Auger Photoelectron Coincidence Research Articles

A Blyholder mechanism in the chemisorption of N2O on Ni(111) – studied with Auger-photoelectron coincidence spectroscopy

In heterogeneous catalysis the surface-adsorbate bond strength is critical for the function of the system. Here we study a series consisting of multilayer, bilayer and monolayer N2O on Ni(111) and employ Auger-photoelectron coincidence spectroscopy (APECS) to study the interaction between the molecule and the substrate directly. We observe intensity in the nitrogen Auger spectra that arise from the interaction between molecule and surface (not observed in free molecules) whereas the oxygen spectra are thickness-independent. Since the two nitrogen atoms of N2O are chemically inequivalent we can assign the intensity present in the bilayer and monolayer cases to orbitals centered on the terminal nitrogen which is closest to the Ni(111) surface. Using ab initio, molecular dynamics and solid-state density functional theory calculations we infer a Blyholder model of the surface bond as consisting of donation from the terminal nitrogen lone-pair valence orbital with back-donation from the metal into the unoccupied orbitals on that nitrogen. This coincidence technique can readily be used to study substrate–adsorbate interactions directly with chemical and orbital specificity — this opens up prospects to study fundamental steps of molecular adsorption and heterogeneous catalysis with unprecedented detail.

The degree of electron itinerancy and shell closing in the core-ionized state of transition metals probed by Auger-photoelectron coincidence spectroscopy.

Auger-photoelectron coincidence spectroscopy (APECS) has been used to examine the electron correlation and itinerance effects in transition metals Cu, Ni and Co. It is shown that the LVV Auger, in coincidence with 2p photoelectrons, spectra can be represented using atomic multiplet positions if the 3d-shell is localized (atomic-like) and with a self-convoluted valence band for band-like (itinerant) materials as explained using the Cini-Sawatzky model. For transition metals, the 3d band changes from band-like to localized with increasing atomic number, with the possibility of a mixed behavior. Our result shows that the LVV spectra of Cu can be represented by atomic multiplet calculations, those of Co resemble the self-convolution of the valence band and those of Ni are a mixture of both, consistent with the Cini-Sawatzky model.

Measurement of the full electron spectrum associated with the Ag N3VV Auger transitions: Evidence for the contribution of multi-electron Auger processes

We report the first measurement of the full energy spectrum (down to 0 eV) of electrons emitted from the surface of Ag(100) as a result of the N3VV Auger transition. A comparison with a Monte-Carlo model was used to obtain an estimate of the contribution of multi-electron Auger processes to the low energy part of the Auger spectrum. The experimental Auger spectrum was obtained in time coincidence with photo-emitted 4p3/2 core electrons from Ag using Auger-Photoelectron coincidence spectroscopy (APECS). The coincidence condition in APECS can remove most, but not all, spectral contributions from events unrelated to the Auger transition. The remaining contributions to the APECS spectrum from Auger-unrelated events, which are significant, were determined from a series of coincidence measurements of inelastically scattered valence band photoelectrons. The contribution from Auger-unrelated events was subtracted from the APECS Auger spectrum of Ag(100) to obtain the full Auger spectrum down to 0 eV. The resulting spectrum includes, in addition to Auger electrons emitted without energy loss, inelastically scattered Auger electrons. We observe that the spectral weight of the low energy tail (LET) in the APECS spectrum is reduced by more than a factor of two after subtracting the Auger-unrelated contributions. However, the intensity of the LET after this subtraction is still two times the integrated intensity in the Auger peak. A Monte Carlo model that includes the inelastic transport of the N3VV Auger electrons through the surface showed that LET produced by the inelastic scattering of the Auger electrons has the same intensity as the integrated intensity in the Auger peak. The excess intensity in the experimental LET provides strong evidence that the Auger related spectrum also includes contribution from multiple electron emissions that are intrinsic to the creation and/or the Auger decay of the core hole. Our estimate shows that as low as 20% to as high as 60% of the core hole decay events result in the emission of more than one Auger electron from Ag (100). This result demonstrates the need for a better understanding of the contribution of valence-electron correlations to the entire CVV Auger spectrum down to 0 eV.

Resonant-Auger-state-selected dissociation dynamics and dissociation limits of N 1s→π* core excited N2 molecules studied using a two-dimensional Auger-electron-photoion coincidence method

The dissociation dynamics of core-excited N2 molecules has been investigated using a two-dimensional Auger electron photoion coincidence method, where the kinetic energies of Auger electrons and ions are measured in coincidence. From the energy correlations between the binding energies of the resonant Auger states and the ion kinetic energy release, the electronic states of fragments at the dissociation limit are investigated for each Auger peak. We also estimate the contribution of double Auger decay to the total Auger spectrum by analyzing events where two N+ ions or metastable doubly charged ions N2++ were detected.

Normal Auger processes with ultrashort x-ray pulses in neon

Modern x-ray sources enable the production of coherent x-ray pulses with a pulse duration in the same order as the characteristic lifetimes of core-hole states of atoms and molecules. These pulses enable the manipulation of the core-hole population during Auger decay processes, modifying the lineshape of the electron spectra. In this work, we present a theoretical model to study those effects in Neon. We identify effects in the Auger-photoelectron coincidence spectrum due to the duration and intensity of the pulses. The normal Auger line-shape is recovered in Auger electron spectrum integrated over all photoelectron energies.

Stability and dissociation dynamics of N2 (++) ions following core ionization studied by an Auger-electron-photoion coincidence method.

An Auger-electron-photoion coincidence (AEPICO) method has been applied to study the stability and dissociation dynamics of dicationic states after the N K-shell photoionization of nitrogen molecules. From time-of-flight and kinetic energy analyses of the product ions, we have obtained coincident Auger spectra associated with metastable states of N2 (++) ions and dissociative states leading to N2 (++) → N(+) + N(+) and N(++) + N. To investigate the production of dissociative states, we present two-dimensional AEPICO maps which reveal the correlations between the binding energies of the Auger final states and the ion kinetic energy release. These correlations have been used to determine the dissociation limits of individual Auger final states.

Local Valence Electronic States and Valence-Band Maximum of Ultrathin Silicon Nitride Films on Si(111) Studied by Auger Photoelectron Coincidence Spectroscopy: Thickness and Interface Structure Dependence

The local valence electronic states of Si3N4 films grown on Si(111)-7 × 7 [Si3N4/Si(111)] have been investigated by coincidence spectroscopy. The Si L23VV Auger electron spectra (AES) measured in coincidence with the Sin+ 2p photoelectrons of β-Si3N4(0001)/Si(111)-8 × 8 indicate that the binding energy of the local valence electronic state at Sin+ increases as n increases. Si4+ L23VV AES measured as a function of the β-Si3N4(0001) thickness show that the binding energy at the valence-band maximum (BEVBM) of β-Si3N4(0001) decreases by 1.6 ± 0.5 eV as the Si3N4 thickness decreases from 3.6 to 0.7 A. The large decrease is attributed to the hybridization of the valence electronic state of Si3N4 with those of neighboring subnitrides and to the formation of β-Si3N4(0001) islands. The BEVBM value of the 3.6-A-Si3N4/Si(111)-quadruplet decreases by 0.7 ± 0.6 eV from that of 3.6-A-β-Si3N4(0001)/Si(111)-8 × 8. The decrease in BEVBM is attributed to the different interface structures.

The LVV Auger line shape of sulfur on copper studied by Auger photoelectron coincidence spectroscopy

We have studied the line shapes of Cu(0 0 1)-p (2 × 2)S L2VV and L3VV Auger decay by means of Auger photoelectron coincidence spectroscopy. Measuring the LVV Auger spectrum in coincidence with S 2p1/2 and 2p3/2 photoelectrons respectively, we have been able to separate the two overlapping Auger spectra and determine their intrinsic line shapes. The two Auger transitions, though shifted in energy, display an identical line shape whose main features can be qualitatively understood considering a single particle approximation but are better described within a Cini-Sawatzky (CS) approach. Comparison between the experimental and the CS calculated spectra confirms that a substantial part of the Auger lines (∼20%) can be ascribed to decay events accompanied by the excitation of one additional electron–hole pair in the valence band. For the first time, the locality of the Auger process combined with the surface sensitivity of the APECS technique and its ability to separate overlapping structures are used to study Auger transitions taking place at the the surface states of a S/noble-metal interface.

Measurement of the background in Auger-photoemission coincidence spectra (APECS) associated with inelastic or multi-electron valence band photoemission processes

Auger photoelectron coincidence spectroscopy (APECS), in which the Auger spectra are measured in coincidence with the core level photoelectron, is capable of pulling difficult to observe low energy Auger peaks out of a large background due mostly to inelastically scattered valence band (VB) photoelectrons. However the APECS method alone cannot eliminate the background due to valence band photoemission processes in which the initial photon energy is shared by two or more electrons and one of the electrons is in the energy range of the core level photoemission peak. Here we describe an experimental method to determine the contributions from these background processes and apply this method in the case of copper M3VV Auger spectrum obtained in coincidence with the 3p3/2 photoemission peak. A beam of 200eV photons was incident on a Cu(100) sample and a series of coincidence measurements were performed using a spectrometer equipped with two cylindrical mirror analyzers (CMAs). One CMA was set at series of fixed energies that ranged between the energy of the core and the VB peaks. The other CMA was scanned over a range corresponding to electrons leaving the surface between 0eV and 70eV. The set of measured spectra were then fit to a parameterized function which was extrapolated to determine the background in the APECS spectra due to multi-electron and inelastic VB photoemission processes. The extrapolated background was subtracted from the APECS spectrum to obtain the spectrum of electrons emitted solely as the result of the Auger process. A comparison of the coincidence spectrum with the same spectrum with background removed shows that in the case of Cu M3VV the background due to the inelastic scattering of VB electrons is negligible in the region of the Auger peak but is more than half the total signal down in the low energy tail of the Auger peak.

Monitoring antiferromagnetism via angle-resolved Auger photoelectron coincidence spectroscopy: The case of NiO/Ag(001)

Spin selectivity in angle-resolved Auger photoelectron coincidence spectroscopy (AR-APECS) is used to probe electronic structure in antiferromagnetic thin films. In particular, exploiting the AR-APECS capability to discriminate Auger electron emission events characterized by a different spin of the ion in its final state, a sharp multiplet structure in the Ni $\mathit{MVV}$ Auger line shape of NiO/Ag(001) thin films is measured below the critical N\'eel temperature. The assignment of multiplet terms follows from a close comparison of the experimental AR-APECS line shapes with the predictions based on semiempirical calculations on a cluster model and an open-band extension of the Cini-Sawatzky approach. In analogy to CoO, also in NiO, above the N\'eel temperature a more featureless Auger spectrum appears and AR-APECS does not disentangle anymore high-spin and low-spin contributions to the total Auger intensity. Such a behavior, which seems to be a general result for metal oxide antiferromagnetic systems, is discussed.

Theory of the APECS spectra of antiferromagnetic transition metal oxides

Angular resolved Auger-Photoelectron Coincidence Spectra (APECS) show a dichroic effect called DEAR-APECS in the antiferromagnetic phase of transition metal oxides and reveal well-defined structures in the Auger line shapes which are not seen otherwise in such partially filled bands. We propose a theory of the XPS and M23M45M45 APECS line shapes. Experimental photoemission and Auger spectra of CoO and NiO are compared with computed profiles. The method takes strong correlation into account, based on the exact diagonalization of octahedral cluster models with different electron numbers, including the on-site repulsion on the central ion expressed in terms of Racah parameters. The hopping parameters are written as combinations of Slater–Coster ones. We compute the relevant Auger matrix elements. The Racah A parameter and the hopping parameters which are known from the literature are refined by fitting the XPS lines; this allows for a parameter-free evaluation of the Auger lines. The Auger line shapes are explained in detail for both compounds, labeling the final states by their total spin. In the case of Co the different spin components peak at separated spectral regions and one sees a transfer of intensity to the high-spin region by varying the photoelectron-Auger electron geometry. In the NiO case the peaks overlap much more but a similar transfer from singlet to triplet final states is nevertheless unambiguously observed, and the theory accounts well for the observed resonance energies. Finally, we highlight the main questions about the dichroic effect that are still unanswered and will require a considerable extension of the theory.

Attempts to Improve the Sensitivity and the Energy Resolution of an Analyzer for Auger Photoelectron Coincidence Spectroscopy and Electron Ion Coincidence Spectroscopy

Sadanori Arae, Rui Kanemura, Kenta Hiraga, Yosuke Ogashiwa, Kohtaro Yanase, Noritsugu Kanayama, Shinya Ohno, Takuhiro Kakiuchi, Kazuhiko Mase, Koji K. Okudaira, Makoto Okusawa, and Masatoshi Tanaka 1 Yokohama National University, Yokohama 240-8501, Japan 2 Gunma University, Maebashi 371-8510, Japan Chiba University, Inage-ku 263-8522, Japan 4 Ehime University, Matsuyama 790-8577, Japan 5 Photon Factory, Tsukuba 305-0801, Japan The Graduate University for Advanced Studies, Tsukuba 305-0801, Japan

オージェ電子-光電子コインシデンス分光法によるSiO2/Si超薄膜の表面界面基板を選別した局所価電子状態の研究

オージェ電子-光電子コインシデンス分光法によるSiO2/Si超薄膜の表面界面基板を選別した局所価電子状態の研究

Spin-Dependent On-Site Electron Correlations and Localization in Itinerant Ferromagnets

Spin selectivity in angle-resolved Auger photoelectron coincidence spectroscopy (AR-APECS) is used to probe electron correlation in ferromagnetic thin films. In particular, exploiting the AR-APECS capability to discriminate Auger electron emission events characterized by valence hole pairs created either in the high or in the low total spin state, a strong correlation effect in the Fe M(2,3)VV Auger line shape (measured in coincidence with the Fe 3p photoelectrons) of Fe/Cu(001) thin films is detected and ascribed to interactions within the majority spin subband. Such an assignment follows from a close comparison of the experimental AR-APECS line shapes with the predictions of a model based on spin polarized density functional theory and the Cini-Sawatzky approach.

Site-specific ion desorption from condensed F3SiCD2CH2Si(CH3)3 induced by Si-2p core-level ionizations studied with photoelectron photoion coincidence (PEPICO) spectroscopy, Auger photoelectron coincidence spectroscopy (APECS) and Auger electron photoion coincidence (AEPICO) spectroscopy

Ion desorption from condensed F3SiCD2CH2Si(CH3)3 induced by Si-2p core-level ionizations of the F3Si and the Si(CH3)3 sites (Si[F] and Si[Me]) was investigated using photoelectron photoion coincidence (PEPICO) spectroscopy, Auger photoelectron coincidence spectroscopy (APECS) and Auger electron photoion coincidence (AEPICO) spectroscopy. A Si-2p-F+ PEPICO peak appeared selectively in the Si[F]-2p region. Si-2p-H+ PEPICO peaks were, on the other hand, observed in both the Si[Me]- and Si[F]-2p regions. The structure of the Si-L23VV-Si[F]-2p Auger photoelectron coincidence spectrum (APECS) was quite different from that of the Si-L23VV-Si[Me]-2p APECS. Ab initio molecular orbital calculations qualitatively reproduced the measured APECSs. The peak positions in the Si-L23VV-F+ AEPICO spectrum were in good agreement with those in the Si-L23VV-Si[F]-2p APECS. The Si-L23VV-H+ AEPICO spectrum, on the other hand, showed no characteristic peaks. Based on these results we have concluded that the site-specific F+ desorption is stimulated by Si[F]-L23VV Auger processes, and that both the Si[Me]- and Si[F]-L23VV Auger decays are responsible for the non-site-specific H+ desorption.

Study of Local Valence Electronic States of SiO2Ultrathin Films Grown on Si(111) by Using Auger Photoelectron Coincidence Spectroscopy: Upward Shift of Valence-Band Maximum Depending on the Interface Structure

To clarify the factors governing the local valence electronic states of SiO 2 ultrathin films, we have measured the Si- L 23 V V –Si n + -2 p Auger-electron photoelectron coincidence spectra ( n = 0, 1, 2, 3, 4, where n represents the number of oxygen atoms bonded to Si) of SiO 2 thermally grown on a Si(111)-7×7 surface [SiO 2 /Si(111)]. The results indicate that the valence electronic states in the vicinity of the Si n + sites shift to deeper binding energies as n increases. Furthermore, Si 4+ - L 23 V V Auger electron spectra, measured as a function of SiO 2 thickness, taken in coincidence with Si 4+ -2 p photoelectron emission show that the valence-band maximum (VBM) of the SiO 2 layer shifts by 2.7 ±1.0 eV toward the Fermi level when SiO 2 thickness is decreased to ≈1 monolayer (ML). This upward shift is much larger than that for a SiO 2 layer with a thickness of ≈1 ML thermally grown on Si(100)-2×1 (about 1.6 eV). We attribute the large shift in the VBM of SiO 2 /Si(111) with a 1-ML-thick SiO 2 layer...

Coincidence photoelectron and Auger-electron spectral behaviors of Auger cascade decay

The Auger-photoelectron coincidence spectroscopy (APECS) and Auger Auger-electron coincidence spectroscopy (AAECS) spectral behaviors of Auger cascade decay are discussed by a many-body theory. There is a significant difference in the coincidence intensity distribution in a two-dimensional map according to whether a pair of coincidence electrons are generated, respectively, by creation and annihilation of the same n-hole state (n⩾1). The difference manifests as a line narrowing and a peak KE shift of the coincidence spectrum compared to the singles (noncoincidence) one. We discussed the inherent mechanism of APECS (AAECS), which causes the difference.

Insight on Hole-Hole Interaction and Magnetic Order from Dichroic Auger-Photoelectron Coincidence Spectra

The absence of sharp structures in the Auger line shapes of partially filled bands has severely limited the use of electron spectroscopy in magnetic crystals and other correlated materials. By a novel interplay of experimental and theoretical techniques we achieve a combined understanding of the photoelectron, Auger, and Auger-photoelectron coincidence spectra (APECS) of the antiferromagnetic CoO. A recently discovered dichroic effect in angle resolved (DEAR) APECS reveals a complex pattern in the Auger line shape, which is here explained in detail, labeling the final states by their total spin. Since the dichroic effect exists in the antiferromagnetic state but vanishes at the Néel temperature, the DEAR-APECS technique detects the phase transition from its local effects, thus providing a unique tool to observe and understand magnetic correlations where the usual methods are not applicable.

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.

Local Valence Electronic States of SiO2 Ultrathin Films Grown on Si(100) Studied Using Auger Photoelectron Coincidence Spectroscopy: Observation of Upward Shift of Valence-Band Maximum as a Function of SiO2 Thickness

The local valence electronic states of the surface, interface, and substrate for SiO 2 ultrathin films thermally grown on a Si(100)-2×1 have been investigated using Si- L 23 V V Auger-electron Si n + -2 p photoelectron coincidence spectroscopy ( n represents the number of oxygen atoms bonded to the Si). A series of Si- L 23 V V Auger electron spectra (AES) measured in coincidence with Si n + -2 p photoelectron indicate that the valence electronic states in the vicinity of the Si n + sites shift to the deeper binding-energy side as n increases. Furthermore, the Si 4+ - L 23 V V AES measured as a function of the thickness of the SiO 2 , show that the valence-band maximum of SiO 2 shifts ∼1.6 eV toward the Fermi level when the thickness of the SiO 2 film is decreased to 1.7–1.5 A. This shift is attributed to a decrease in the number of Si 4+ and an increase in the number of Si 3+ , Si 2+ , Si 1+ , and Si 0 in the vicinity of the topmost SiO 2 layer.