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 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.     

Competition between relaxation and decay of a resonantly excited core-hole state

By a many-body theory it is shown that in general when the time scale of the relaxation from the resonantly excited core-hole state to the fully relaxed core-hole state is much shorter than that of core-hole decay, the autoionization (deexcitation) spectrum is more or less identical to the Auger-electron spectrum by core-hole decay of the fully relaxed core-hole state. Here, the fully relaxed core-hole state is the lowest core-hole state in the X-ray photoelectron spectroscopy spectrum. The present theory explains why the C 1s autoionization spectrum of CO molecule adsorbed on Ni(1 0 0) surface measured at the resonant core-level electron excitation energy is more or less indentical to the Auger-electron spectrum measured at far above the core-level electron ionization limit.     

The M5 photoelectron line of Ag metal measured in coincidence with the M5–N45N45 Auger-electron spectral line

The M₅ (3d_5/2) photoelectron line of Ag metal was measured in coincidence with the M₅–N₄₅N₄₅ (¹G or ³F) Auger-electron line by Auger-photoelectron coincidence spectroscopy (APECS). The M₅ photoelectron line of Ag metal measured in coincidence with either the ¹G or the ³F Auger-electron line is unshifted, within the accuracy of the experiment. As the theory [M. Ohno, J. Electron Spectrosc. Relat. Phenom. 124 (2002) 53] predicts, the energy shift and asymmetrical narrowing of the coincidence photoelectron line compared to the singles one is much smaller than that in Cu metal.     

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.     

Inherent post-collision interaction in spectator: Auger-transitions in metals and charge-transfer systems

The inherent post-collision interaction (PCI) between an Auger-electron and a spectator electron in core-hole decay in metals and charge-transfer systems is described in a systematic maimer by a many-body theory. The inherent PCI effect on spectator Auger-transition spectral lineshapes of metals and charge-transfer systems is discussed. The spectator Auger-transition energy shift due to a change of chemical environment is discussed in the light of the inherent PCI.     

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 Lx (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.     

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 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.).     

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.     

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.     

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.     

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.     

Why are the band-like states suppressed in the M3–M4,5M4,5 super Coster–Kronig electron spectrum of Cu metal compared to those in the L3–M4,5M4,5 Auger electron spectrum?

The relative spectral intensity of the band-like two M_4,5-hole state to the atomic-like localized one is much suppressed in the coincidence M₃–M_4,5M_4,5 super Coster–Kronig (sCK) electron spectrum of Cu metal compared to the one in the coincidence L₃–M_4,5M_4,5 Auger electron spectroscopy (AES) spectrum. The M₃-hole lifetime width of Cu metal is calculated by an ab initio atomic many-body theory (the extended relaxed core random phase approximation with exchange). The calculated M₃-hole lifetime width of Cu metal agrees well with the experimental one. The M₃–M_4,5M_4,5 sCK decay width of Cu metal decreases much with delocalization of the two M_4,5 holes in the sCK final state, whereas the Auger decay width is fairly independent of localization and delocalization of the two M_4,5 holes in the Auger final state. Thus, the relative spectral intensity of the band-like state is much suppressed in the coincidence M₃–M_4,5M_4,5 sCK-electron spectrum of Cu metal compared to the one in the coincidence L₃–M_4,5M_4,5 AES spectrum.     

A theoretical study of the photoelectron spectrum of atomic lithium

The 1s photoelectron spectrum of atomic lithium has been calculated for incident photon energies of 151 and 1487 eV. For this open-shell system there are four possible final-state manifolds for the residual ion, with symmetries ³S, ¹S, ³P^o and ¹P^o. The line energies for the first four members of each manifold have been calculated using relaxed Hartree—Fock or configuration interaction wavefunctions as required. The agreement with available optical data is excellent.The intensity expressions neglecting interchannel coupling have been evaluated to allow a comparison of line intensities between final-state manifolds and hence the synthesis of theoretical spectra. The variation of the spectral profile with photon energy is clearly demonstrated. Conjugate shake-up, as well as shake-up processes are predicted to be important at high photon energy.     

The effect of localization on interference. II. Bearing on locality violation and the interpretation of quantum mechanics

In a two-channel interference experiment such as that considered in the preceding companion paper, a quantum may be localizable predominantly in one channel by a time-coincident experiment on a correlated quantum. The Copenhagen interpretation of quantum mechanics then requires a coincidence intensity prediction having the same reduced interference between channels as if the probability amplitude in the other channel had been attenuated by a filter. The quantum mechanical treatment of correlated systems originated by von Neumann does predict this reduced interference, but avoids requiring a resulting direct locality violation by predicting that this reduced interference also occurs in a simple singles intensity observation. In contrast, de Broglie's locally causal interpretation of quantum mechanics requires that the experiment on the remote correlated system cannot change the amplitudes or phase relationship of coherent space-time wave propagating through the two channels, so that the full classical optics interference effect should be predicted for both singles and coincidence intensities.     

Anomalous Auger-electron spectra of metallic calcium

A1Kα-excited L_{2, 3}MM and L_{2, 3}MV Auger-electron spectra of Ca have been measured in ultrahigh vacuum from a metallic sample evaporated onto an Ag substrate. An interpretation of the spectra is made by applying a line-fitting procedure. The lineshape and the solid-state—free-atom kinetic-energy shift are also studied. The extrinsic loss structure in the L_{2, 3}MM Auger-electron emission is found to be similar to that in 2p photoelectron emission. Spin—density-functional (SDF) calculations for the singularity index describing the intrinsic lineshape give a value of ～ 0.35 for both processes. Thus the experimental 2p_3/2 photoelectron line broadened from 1.2 to ～ 5 eV FWHM has been used as a standard line in the line fitting of the L_{2, 3}MM transitions. The term splitting of the L_{2, 3}M_{2, 3}M_{2, 3} transition is larger than in the corresponding free-atom spectrum. This result is also supported by the SDF calculations. The L_{2, 3}M_{2, 3}V spectrum is anomalously sharp, probably both because of the structure of the local density of states at the site of the core-ionized atom and because of differences in the transition probabilities into the different parts of the band. The experimental solid-state shift is 20.3 eV for the L_{2, 3}M_2,3M_{2, 3}:¹D transition, and the binding-energy shifts are 8.3 and 6.1 eV for the 2p and 3p levels, respectively. The SDF shifts for the above transitions are 19.9 (configurational average), 9.4 and 8.0 eV, consecutively, in agreement with the experimental values. The calculations also show a localized d-type (atomic-like) structure for the screening of the initial- and final-state core hole (s). This is the origin of the large values of both the singularity index and the solid-state shift.     

高频激光脉宽对原子光电子发射谱的影响

采用广义含时伪谱方法数值求解原子在激光脉冲作用下的动量空间含时薛定谔方程,研究了高频激光脉宽对原子光电子发射谱的影响.数值模拟表明,随着激光脉冲宽度的增加,光电子谱干涉结构的振荡幅值逐渐减小,其最大峰值的强度和位置取决于产生有效电离的最大即时强度.通过分析光电子谱的变化规律能进一步加深对高频强场电离产生的动力学干涉效应的理解.     

电光调制器低频控制对受残余幅度调制影响的频率调制光谱线型优化研究

频率调制(FM)光谱技术中由于激光偏振态变化产生的残余幅度调制(RAM)使其在微量气体检测中的应用受到极大的限制。理论上详细分析了这一过程产生的原因,获得了存在RAM时FM光谱线型的表达式,同时给出N.C.Wong和J.L.Hall(W-H)方案抑制RAM后的FM光谱线型表达式;在实验上通过对乙炔气体的测量获得了存在RAM时的光谱线型,同时采用W-H方案对RAM进行了抑制,并获得了优化的光谱线型;最后基于理论结果对实验线型进行了拟合,两者差值小于信号峰峰值的4%。