Many-body effect in the N23-VV Auger-photoelectron coincidence spectroscopy (APECS) spectra of Ag metal

The coincidence N23-VV Auger-electron spectroscopy (AES) spectra and N23 photoelectron spectroscopy (PES) spectra of Ag metal are analyzed. Here NX is the notation for atomic shell Nx (X = 2, 3). The band-like feature in the coincidence N23-VV AES spectra is much more intense than that in the coincidence M45-VV ones because the potential in the delocalized two-hole state is less attractive than that in the localized one. The partial N23-VV super Coster–Kronig (sCK) transition rate depends critically on both the final-state potential and the sCK-electron kinetic energy (KE) because the KE is low, whereas the partial M45-VV Auger-transition rate is fairly independent of them because the KE is very high. As a result, the partial sCK-transition rate to the band-like state is enhanced compared to that to the atomic-like localized state. The low KE tail in the coincidence N23-VV AES spectra which is likely due to the sCK transition involving more than two electrons, is more enhanced than that in the coincidence M45-VV ones. This is due to the enhancement of the partial sCK-transition rate by the presence of extra holes in the final state. The sharp peaks of small intensity on the lower KE side of the main line in the coincidence N2 PES spectrum are tentatively attributed to the shakeup satellites.     

Analysis of the Auger-photoelectron coincidence spectroscopy (APECS) spectra of metallic Pd,Ag, and Sn

The M45-level photoelectron spectrum of Ag metal measured in coincidence with the M45–VV(¹G) Auger-electron line is analyzed by taking into account the possibility of the M4–M5–VV Coster–Kronig (CK)-transition preceded Auger transition. We denote the atomic shells Mx(Mxy) and Nxy (x, y = 4,5) by MX(MXY) and V, respectively. The M4–M5 CK-transition rate is very small. The M45–VV Auger-electron spectra of metallic Pd and Sn measured in coincidence with the M4 (or M5)-level photoelectron line are analyzed. The M4–M5 CK-transition rates are also very small in metallic Pd and Sn. The coincidence Auger-electron line previously interpreted as the M4–M5–VV (or M4–M5V–VVV) Auger-electron line is largely due to the inelastically scattered M5-level photoelectron background beneath the M4-level photoelectron line. The APECS spectrum of Pd metal shows the first evidence of the M5V–VVV transition of the localized M5V shakeup two-hole state. The intensity ratio of the inelastically scattered Auger-electron background to the M5–VV Auger-electron main line of Ag metal measured in coincidence with the inelastically scattered M5-level photoelectron background beneath the M4-level photoemission line increases, as compared to that measured in coincidence with the M5-level photoelectron main line. This is because when the probability of the photoelectron being inelastically scattered increases, that of the Auger electron emitted by the same ionized atom, being inelastically scattered increases. In other words the photoelectrons and the Auger electrons are originated from the deeper atomic sites (longer pathlength).     

Many-electron effect in the angle resolved L3-M23M23 Auger-photoelectron coincidence spectroscopy (APECS) spectrum of metallic Zn

The many-body effect in the L₃-M₂₃M₂₃ Auger-electron spectroscopy (AES) spectrum of metallic Zn is discussed. The lifetime width and residual relaxation energy shift of the two M₂₃-hole state are governed by the (super) Coster-Kronig (sCK) transitions of two M₂₃-hole state. The residual relaxation energy shift and decay width of the two M₂₃-hole state are calculated in an average configuration by an ab initio atomic many-body theory. The agreement with experiment is good. To elucidate the many-body effect in the two-hole states, it is necessary to be able to discriminate individual components of the multiplet-split AES spectrum. We discuss how to discriminate individual components of the multiplet-split L₃-M₂₃M₂₃ AES spectrum of metallic Zn by angle-resolved Auger-photoelectron coincidence spectroscopy (AR-APECS) in order to determine accurately their line shapes, multiplet splitting energies, and spin states (singlet etc.).     

Many-body effect in the M45–N23N45–N45N45N45 coincidence spectroscopy spectra of metallic elements around Cd

The quasi-particle approximation for the 4p4d state of the metallic elements around Cd breaks down because of very rapid 4p4d–4d³ super Coster–Kroning (sCK) decay of the 4p hole in the presence of the spectator 4d hole. Here the underbar is a hole. As a result, the 4p4d multiplet coupling breaks down. We can examine the presence or absence of the 4p4d multiplet by Auger-electron sCK-electron coincidence spectroscopy measurement of the 3d–4p4d–4d³ Auger-preceded sCK transitions. We collect the sCK-electrons in coincidence with the Auger-electrons of a selected kinetic energy (KE) and vice versa. If the multiplet coupling breaks down and does not exist, the coincidence sCK-electron (or Auger-electron) lines shift as much as the Auger-electron (or sCK-electron) analyzer's selected KE is varied. We can determine not only the three 4d-hole sCK final-state energy but also the presence or absence of the 4p4d multiplet by Auger-electron sCK-electron coincidence spectroscopy. The unique capability of the coincidence measurement by which one can determine the correlation between an Auger-electron and a sCK electron generated, respectively, by creation and annihilation of the same Auger two-hole final state is very useful, even when the quasi-particle approximation of the two-hole state breaks down.     

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.     

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.     

Many-body effect in the coincidence L3 and M3 photoelectron spectroscopy spectra of Cu metal

The coincidence L₃ and M₃ photoelectron spectroscopy (PES) main lines of Cu metal are calculated by a many-body theory. There is no peak-energy shift between the singles PES main line and the coincidence one. The asymmetric narrowing of the coincidence PES main line on the low kinetic energy (KE) side is very small. This is in accord with recent experimental findings. In Cu metal, the shakeup satellite intensity is small and the main-line satellite separation energy is much larger than the core–hole lifetime width. The interference via the final-state interaction is negligible. In the PES main line, the imaginary part of the self-energy by shakeup excitations, which is very small compared to the core–hole lifetime width, decreases very slowly in linear with photoelectron KE. The branching ratio of Auger decay of a single hole state then increases very slowly in linear with photoelectron KE so that the deviation of the coincidence PES main line from the singles one is very small. The 939 eV structure seen only in the coincidence L₃ PES spectrum of Cu metal is attributed to the enhancement of the inelastic peak of a smaller energy loss due to electrons of a smaller average emission depth measured in coincidence with the elastic Auger peak. The structure will not be enhanced in the singles PES spectrum. The background subtraction in the coincidence spectrum cannot be the same as that in the singles one. Such consideration is necessary before we can conclude about the asymmetric narrowing on the low KE side. A unique capability of APECS by which one can determine the photoelectron KE dependent part of the imaginary part of the self-energy is pointed out.     

The participant Coster–Kronig preceded Auger transition in the resonant L2,3-M2,3V Auger electron spectrum of Ti metal

The L_2,3-M_2,3V resonant Auger electron spectroscopy (RAES) spectrum of Ti metal measured by Le Fêvre et al. [P. Le Fêvre, J. Danger, H. Magnan, D. Chandesris, J. Jupille, S. Bourgeois, M.-A. Arrio, R. Gotter, A. Verdini, A. Morgante, Phys. Rev. B69 (2004) 155421] is analyzed in the light of relaxation and decay of the resonantly excited L_2,3-hole states. The relaxation time of the resonantly excited L_2,3-hole state to the fully relaxed (screened) one is much shorter than the L_2,3-hole Auger decay time, whereas the participant Coster–Kronig (CK) decay time of the resonantly excited L₂-hole state to the fully relaxed L₃-hole state at the L₂ resonance is as short as the relaxation time of the resonantly excited L₂-hole state to the fully relaxed one. The excited electron is predominantly either rapidly decoupled from the L_2,3-hole decay or annihilated by the participant CK decay. Thus, near the L_2,3 edges the L_2,3-M_2,3V RAES spectral peak appears at constant kinetic energy. The L_2,3-M_2,3V RAES spectrum shows a normal L_2,3-M_2,3V Auger decay profile not modulated by the density of empty d states probed by the resonant excitation. Not only the relaxation time but also the participant CK decay time depends on photon energy because they depend on the density of empty d states probed by the resonant excitation. As a result, the L_2,3 X-ray absorption spectroscopy spectral line broadening depends on photon energy.     

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.     

The origin of narrowing of the Si 2p coincidence photoelectron spectroscopy main line of Si(1 0 0) surface

The Si 2p photoelectron spectroscopy (PES) main line of Si(1 0 0) surface measured in coincidence with the singles (noncoincidence) Si L_2,3-VV Auger-electron spectroscopy (AES) elastic peak is calculated. The agreement with the experiment is good. The present work is the first many-body calculation of the experimental coincidence PES spectrum of solid surface. The narrowing of the coincidence Si 2p PES main line compared to the singles one is due to the mechanism inherent in the coincidence PES. The inherent mechanism is explained by a many-body theory by which photoemission and Auger-electron emission are treated on the same footing.     

Many-electron effect in the Fe L3-M4,5M4,5 Auger electron spectrum of ultrathin Fe films grown on Cu(1 0 0)

D’Addato et al. [S. D’Addato, P. Luches, R. Gotter, L. Floreano, D. Cvetko, A. Morgante, A. Newton, D. Martin, P. Unsworth, P. Weightman, Surf. Rev. Lett. 9 (2002) 709] studied the variation with Fe coverages in the relative Fe L₃-M_4,5M_4,5 Auger electron spectroscopy (AES) spectral satellite intensity of ultrathin Fe films grown on Cu(1 0 0) by sweeping photon excitation energy through the Fe L₂-level ionization threshold. They interpreted that the M_4,5 hole in the L₃M_4,5 double-hole state created by the L₂-L₃M_4,5 Coster–Kronig (CK) decay remains localized for longer than the L₃-hole lifetime for the 0.3 and 10 ML coverages but has a lifetime comparable to the L₃-hole lifetime for the 1 ML coverages. The present many-body theory shows that when the M_4,5 hole created either by the CK decay or by the L₃M_4,5 shakeoff hops away from the ionized atomic site and becomes completely screened out prior to the L₃-hole decay, the Fe L₂-L₃M_4,5-L₃-M_4,5M_4,5 AES main line as well as the Fe L₃ M_4,5 (satellite)-L₃-M_4,5M_4,5 one, both of which are identical in line shape to the Fe L₃-M_4,5M_4,5 one, dominate in the Fe CK preceded AES spectrum. The present analysis shows that the delocalization time of the M_4,5 hole created in the 1 ML Fe/Cu(1 0 0) system by the L₂-L₃M_4,5 CK decay is much shorter than the L₃-hole lifetime so that the Fe L₃-M_4,5M_4,5 AES spectral line shape hardly changes, except for the presence of a very weak spectator L₂-L₃M_4,5-M_4,5M_4,5M_4,5 AES satellite, when the photon excitation energy is swept through the Fe L₂-level ionization threshold. For the 0.3 ML coverages the M_4,5-hole delocalization time is still shorter than the L₃-hole lifetime.     

On the L3 level photoelectron spectrum of Cu metal measured in coincidence with the L3–VV Auger-electron spectrum

We provide an answer to the question why the L₃ photoelectron line of Cu metal measured in coincidence with the L₃–VV (³F or ¹G) Auger-electron line, does not line up with the L₃ single photoelectron line. We provide also an answer to the question why the L₃ coincidence photoelectron line is unshifted when the Auger-electron analyzer is moved away from the Auger-electron line. We show that it is the initial core–hole self-energy by the monopole excitation (screening) and the density of final states which play an important role in the shift and narrowing of Auger-photoelectron coincidence spectroscopy (APECS) spectral line. To explain the shifted APECS spectral line, Thurgate and Jiang (Surf. Sci. 466 (2000) L807) recently proposed the presence of two different core–hole states in the main-line state upon the L₃ level ionization in Cu metal. However, their explanation appears to be incorrect.     

Study of ion desorption induced by a resonant core-level excitation of condensed NH3 using Auger-electron photo-ion coincidence (AEPICO) spectroscopy combined with synchrotron radiation

Photostimulated ion desorption at the 4a₁ ← N 1s resonant transition of condensed NH₃ was studied using electron emission spectroscopy and Auger-electron photoion coincidence (AEPICO) spectroscopy. The total ion yield divided by the Auger-electron yield exhibited a threshold peak at hν = 399 eV which is ascribed to the resonant transition from the N 1s to the N---H antibonding 4a₁ orbital. The electron emission spectrum at the 4a₁ ← N 1s resonance is decomposed into three components: a valence photoelectron emission spectrum, and normal- and resonant-Auger-electron emission spectra. We ascribe the resonant-Auger-electron emission spectrum mainly to spectator-Auger transitions on the basis of the peak assignment. A series of AEPICO spectra at the 4a₁ ← N 1s resonance was also measured as a function of the Auger-electron kinetic energy. The electron kinetic energy dependence of the H⁺ AEPICO yield displays a shape approximately similar to that of the mixed spectrum of normal- and spectator-Auger-electron emission spectra. Based on this result the H⁺ desorption at the 4a₁ ← N 1s resonance is concluded to originate from the spectator-Auger transitions and from the normal-Auger transitions following the delocalization of the excited electron.     

Effect of the final-state interaction on the initial core–hole lifetime: the case of the 4s-hole lifetime of Sn metal

The first theoretical study of the effect of the final-state interaction on the initial core–hole lifetime is presented. The 4s-hole lifetime width of Sn metal is calculated by an ab-initio atomic many-body theory (Green’s function method). When the final-state interaction in the 4p4d two-hole state, created by the 4s⁻¹−4p⁻¹4d⁻¹ f super Coster–Kronig (CK) transition of the initial 4s hole, is explicitly taken into account, the ab-initio atomic many-body calculation of the 4s-hole X-ray photoelectron spectroscopy (XPS) spectrum of Sn atom can provide excellent agreement with experiment in both the 4s-hole energy and the 4s-hole lifetime width. Otherwise, the many-body calculation underestimates considerably the 4s-hole lifetime width. The 4p4d two-hole state interacts strongly with the 4d triple-hole state by the 4p⁻¹4d⁻¹−4d⁻³ f super CK transition. The interaction affects greatly the initial 4s-hole lifetime width.     

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

The M₃–VV Auger-photoelectron coincidence spectroscopy (APECS) spectrum of Cu(100) and the L₃–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 M₃–VV APECS spectrum is well predicted by the present theory. The spectrum shows the presence of the M₂–M₃(V)–VV(V) decay in which a hole in the 4s band hops away prior to the decay of M₃ hole. The analysis of the L₃ 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 L₃ level ionization recently proposed by Thurgate and Jiang [Surf. Sci. 466 (2000) L807]. The analysis of the L₃–VV APECS spectrum of CuOx/Cu shows that the final-state charge–transfer interaction plays an important role in CuO.     

Fe valence determination and Li elemental distribution in lithiated FeO₀.₇F₁.₃/C nanocomposite battery materials by electron energy loss spectroscopy (EELS)

Electron energy loss spectroscopy (EELS) is a powerful technique for studying Li-ion battery materials because the valence state of the transition metal in the electrode and charge transfer during lithiation and delithiation processes can be analyzed by measuring the relative intensity of the transition metal L₃ and L₂ lines. In addition, the Li distribution in the electrode material can be mapped with nanometer scale resolution. Results obtained for FeO_0.7F_1.3/C nanocomposite positive electrodes are presented. The Fe average valence state as a function of lithiation (discharge) has been measured by EELS and results are compared with average Fe valence obtained from electrochemical data. For the FeO_0.7F_1.3/C electrode discharged to 1.5 V, phase decomposition is observed and valence mapping with sub-nanometer resolution was obtained by STEM/EELS analysis. For the lowest discharge voltage of 0.8 V, a surface electrolyte inter-phase (SEI) layer is observed and STEM/EELS results are compared with the Li-K edges obtained for various Li standard compounds (LiF, Li₂CO₃ and Li₂O).     

Charge-transfer hole-lifetime broadening of the Cu Auger-electron spectra of high Tc superconductors

The satellite intensity in the Cu L₂₃-VV Auger-electron spectrum of the high T_c superconductor (YBa₂Cu₃O_7−x, 123) is much more enhanced as compared to that of CuO. This enhancement was previously interpreted by Ramaker et al. [D.E. Ramaker, N.H., Turner, F.L. Hutson, Phys. Rev. B 38 (1988) 11368] as a result of the mixing between the ddp and dpp states. Here, d is a hole in the Cu 3d band and p is a hole in the O 2p band. However, the dramatic Auger electron spectroscopy (AES) spectral lineshape change from CuO to 123 is not only in the charge-transfer (CT) satellite but also in the main-line width. The change arises from the transit of the “pairing” of two bound d holes in the ddp state to that of two bound p holes in the dpp state. As a result, in CuO there is no CT satellite but the dd state becomes a resonant state broadened by the CT hole-lifetime broadening, whereas in 123 the dd state becomes a mixture of a resonant-like state and nonresonant band states. The present many-body theory can explain the overall AES lineshape change from CuO to 123.     

Selection rules for bound exciton Auger recombination in semiconductors

We use selection rules developed for Auger-electron spectroscopy to study non-radiative transitions of neutral acceptor bound excitons. We show that (total) angular momentum selection rule implies that excitons originating from the J = 0 and the J = 2 two-hole states have different Auger transition probabilities. The results are shown to be in agreement with photoluminescence experiments.     

The initial oxidation of Cu(100) single crystal surfaces: an electron spectroscopic investigation

The total energy distribution of electrons emitted from clean Cu(100) and oxygen covered surfaces is analysed. A primary electron energy of 400 eV enabled the investigation of characteristic losses (ELS), Cu MVV Auger transitions and true secondary electrons in a single spectroscopic run. Oxygen exposure up to 10⁸ L at elevated temperature (~400 K) results in a Cu density of states (DOS) strongly affected by O(2p) electrons. The Auger lines of Cu, atomic-like for clean surfaces, reveal DOS effects after some 10⁷ L oxygen exposure: all MVV transitions shift down by ~2 eV in spite of a fixed M₂₃ level; the M₂₃VV Auger line splitting is vanishing due to a broadened valence band maximum allowing the deexcitation of the final two-hole state of intraatomic transitions. Heating the oxygen covered crystal to 820 K is accompanied by the removal of much surface oxygen and an electronic state resembling an earlier oxidation state without DOS effects in the Cu Auger spectrum.     

Hydrogen chemisorption on silicon (100) 2 × 1

Changes in the valence band density of states (DOS) of a (100) silicon surface that accompany he chemisorption of atomic hydrogen onto that surface are deduced from a study of the changes in the L_2,3VV Auger lineshape. Complementary changes in the conduction band DOS are inferred from changes in L_2,3VV-core-level characteristic loss spectra (CLS). The chemisorbed hydrogen layer is identified as the dihydride phase from low energy electron diffraction measurements. Upon hydrogen adsorption the DOS at the top of the valence band decreases and new energy levels associated with the Si-H bonds appear lower in the band. Assuming that the Auger signal from the hydrogen covered sample consists of a superposition of a signal from silicon atoms bonded to hydrogen in the dihydride layer and an elemental-Si signal from the substrate, a N(E) difference spectrum with features due only to the dihydride is obtained by subtracting the background corrected, loss deconvoluted L_2,3VV signal for a clean (100)Si surface rom the corresponding signal for the hydrogen covered surface. Comparisons of the energy position of the major peak in this difference spectrum with that of the main peak in a gas phase silane Si-L_2,3VV spectrum, and of the corresponding Auger energy calculated empirically, indicate a hole—hole interaction energy of ~8 eV for the two-hole final state in the gaseous system and zero for the dihydride surface system. Hydrogen induced changes in the conduction band DOS are less apparent than those of the valence band DOS with only the possibility of a decrease in the DOS at the bottom of the conduction band being inferred from the CLS measurements. Electron stimulated desorption of hydrogen from the dihydride layer is adduced from changes in the Auger lineshape under electron beam irradiation of the surface. Hydrogen induced changes in the near-elastic electron energy loss spectra (ELS) are also reported and compared with previously published ELS results.