Auger-photoelectron Coincidence Spectroscopy Spectra Research Articles

A many-body theory of the Ti M 2,3-VV Auger-photoelectron coincidence spectroscopy (APECS) spectrum of TiO 2(1 1 0)

We calculated the Ti M 2,3-VV Auger-photoelectron coincidence spectroscopy spectrum of TiO 2(1 1 0) by a many-body theory. The spectral main line is governed by the DOS of the two O 2p holes living longer than the Ti ∣ cd 2 L 2〉 → ∣ L 2〉 super Coster-Kronig (sCK) decay. The two O 2p holes are created by the charge transfer core–hole screening at the Ti atomic site. Here c and L are the Ti 3p hole and the ligand O 2p hole, respectively. Analysis of the spectrum shows that the two (or three) CT O 2p holes in the π bonding states are localized, whereas those in the σ bonding states are delocalized. The three localized CT O 2p ( π) holes in ∣ cd 3 L 3〉 in Ti M 2,3 main line (or satellite) of TiO 2(1 1 0) live longer than the Ti ∣ cd 3 L 3〉 → ∣d 1 L 3〉 sCK decay so that the Coulomb repulsion from the surrounding Ti ions gives the O + ion desorption from the surface.

Effect of relaxation and decay of a charge transfer shakeup satellite on Auger-electron spectroscopy spectra and Auger-photoelectron coincidence spectroscopy spectra of adsorbates

An electron excited to an unoccupied part of adsorbate–substrate hybrid states in a chemisorbed molecule by a resonant core electron excitation or charge transfer (CT) shakeup may delocalize on time scale of core-hole decay so that the excited core-hole state relaxes partly or completely to a fully relaxed one. The Auger decay of the fully relaxed core-hole state via the relaxation of the excited one introduces an additional feature in the resonant Auger-electron spectroscopy (RAES) spectrum and the AES spectrum. However, the additional feature in the RAES spectrum is a normal AES spectrum by decay of the fully relaxed core-hole state, whereas the one in the AES spectrum is the AES spectrum by decay of the fully relaxed core-hole state broadened by the photoelectron spectroscopy (PES) CT shakeup satellite weighted by the branching ratio of the relaxation width. The discrepancies between the AES spectrum measured at high above the ionization threshold and the additional feature in the RAES spectrum consist of the symmetric-like part by the decay of the fully relaxed core-hole state via the relaxation of the CT shakeup state and the asymmetric part by the direct decay of the shakeup states. The asymmetric part increases with a decrease in the hybridization strength. This explains the variation with the hybridization strength in the discrepancies between the RAES spectra and the AES spectra of chemisorbed molecules such as CO/Ni, CO/Cu and CO/Ag. A comparison of the singles PES spectrum with the one measured in coincidence with the AES main line of a selected kinetic energy (KE) provides the delocalization rate of the excited electron in the CT shakeup state as a function of photoelectron KE. The coincidence measurement to obtain the partial singles PES spectrum is discussed.

Many-body effect in the resonant Ti L 23–M 23V Auger-electron spectroscopy spectra and Auger-photoelectron coincidence spectroscopy spectra of Ti oxides

Recently Danger et al. [J. Danger, H. Magnan, D. Chandesris, P. Le Fèvre, S. Bourgeois, J. Jupille, A. Verdini, R. Gotter, A. Morgante, Phys. Rev. B 64 (2001) 045110] and Le Fèvre et al. [P. Le Fèvre, J. Danger, H. Magnan, D. Chandesris, J. Jupille, S. Bourgeois, M.-A. Arrio, R. Gotter, A. Verdini, A. Morgante, Phys. Rev. B 69 (2004) 155421] showed the absence of resonant Raman scattering feature in the Ti L 23–M 23V resonant Auger-electron spectroscopy (RAES) spectra of Ti oxides measured across the Ti 2p edges. They attributed the absence to the covalent character of the Ti–O bond which allows an effective delocalization of 3d electrons. It is shown by a many-body theory that when the time scale of relaxation of the resonantly excited core-hole state to the fully relaxed core-hole state is much shorter than that of core-hole decay, any sizeable Raman scattering is absent in the RAES spectra measured across the Ti 2p edges. The relaxation width depends on the hybridization strength and the charge transfer (CT) energy between the two states. The L 2–L 3V Coster–Kronig (CK) decay widths of TiO 2 and TiO 2− x are determined from the L 23–M 23V Auger-photoelectron coincidence spectroscopy (APECS) spectra reported in the aforementioned papers. They are about 0.18 and 0.35 eV, respectively. The CK-decay width in the reduced Ti oxide increases compared to that of TiO 2 in rutile because of filling of the 3d states just below the Fermi level in the former.

Many-body effect in Auger-photoelectron coincidence spectroscopy spectra

When the shakeup/down excitations are not negligible in the core-level electron ionization, the photoelectron spectral peak measured in coincidence with a selected singles (noncoincidence) Auger-electron spectral peak does not necessarily coincide with the singles one. We discuss how the interference between the core-hole decay of a fully relaxed core-hole state and that of an incompletely relaxed one via the interaction between the final states created by the respective core-hole decays, affects the kinetic energy shift and asymmetrical lineshape change of the coincidence photoelectron spectrum compared to the singles one. When the final-state interaction is considerable, the interference reduces much the energy shift and the asymmetrical lineshape change. By the Auger-photoelectron coincidence spectroscopy (APECS) we can study the interference effect which does not manifest in the singles photoelectron spectrum. We discuss also the interference effect when the core-hole decay rates of both the fully relaxed core-hole state and the incompletely relaxed one depend critically on the changes in both the Auger-electron kinetic energy and the final-state potential. The effect is fairly independent of the changes.

Effect of the competition between relaxation and core hole decay on the Auger-photoelectron coincidence spectroscopy (APECS) spectrum of Ni metal

The valence hole created in Ni metal either by the L2-L3V Coster–Kronig (CK) transition or by the L3V shakeup/off becomes screened out prior to the L3-hole decay. We denote the atomic shell L x ( x = 2, 3) by LX. The metastable two-hole L3V state relaxes to the fully relaxed single L3-hole state before the L3-hole decays. Thus, the coincidence L2-L3(V)-VV(V) Auger-electron spectrum resembles closely the coincidence L3-VV Auger-electron spectrum. The final state of the CK transition preceded Auger transition is a two-hole state rather than a three-hole state. The four-hole satellite about 8 eV below the L3-VV main line in the singles (non-coincidence) Auger-electron spectrum is partly due to the L3VV-VVVV transition and the L2-L3VV-VVVV transition. The valence holes created either by the L2-L3VV transition or by the L3VV double shakeup/off remain localized during the L3-hole decay. The L3-hole lifetime widths of Fe, Co and Ni metals are determined from the APECS spectra. The agreement between experiment and theory (the independent-particle approximation) is poor.

Many-electron effects in the Auger-photoelectron coincidence spectroscopy spectra of the late 3d-transition metals

The valence hole created by the L2–L3 M45 Coster–Kronig (CK) transition may hop away from the ionized atomic site before the L3-hole decays. Then when the third (Auger) electron emitted by the L3-hole decay is measured in coincidence with the photoelectron emitted by the initial L2-level electron ionization, the coincidence spectrum becomes similar or identical to the singles spectrum of the secondary (Auger) electron emitted by the L3-hole decay as if it decayed as an initial single core hole. Thus the coincidence spectrum is essentially governed by the valence-hole dynamics of both the intermediate states and the final states of the L2–L3 (M45) CK-transition preceded Auger transition. In the present paper the Auger-photoelectron coincidence spectroscopy (APECS) spectra of Fe, Co, and Ni metals reported by C.P. Lund et al. (Phys. Rev. B55 (1997) 5455) are analyzed in light of the delocalization and localization of the valence hole(s) created by the CK transition or the CK-transition preceded Auger transition.

The effect of Coster–Kronig transition on the Auger-photoelectron coincidence spectroscopy spectra of early 3d-transition metals

The singles L23–M45M45 Auger-electron spectroscopy (AES) spectrum of early 3d-transition metal can be fitted by a weighted sum of the density of the single-hole states and that of the two-hole states, broadened by the initial L23-hole lifetime width, respectively (in the present paper we denote the atomic shells Lx, My, and Nz by LX, MY and NZ, respectively). With increasing occupancy of the 3d band the probability of creating the two-hole states by the L23–M45M45 Auger transition and the L2–L3M45 Coster–Kronig (CK) transition increases. However, the M45 hole created by the CK transition is delocalized and becomes decoupled (screened out) from the L3-hole decay so that the L3M45 two-hole state “decays” to the single L3-hole state before the L3-hole decays. Thus the singles AES spectrum by the L2–L3–M45(M45) CK-transition preceded Auger transition and the singles one by the L3–M45(M45) Auger-transition overlap. We can study the M45-hole dynamics by Auger-photoelectron coincidence spectroscopy because the coincidence spectral lineshape depends on the dynamics of the M45 hole created by the CK transition.

On the Auger-photoelectron coincidence spectroscopy spectra of Cu(100), Cu metal and CuOx/Cu surface

The M 3–VV Auger-photoelectron coincidence spectroscopy (APECS) spectrum of Cu(100) and the L 3–VV APECS spectra of Cu metal and CuOx/Cu surface are analyzed in detail. The narrowing and energy shift of the photoelectron line in the M 3–VV APECS spectrum is well predicted by the present theory. The spectrum shows the presence of the M 2–M 3(V)–VV(V) decay in which a hole in the 4s band hops away prior to the decay of M 3 hole. The analysis of the L 3 photoelectron spectra of Cu metal measured in coincidence with the 3F or 1G Auger line raises a question concerning the presence of two different core–hole states upon the L 3 level ionization recently proposed by Thurgate and Jiang [Surf. Sci. 466 (2000) L807]. The analysis of the L 3–VV APECS spectrum of CuOx/Cu shows that the final-state charge–transfer interaction plays an important role in CuO.

The use of Auger photoelectron coincidence spectroscopy to deconvolute the M45N45N45 AES of Palladium

Palladium (Pd) has a complex Auger spectrum. It is difficult to fully describe the M45N45N45 Auger spectra of its surface atoms because of the many possible cascade processes placing intensity in this energy region. Weightman et al. described this Auger peak from a multiplet-splitting point of view and we used this model as a starting point. The model was compared to Auger photoelectron coincidence spectroscopy (APECS) data of the same peak and intensity was found in the M4N45N45 APECS spectra that could not be accounted for by Weightman's model. Thus new model curves for each component of the Auger peak were created by applying the Cini-Sawatzky Theory to the convoluted one-electron density of states. Experimental APECS data was used to verify the model curves and the final composite picture was compared to a high resolution AES spectrum. There was a good fit between the final model, the APECS data and high resolution AES data. The model also provided evidence for an M4-M5N45-N45N45(N45) Coster-Kronig process.

More Auger photoelectron coincidence spectra from copper

We have recently re-built our Auger photoelectron coincidence spectroscopy (APECS) system at Murdoch University. The new instrument counts at much higher rates and with greater energy resolution. With this we have looked again at the APECS spectra from Cu. We have been able to see more clearly the L2–L3V–VV(V) Coster–Kronig satellite and, in a reverse experiment, to find out which parts of the photoelectron spectrum are responsible for the satellite. The data also show the importance of shake-up/off processes in the background of the L3–VV line. We will present information on the changes we have made to the apparatus as well as the implications of the data we have collected with it.