Core-level Threshold Research Articles

The bulk of evidence for novel electron transitions above the core level threshold

Standard equipment for electron spectroscopy empowers fingerprinting the adsorbed layer at atomic-molecular level. The resource results from the novel route for primary electron energy dissipation through a set of shake-off and shake-up electron transitions, each coupled with the threshold core level excitation. Experimental evidence for the route is based on regular fine structures in extended elastic electron spectra from the Pt(100) single crystal fitting the valence state structures of adsorbed species and substrate atoms as well as plasmon excitations; in total over 20 particular satellites have been detected in conventional experimental conditions. Theoretical justification of a route consists in the mechanism being a combination of well-known electron transitions. The phenomenon is considered as a fundamental regularity of electronsolid interaction disclosing the outer shell structure of near-surface atoms alike an Auger effect brings to light the inner shell structure.

Mechanism of conjugate electron transitions on the surface of a solid

The conjugate electron excitation (CEE) above the core-level thresholds of substrate atoms is a combination of the known electronic shake-off and shake-up electron transitions resulting in the location of excited electrons can at both vacuum level and vacant states near the Fermi level. The CEE satellites in elastic electron spectra provide the direct experimental data on the valence state structure of surface atoms and components of the adsorbed layer. The CEE control can be implemented as an additional option of the standard equipment for electron spectroscopy.

Resonant valence-band photoemission spectroscopy on the Fe62Ni20Cr18 alloy

The Fe62Ni20Cr18 valence band was studied by scanning the photon energy across the 2p3/2 core-level threshold of each element of the alloy. A resonant enhancement of the weak 3d-like features was observed. In pure transition metals, similar valence band resonances are explained by a radiation-less Raman de-excitation emission, which is active at threshold and degenerate with the two-hole satellite of direct photoemission. Present structures are associated to satellite features occurring in Fe62Ni20Cr18, and their intensities and binding energies are compared to those of the pure metal components. The alloy satellite resonant behaviour reveals some peculiar modifications of: a) the crossover between the radiation-less Raman scattering and the Auger emission regimes; b) the ratio of the relative intensities of the main and satellite peaks. We mainly assign these differences to the hetero-nuclear bonds in the alloy.

Core level ionization dynamics in small molecules studied by x-ray-emission threshold-electron coincidence spectroscopy

X-ray-emission threshold-electron coincidence spectroscopy has been applied to ${\mathrm{N}}_{2}$, ${\mathrm{O}}_{2}$, and ${\mathrm{N}}_{2}\mathrm{O}$. The main features of the spectra can be interpreted as conventional core level threshold electron spectra, free from postcollision interaction effects. The relative cross sections for adiabatic ionization of close-lying core hole states and vibrational substates are presented. The results are compared to theoretical predictions and state-of-the-art photoelectron spectra and are discussed in terms of direct threshold ionization and core vacancy rearrangement processes.

Micromagnetism of a Permalloy stripe array investigated by soft-X-ray T-MOKE

Using the soft-X-ray transverse magnetooptic Kerr-effect at core level thresholds, we have measured hysteresis loops at different locations on a Permalloy stripe structure, supplied by Philips Semiconductors Hamburg. The Permalloy stripes are part of the magnetic field sensors, utilizing the AMR-effect, that are manufactured by Philips Semiconductors. This structure comprised of a stripe array and several additional stripes of different aspect ratios.

Franck–Condon breakdown in core-level photoelectron spectroscopy of chemisorbed CO

The photon energy dependence of the vibrational fine structure in the C1s and O1s X-ray photoelectron main lines of chemisorbed CO on Ni(100) and Ru(0001) has been measured from 6 to 150 eV above the core-level thresholds. Significant deviations from the behavior in gas-phase CO are found. A strong dominance of the adiabatic peak towards threshold is found for the C1s, but not the O1s, lines. In the C1s lines, we observe a broad maximum of vibrational excitation 5 eV above the shape resonance. At high photon energies, Franck–Condon behavior is observed in both the C1s and O1s lines. This behavior is discussed in terms of the adsorbate electronic structure and the dynamic metallic screening upon core ionization.

Angular-dependent resonant-photoemission processes at the2pthresholds in nickel metal

Angle-resolved valence-band resonant photoemission of nickel metal has been measured close to the 2p core-level thresholds with synchrotron radiation. The well-known 6-eV correlation satellite has an intensity enhancement of about two orders of magnitude at resonance. The angular dependence of the photoemission intensity has been studied as function of photon energy and provides unambiguous evidence for interference effects all the way up to the resonance maximum. The observation of different angular asymmetries, {\beta}, for the valence band and the satellite is discussed in connection to the origin of the resonant photoemission process and the character of the satellite.

Scanning microscopy of magnetic domains using the Fe 3p core level transverse magneto-optical Kerr effect

We demonstrate the feasibility of the vacuum ultraviolet analog to visible-light magneto-optical imaging of magnetic structures using the resonantly enhanced transverse magneto-optical Kerr effect at core level thresholds with incident p-polarized radiation. The advantages are element specificity and a variable information depth. We used the scanning x-ray microscope at HASYLAB capable of obtaining about 1 μm resolution by means of its focusing ellipsoidal ring mirror. The p-polarized component of the reflected light was selected using multilayer reflection at an additional plane mirror downstream to the sample. Micrographs of the optical reflectivity were taken in the vicinity of the Fe 3p core level threshold at 53.7 and 56.5 eV photon energy where the magneto-optical effect is of opposite sign. Magnetic domains are visible in the difference of both recorded images.

Spectroscopy of near-threshold electrons from surfaces

A time-of-flight technique for two-dimensional spectroscopy of near-threshold electrons from surfaces is described, and the properties of the method are illustrated using selected results from clean surfaces, and mono- and multilayer adsorbates. The analysis of two-dimensional data yields easy access to inner (bulk) and outer (surface) ionization edges and exciton series, for valence and core electron levels including satellite states, and to final state effects. Strong interaction between bulk and surface states is revealed in particular for samples with negative electron affinity. Core-level threshold electron spectra from monolayers adsorbed on metal surfaces exhibit strong substrate-mediated suppression of post-collision interaction-induced line broadening and shift, each of which can be quantitatively explained by image charge screening.

Characteristic losses in metals: Al, Be, and Ni

Information about the occupied portion of the surface density of states of materials can be derived from electron-excited Auger electron spectroscopy (AES), which is a standard experimental technique in most surface science laboratories. Surface sensitive experimental techniques that provide information regarding the unoccupied portion of the surface density of states are often not standard and are not so readily available. Here we explore the possibility of utilizing the same experimental equipment as in AES to derive information about the unoccupied portion of the surface density of states from a characteristic loss spectroscopy, in particular, from core-level inelastic electron-scattering spectroscopy (CLIESS). An important application of this technique is in comparative studies. CLIESS spectra from clean surfaces of aluminum, beryllium and nickel are presented. These data were taken in the first-derivative mode using the reflection of monoenergetic primary beams of 450 eV energy for Be, and 300 eV for Al and Ni. The Al and Be spectra had to be extracted from overlapping plasmon signals using synthesized plasmon spectra based on the behavior of these spectra between the elastic peak energy and the respective core level threshold energies. After applying loss-deconvolution techniques to remove secondary loss spectral distortions, integral spectra were obtained which compared well to corresponding experimental soft x-ray absorption and transmission electron-energy loss data as well as to theoretical calculations of the unoccupied density-of-states for these materials. Comparison similarities as well as some differences are discussed. Finally, in order to illustrate the potential these signals have in serving as “fingerprints” of surface chemistry, derivative metal-CLIESS curves for the three oxide surfaces of the metals are also presented.

Comparative study of the L23–M45M45 Auger decay in CuO and Cu using synchrotron radiation

Abstract We report on the evolution of L 3 – and L 2 – M 45 M 45 Auger spectral regions in Cu and CuO as a function of photon energy used to create the initial hole states. It is shown that the satellite features in Auger spectra are completely suppressed close to the threshold; intensities of these satellites increase dramatically when photon energy is swept through deeper core level thresholds. We discuss various decay channels responsible for the spectral features in the L 23 – M 45 M 45 regions of Cu and CuO and obtain quantitative estimates for their contributions to the total spectrum.

Energy dependence of CuL2,3satellites using synchrotron excited x-ray-emission spectroscopy

The L2,3 X-ray emission of Cu metal has been measured using monochromatic synchrotron radiation. The self-absorption effect in the spectra is shown to be very small in our experimental geometry. From the quantitative analysis of spectra recorded at different excitation energies, the L3/L2 emission intensity ratio and the partial Auger-width are extracted. High-energy satellite features on the L3 emission line are separated by a subtraction procedure. The satellite intensity is found to be slowly increasing for excitation energies between the L3, L2 and L1 core-level thresholds due to shake-up and shake-off transitions. As the excitation energy passes the L2 threshold, a step of rapidly increasing satellite intensity of the L3 emission is found due to additional Coster-Kronig processes.

Valence-band and resonance-photoemission study ofLa2CuO4+xsingle crystals

Angle-resolved resonance-photoemission measurements at 15{endash}30 and 100{endash}130 eV photon energy ranges were performed from different principal planes of La{sub 2}CuO{sub 4+x} single crystals. Resonance photoemission near the La-4d threshold shows the presence of states in the valence band associated with La within the energy region 2{endash}5.5 eV. At low photon energies near the La-5p threshold (16.5 eV) with the normal emission aligned in the (001) direction (c{perpendicular} surface), valence-band spectra display nonmonotonic photon energy dependence of a 6-eV binding-energy feature, i.e., beyond the main La density of states in the valence band. Along with the absence of such behavior for normal emission from the other principal planes (100) and (010) (c{parallel} surface geometry), this fact implies that surface termination effects need to be taken into account to explain these results. The binding energy of the resonant feature and the resonance photon energy are slightly different for photon polarization vector parallel and perpendicular to the Cu-O planes of the crystal. Measurements were also done on another La-based compound, La{sub 0.7}Sr{sub 0.3}MnO{sub 3}, having similar La-O plane but much smaller c-axis parameter. This manganese oxide demonstrates the same nonmonotonic dependence of the 6-eV binding-energy feature, but in a wider photon energy region.more » On the basis of this comparison, it is tentatively suggested that the intensity modulation is governed not only by the core-level threshold, but may carry information of bulk Brillouin zones as well. {copyright} {ital 1997} {ital The American Physical Society}« less

Transverse magneto-optical Kerr effect of Fe at the Fe 3p threshold

We have studied the transverse magneto-optical Kerr effect of a Fe[(5.7% Si)001] sample and of ultrathin films of Fe evaporated onto Ag(001) in the soft-x-ray regime at the 3p core level threshold of Fe. We test the relationship between the asymmetries in the reflectivity and in the total photoyield which have been measured simultaneously.

Electronic structure of buried Si layers in GaAs(001) as studied by soft-x-ray emission.

This thesis addresses the electronic structure of molecules and solids using resonant X-ray emission and photoemission spectroscopy. The use of monochromatic synchrotron radiation and the improved performance of the instrumentation have opened up the possibility of detailed analyses of the response of the electronic systems under interaction with X-rays. The experimental studies are accompanied by numerical ab initio calculations in the formalism of resonant inelastic scattering. The energy selectivity has made it possible for the first time to study how the chemical bonds in a molecule break up during resonant inelastic X-ray scattering. In the conjugated polymer systems, the element selectivity of the X-ray emission process made it possible to probe the different atomic elements separately. The X-ray emission technique proved to be useful for extracting isomeric information, and for measuring the change in the valence levels at different degrees of doping. In this thesis, spectral satellite features in transition metals were thoroughly investigated for various excitation energies around a core-level threshold. By measuring the relative spectral intensity of the satellites it was possible to extract information on the partial core-level widths. Using the nickel metal system as an example, it was shown that it is possible to probe the different core-excited states close toshake-up thresholds by measuring the relative spectral intensity variation of the Auger emission.Resonant photoemission measurements showed unambiguous evidence of interference effects. Theseeffects were also thoroughly probed using angle-dependent measurements. The combination of X-rayemission and absorption were useful for studying buried layers and interfaces due to the appreciable penetration depth of soft X-rays. X-ray scattering was further found to be useful for studying low-energy excited states of rare earth metallic compounds and transition metal oxides.

Sodium-induced H + ion resonance on silicon surfaces

We present the first observation of an ion resonance, H +, desorbed from Na overlayers on Si(100)2 × 1 and Si(111)2 × 1 surfaces by means of photon stimulated ion desorption (PSD). This resonance occurs at a photon energy corresponding to the excitation of the Na2p core level and is much stronger than the corresponding interband electronic excitations initiating the PSD process. This H + ion resonance, which is unique to Na on Si surfaces, is quenched by coadsorption of other alkali metals, while it is strongly enhanced by the presence of an electron acceptor specie as oxygen. It is associated with the dielectric response of the near surface region (including a spectator photoelectron within the SiNaH interface complex) at the Na 2p core level threshold.

Multiple-scattering theory of X-ray absorption: a review

The author reviews the basic elements of the theory of X-ray absorption using the tools provided by the theory of multiple scattering. He first uses an approximate momentum space approach which gives a very clear physical insight where the final formulas expressing EXAFS and XANES, i.e. the structures appearing in the absorption coefficient above the edge of a deep core-level threshold, are given in terms of eigenstates of the photoelectron momentum. He then reviews the correct formalism, the curved-wave theory for which he needs to work in angular momentum space. Simple graphic representations are given for the multiple-scattering function.

Photon excitation for satellite-free X-ray spectroscopy: Instrumentation challenges

Abstract Soft X-ray emission spectra (SXES) obtained using monochromatic synchrotron radiation (SR) provides important advantages as compared with electron excitation. Photons can penetrate deep into the sample providing nondestructive information from bulk samples. With availability of tunable monochromatic photons from synchrotron sources, it is now possible to selectively ionize core electrons near enough to the core level threshold energy producing singly ionized atoms and molecules resulting in satellite-free X-ray emission spectra. This offers the ability to separate satellite structure from diagram lines and also allows more detailed studies of the satellite processes. Recent developments in satellite-free SXES will be reviewed along with instrumentation challenges in utilizing the small phase-space attributes of high brightness from third-generation SR sources producing soft X-ray and vacuum ultraviolet wavelengths.

Parity-unfavored transitions in resonant photoemission from Ar, Kr, and Xe: experimental and theoretical results

High-resolution angle-resolved electron spectroscopy with synchrotron radiation has been used to study the angular distributions of resonant-Auger processes near the core-level thresholds Ar2p, Kr3d and Xe4d. Angular-distribution parameters (β) were measured for all resolved peak in the electron spectra and large negative values of β were found for some peaks. A quantitative method for calculating the angular-distribution parameters is described and results of calculations for the Ar2p 3 2 →4s resonant-Auger spectrum are compared with the experimental results.

Multielectron excitations in high-energy photoelectron spectra of CO adsorbed on Ni(100).

High-resolution x-ray-excited core-level shake-up and valence-electron spectra of CO adsorbed on Ni(100) in the c(2\ifmmode\times\else\texttimes\fi{}2) structure are presented. A number of newly resolved structures are reported. The C 1s and O 1s shake-up spectra show pronounced differences. The shake-up transitions are treated as local molecular excitations in the presence of the core hole. It is proposed that transitions from the 2${\ensuremath{\pi}}^{\mathrm{*}}$ screening orbital into 3p and other higher states with Rydberg character give rise to the most intense shake-up peaks in adsorbed CO. In the C 1s spectrum a low-energy satellite is identified as an excitation from bonding to antibonding 2${\ensuremath{\pi}}^{\mathrm{*}}$-3d combinations. The high-energy shake-off continua reveal several broad structures which are interpreted as autoionizing or shape resonances in the shake-off continua. These features are compared with thresholds observed in photon-stimulated-ion-desorption spectra for adsorbed CO. The difference spectrum between the valence region of Ni with and without adsorbed CO reveals several features over an energy range of 40 eV. The 3\ensuremath{\sigma} state and the correlation satellites are compared with features in the autoionizing spectra at the core-level thresholds.