystal quality, and cleaned by means of cyclic argon-ion bombardment and annealing to remove surface contaminants. The sample was oriented so that the photons were incident at about 7o grazing angle with the polarization vector of the x-rays in the plane of the sample. The electron spectrometer was oriented near the sample normal and perpendicular to the photon beam. This geometry increases the weight of the Auger matrix element relative to the direct photoemission matrix element. During the Auger measurements the resolution of the monochromat

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