Auger Line Shape Research Articles

Partial densities of states from x-ray absorption and Auger electron spectroscopy: Use of core-hole memory.

Ag L 3 edge x-ray absorption and L 3 - M 4,5 M 4,5 Auger electron spectra have been measured and simulated for a variety of x-ray electric-field polarization and Auger electron emission directions. The theory relies on density-functional theory, use of the Bethe-Salpeter equation, atomic multiplet theory, and a simplified model for the Auger line shape and its dependence on photon energy. We also demonstrate that partial densities of states for d T 2g , d Eg , and s Ag symmetry partial-wave channels at the Ag site in the solid can be deduced from the experimental measurements with only atomic theoretical input, i.e., with no solid-state theory other than assuming a Fermi edge.

Formation of graphitic films on Cu(111) via electron beam induced deposition

Electron beam induced deposition using ethylene as precursor gas is used to generate carbonaceous films on Cu(111). The study was performed in an ultrahigh vacuum chamber for two different precursor gas pressures, and two substrate temperatures. Auger electron spectroscopy and reflection electron energy loss spectroscopy were used to characterize the film growth process in-situ. These techniques allowed us to determine how the deposited film covers the Cu(111) substrate as a function of the ethylene exposure, obtaining a final coverage of up to 0.8 monolayer, depending on the growing conditions. Based on the CKLL Auger line shape and on the reflection electron energy loss (REELS) spectra we can conclude on the graphitic characteristics of the grown film and on the optimum growing conditions: lower substrate temperature and higher ethylene pressure. Raman spectroscopy and scanning tunneling spectroscopy were used as ex-situ techniques to assess the properties of the grown film. Results from both techniques indicate that the film consists of small (10–20 nm in size) nanocrystals of few (1–2) layers graphene. Once optimized, this method may allow growing graphene with a predesigned pattern, without the need to heat the substrate at high temperatures.

Recoil-induced ultrafast molecular rotation probed by dynamical rotational Doppler effect

Observing and controlling molecular motion and in particular rotation are fundamental topics in physics and chemistry. To initiate ultrafast rotation, one needs a way to transfer a large angular momentum to the molecule. As a showcase, this was performed by hard X-ray C1s ionization of carbon monoxide accompanied by spinning up the molecule via the recoil "kick" of the emitted fast photoelectron. To visualize this molecular motion, we use the dynamical rotational Doppler effect and an X-ray "pump-probe" device offered by nature itself: the recoil-induced ultrafast rotation is probed by subsequent Auger electron emission. The time information in our experiment originates from the natural delay between the C1s photoionization initiating the rotation and the ejection of the Auger electron. From a more general point of view, time-resolved measurements can be performed in two ways: either to vary the "delay" time as in conventional time-resolved pump-probe spectroscopy and use the dynamics given by the system, or to keep constant delay time and manipulate the dynamics. Since in our experiment we cannot change the delay time given by the core-hole lifetime τ, we use the second option and control the rotational speed by changing the kinetic energy of the photoelectron. The recoil-induced rotational dynamics controlled in such a way is observed as a photon energy-dependent asymmetry of the Auger line shape, in full agreement with theory. This asymmetry is explained by a significant change of the molecular orientation during the core-hole lifetime, which is comparable with the rotational period.

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.

Oxygen vacancy induced structure change and interface reaction in HfO2 films on native SiO2/Si substrate

The HfO2 films were deposited on SiO2 (native)/n-Si (100) substrates by electron beam evaporation (EBE) technology. The structural evolution of the films during thermal annealing were studied by using grazing incidence X-ray diffraction (GIXRD), Raman spectra and Auger electron spectroscopy (AES), while the change of bonding structure and interface products were obtained by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The existence of oxygen vacancies in the surface layer of the as-deposited HfO2 film was confirmed by Auger line shapes, which could facilitate the adsorption of ambient oxygen on the free surface and induce the diffusion of oxygen atoms to the surface layer during thermal annealing. Meanwhile, the newly formed defects in the films could provide more passageways for the diffusion of oxygen atoms to the film-substrate interface layer. The oxygen that diffused to the interface layer was the key factor of the change in the interface structure, which participated in the interface reaction. In addition, the formation amount and bonding structure of the interface products such as suboxide of silicon and hafnium silicate were influenced by the oxygen concentration of the external environment.

Simulation of molecular Auger spectra using a two-electron Dyson propagator

In order to simulate Auger electron spectra (AES), we propose the use of the two-electron Dyson propagator with the shifted denominator approximation (SD2). The double ionization potentials (DIPs) of molecules calculated using the SD2 method have shown good agreement with experimental data. This method can be used to calculate each DIP separately, and reducing the matrix dimensionality into that of only a two-hole configurations. We carried out AES simulations of water (H2O), ethylene (C2H4), and formaldehyde (H2CO) molecules and compared with the observed spectra. Furthermore Auger line shapes of glycine and hydrated glycine molecules were simulated, it found out that the peaks of nitrogen K-LL Auger were broadened due to hydration. From these results, we conclude that the SD2 method is very useful for the calculation of DIPs to investigate the properties of a double ionized molecule.

X-ray excited Auger transitions of Pu compounds

X-ray excited Pu core–valence–valence and core–core–valence Auger line-shapes were used in combination with the Pu 4f photoelectron peaks to characterize differences in the oxidation state and local electronic structure for Pu compounds. The evolution of the Pu 4f core-level chemical shift as a function of sputtering depth profiling and hydrogen exposure at ambient temperature was quantified. The combination of the core–valence–valence Auger peak energies with the associated chemical shift of the Pu 4f photoelectron line defines the Auger parameter and results in a reliable method for definitively determining oxidation states independent of binding energy calibration. Results show that PuO2, Pu2O3, PuH2.7, and Pu have definitive Auger line-shapes. These data were used to produce a chemical state (Wagner) plot for select plutonium oxides. This Wagner plot allowed us to distinguish between the trivalent hydride and the trivalent oxide, which cannot be differentiated by the Pu 4f binding energy alone.

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.

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.

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.

Theory of ac Stark splitting in core-resonant Auger decay in strong x-ray fields

In this work we report the modification of the normal Auger line shape under the action of an intense x-ray radiation. Under strong Rabi-type coupling of the core, the Auger line profile develops into a doublet structure with an energy separation mainly determined by the relative strength of the Rabi coupling. In addition, we find that the charge resolved ion yields can be controlled by judicious choice of the x-ray frequency.

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.

Probing dissociative molecular dications by mapping vibrational wave functions

We present high-resolution photoelectron--Auger-electron coincidence spectra of methane (${\mathrm{CH}}_{4}$). Since the vibrational structure in the photoelectron spectrum is resolved, the Auger spectra corresponding to different vibrational levels can be separated. The seven final states of ${\mathrm{CH}}_{4}^{2+}$ are either dissociative or metastable, but in any case are populated in a repulsive part of their potential-energy curve via the Auger decay. The Auger line shapes can therefore be obtained by mapping the vibrational wave functions of the core-hole state into energy space. We have implemented this connection in the data analysis. By simultaneously fitting the different Auger spectra, detailed information on the energies of the dicationic states and their repulsive potential-energy curves is derived.

On the theory of the KVV auger spectrum of Li

Many-body effects on the core-valence-valence Auger lineshape of Li are evaluated in a change-of-mean-field, free-electron model.

Mixedab initioquantum mechanical and Monte Carlo calculations of secondary emission fromSiO2nanoclusters

A mixed quantum mechanical and Monte Carlo method for calculating Auger spectra from nanoclusters is presented. The approach, based on a cluster method, consists of two steps. Ab initio quantum mechanical calculations are first performed to obtain accurate energy and probability distributions of the generated Auger electrons. In a second step, using the calculated line shape as electron source, the Monte Carlo method is used to simulate the effect of inelastic losses on the original Auger line shape. The resulting spectrum can be directly compared to ``as-acquired'' experimental spectra, thus avoiding background subtraction or deconvolution procedures. As a case study, the $\text{O}\text{ }K\text{\ensuremath{-}}LL$ spectrum from solid ${\text{SiO}}_{2}$ is considered. Spectra computed before or after the electron has traveled through the solid, i.e., unaffected or affected by extrinsic energy losses, are compared to the pertinent experimental spectra measured within our group. Both transition energies and relative intensities are well reproduced.

The line shape of L2 - L2L3M4,5 - M4,5M4,5(M4,5) satellites in the auger spectra of solids

We propose the foundations of an extended Auger Line Shape Analysis aiming to include the Coster-Kronig (CK) satellites in solid state spectra. In many solids like Cu and Zn metals they show up as intense high binding energy satellites of the Core-Valence-Valence (CVV) lines. Our theory covers the whole range between weak and strong correlation. We find that the satellites display three-hole and distorted two-hole features along with uncorrelated band-like continua.

Phase identification from electronic structures by Auger electron spectroscopy

A technique of phase identification from the characteristics of electronic structures is established by Auger electron spectroscopy. GaN epilayers in wurtzite and zinc-blende polytypes are used for practical investigations. Auger spectra show phase-dependent energetic shifts and peak intensity variations. Simulation of theoretical spectra reveals the substantial correlation of the Auger line shape with the bonding electronic states. This approach demonstrates the correspondence between electronic structure and atomic structure and hence provides criteria for phase identification.

Interfacial Structure of Ta 2O 5/Si Film and Photoactivity

Tantalum oxide (Ta 2O 5) thin film photocatalysts were prepared on single crystal Si(110) substrates via sol-gel and spin coating methods. Ta 2O 5 crystallinity was improved, and the crystal size became larger with the increase of heat-treating temperature. The interface diffusion and reaction of the film were studied by using Auger electron spectroscopy (AES) depth profile and line shape analysis. Diffusion was dominant at the interface layer when the calcination temperature was below 700 °C. When the temperature reached 800 °C, both interface diffusion and reaction occurred. The photocatalytic activity was studied using aqueous salicylic acid as a degradation probe molecule under UV-light irradiation. It was found that Ta 2O 5/Si film showed a photocatalytic activity similar to that of TiO 2/Si film.

Auger-photoelectron coincidence spectroscopy of SiO 2

The O K-LL spectrum from a SiO 2 surface was measured in coincidence with O 1s photoemission in SiO 2. The coincidence lineshapes are compared to the simultaneously obtained conventional Auger lineshapes and to model lineshapes. The low-energy tail of the O K-LL lineshape from SiO 2 is suppressed in the APECS spectrum due to discrimination against extrinsic loss and initial-state shake processes, but remaining intensity indicated significant contribution from final-state shake-up/off processes. The results also confirm that a low energy satellite to the O K-L 2,3L 2,3 Auger line that is eliminated in the coincidence spectrum, and a high-energy shoulder that is enhanced in the coincidence spectrum, arise from an initial-state shake process, and final-state correlation effects, respectively. The results demonstrate the viability of applying APECS to the study of other metal oxides via the anion.