Search

Nowadays cutting edge femtosecond EUV and soft X-rays radiation sources are the driving force of groundbreaking time resolved X-ray spectroscopies. These new light sources are allowing pioneering experiments in the field of ultrafast phenomena and disclosing new insights about the physics of the out-of-equilibrium matter. Here we report an introductory and concise outlook about some possible perspectives in this field. (C) 2015 Elsevier B.V. All rights reserved.

In this review we describe the development of ultrafast non-linear photoemission spectroscopy on metals from the first historic observations in the sixties to state-of-the-art experiments. We present an account that is focused on electron spectroscopy experiments that use short laser pulses to investigate the non-equilibrium response of electrons in metals. Several examples of the application of non-linear spectroscopy to the investigation of many-body effects and highly non-equilibrium processes will be illustrated. Furthermore, we give a brief overview of the wide spectrum of experimental methods based on non-linear photoemission spectroscopy.

A comparative experimental study is presented of the electronic properties of MnSi films grown on Si(111) and of MnSi single crystals, using X-ray absorption spectroscopy (XAS), and core level and valence band photoemission spectroscopy (PES). No significant differences in the electronic structure of the two systems can be found. Absorption measurements on the Mn 2p threshold show a mixed valence ground state, where the multiplet structure is washed out by the hybridisation of the Mn 3d states with the Si sp states. These results are also confirmed by photoemission (PE) spectra from the valence band and the Mn 3s, 3p and 2p core levels. Strong attention has been paid to the effect of contamination. The occurrence of multiplet effects in the absorption spectra indicates unambiguously the localisation of the Mn 3d electrons in Mn-O bonds, which strongly influences the electronic properties of these systems. (C) 2007 Elsevier B.V. All rights reserved.

Angle resolved non-linear photoemission induced by short laser pulses is used to investigate both the occupied surface state and the unoccupied image potential states on Cu(1 1 1). The image potential state effective mass measured at zero parallel momentum with a non-resonant photon energy between the surface state and the image potential state is significantly different from the effective mass measured at resonance in the same conditions. The surface state effective mass shows no variation with photon energy. To explain the modification of the image potential state effective mass in the non-resonant excitation, the influence of the high energy electron population in the bulk bands near the image potential state must be taken into account.

MnSi is a ferromagnetic compound with a Curie temperature of 29 K. Recent theoretical studies predict that 2 ML of MnSi epitaxially grown on Si show a ferromagnetic metallic ground state with spin polarization. of about 50%. This would allow the development of spintronic devices based on the injection of spin-polarized current from a ferromagnetic metal into a semiconductor. In this context the possibility of growing in situ MnSi on the Si(111) surface has been explored. Thermal reaction, crystalline structure and electronic properties of the grown films have been studied in situ by photoemission spectroscopy (PES), X-ray absorption spectroscopy (XAS), and low energy electron diffraction (LEED). Depositing a thin film of Mn on Si(111) the formation of ordered islands (with dimensions dependent on the amount of deposited Mn) is driven by annealing at selected temperatures, as already observed. Our preliminary studies show that by simultaneously depositing Mn and Si in 1:1 stoichiometry on Si(111) a large improvement in the homogeneity of the MnSi films is achieved. (c) 2006 Elsevier B.V. All rights reserved.

Image potential states are populated through indirect, scattering-mediated multiphoton absorption induced by femtosecond laser pulses and revealed by single-photon photoemission. The measured effective mass is significantly different from that obtained with direct, resonant population. These features reveal a strong coupling of the electrons residing in the image potential state, outside the solid, with the underlying hot electron population created by the laser pulse. The coupling is mediated by a many-body scattering interaction between the image potential state electrons and bulk electrons in highly excited states.

X-ray photoabsorption and resonant X-ray emission at the oxygen K edge of haematite (Fe 2 O 3 ) single crystals have been measured with linearly polarized light. The X-ray absorption is strongly dichroic, in agreement with previous results from thin films: there are substantial differences between spectra with the electric vector of the incident light parallel and perpendicular to the c axis. This is explained in terms of published electronic structure calculations. The X-ray emission under resonant Raman conditions however is only weakly dichroic, and this is ascribed to delocalization of the excited electron in the conduction band.

Intrinsic linewidths of electronic image-potential states versus the momentum parallel to the surface on Ag(1 0 0) have been measured with angle resolved two-photon photoemission using the time of flight technique. The influence of the time of flight spectrometer angular acceptance on intrinsic linewidth measurements is taken into account and a proper data analysis is proposed. The inverse linewidth as a function of parallel momentum is compared with time-resolved measurements of image-state lifetimes on Ag(1 0 0) and Cu(1 0 0) reported in the literature. The data analysis shows that decay dynamics of image potential states in Ag and Cu may be determined by different physical mechanisms.

The ultrafast electron and spin dynamics of Co films was investigated using a femto-second pump-probe technique. The samples were magnetically characterized by means of magnetooptical Kerr effect and non-optical magnetometers, i.e. superconductive quantum interference device and alternating gradient field magnetometer. Time evolution of both, Kerr rotation and ellipticity, was measured at different points of the static hysteresis cycle. This allowed separating magnetic and non-magnetic contributions to the signal. The sample magnetization was found to drop within 200 fs, following the pulse cross-correlation trace, while the magnetization recovery time was found to last few picoseconds.

The electronic properties of a C70 single layer chemisorbed on Cu(1 1 1) surface, in which the charge state has been modified by Na doping, were studied via synchrotron radiation photoemission and absorption spectroscopy. A two-dimensional metallic phase is observed in the whole range of charge states investigated (from 1 up to 3 electrons/C70). Moreover, all the results suggest that the orientation of the molecules, with the C5v axis perpendicular to the surface, is not affected by Na doping.

This paper reports an ultra-violet photoemmision spectroscopy study on K 3 C 60 in the temperature range 30–600 K. The experiment shows that the temperature dependence of the conduction band is consistent with the behaviour of a strongly correlated metal. In addition, the photoemission intensity near the Fermi edge decreases as the temperature is increased. This trend is similar to the vanishing behaviour of the quasi-particle weight predicted for strongly correlated metals with increasing disorder. The experimental evidence that the shoulder visible in the highest occupied molecular orbital region, unexpected on the basis of conventional band calculations, is coupled with the sharp peak at the Fermi level is also provided.

Zhang–Rice singlet states, clearly detected as first ionization state in the photoemission spectra of CuO are not observed in other cuprates such as CuGeO3 and Bi2CuO4. This paper reports a study, based on X-ray photoemission valence band spectra of CuGeO3, Bi2CuO4, and CuO, focused to enquire about the nature of the Zhang–Rice singlets in these compounds. Impurity cluster calculations, that reproduce the correlated satellites and the low lying binding energy states well, in agreement with photoemission experiments, clearly indicate a strong dependence of the Zhang–Rice singlets behaviour on the Cu–O bond ionicity. This finding fully explains the experimental data. The stability of the Z–R singlet can be considered as a peculiar feature of the low-gap cuprates (e.g. CuO) whereas the energy gap increase reduces the singlet–triplet separation affecting the Z–R singlet stability in the more insulating cuprates such as CuGeO3.

A total quenching of the charge transfer related emission is observed in the 3s photoelectron spectra of NiO single crystals taken at hν=180 eV photon energy. This finding is related to an efficient screening mechanism of the 3s core hole which prevents the stabilization of charge transfer derived configurations in the final state of the photoemission process, leading to a spectrum with the lone high and low-spin exchange components.

We investigated the far- and mid-infrared reflectivity (20-6000 wavenumber) of several pure and doped CuGeO3 single crystals. The b-axis and c-axis optical response is presented for different temperatures between 4 K and 300 K. Moreover, a full group theoretical analysis of the lattice vibrational modes of CuGeO3 in the high temperature undistorted phase as well as in the low temperature spin-Peierls phase is reported and compared to the experimental results. We observe the activation of zone boundary phonons along the b axis of the crystal below the spin-Peierls transition temperature., Comment: Revtex, 10 pages, 8 postscript pictures, Physica B, in press (1997)

The temperature effects on sub-monolayers of V deposited at the TiO2(001) surface have been studied by ultraviolet and X-ray photoelectron spectroscopies, UPS and XPS, from ∼ 300 up to 623 K. V coverages, Θv, between 0.2 and 0.7 monolayers (ML) were deposited by an e-beam evaporator at 300 K. The V 2p32 core line region exhibits two well-defined components whose relative intensity depends on Θv. These two components, assigned to different oxidation states of V, are correlated with two features, with a dominant V 3d character, detected within the TiO2(001) band gap of the UPS valence band spectra. UPS and XPS measurements performed after in-situ thermal treatments show unambiguous and reproducible changes of these spectral components. After annealing at 623 K only the higher binding energy component is present in the V 2p32 spectra; the Ti 2p core lines recover the typical symmetry of the clean and stoichiometric TiO2(001) surface and the higher binding energy feature only is detected in the TiO2 band gap. These data suggest that, within the volume probed by XPS and UPS, Ti ions have a mainly d0 configuration, while V has a single and stable open-shell configuration, as revealed by the significant intensity detected within the TiO2 band gap. These annealing-induced changes are due, as suggested by the O 1sTi 2p core line intensity trend, to an oxygen diffusion from the TiO2 bulk to the surface. Finally, a detailed analysis of the data indicates that different VTiO2(001) interfaces exhibit different behaviours after annealing treatments, depending on Θv. For Θv = 0.7 ML, V interdiffuses into the TiO2 sub-surface layers, whereas for Θv = 0.2 ML it remains at the surface. This finding is consistent with a rearrangement of V atoms. which under annealing occupy first the energetically most favorable surface sites (Θv = 0.2 ML) before interdiffusing into the TiO2 lattice (Θv = 0.7 ML).

This Letter reports an X-ray photoelectron diffraction study of ∼5 V monolayers deposited by electron beam evaporation at the TiO 2 (110) surface. Data clearly show that V grows as islands with a bcc structure having the [100] direction normal to the substrate surface. A detailed analysis of the V″2p and Ti″3p azimuthal curves indicates also an alignment of the [001] overlayer azimuth with the [1¯10] direction of the TiO 2 substrate. These results are consistent with an epitaxial growth of bcc V at the TiO 2 (110) surface where a double V surface unit cell is matched to the TiO 2 (110) rectangular cell.

A study of multi-electron excitations in the O 1s X-ray photoemission (XP) region of NiO single crystal is reported. The observed spectral lines are discussed and assigned to surface and bulk plasmon losses on the basis of a Kramers-Kronig analysis of the 0–30 eV range reflectivity data. These results, in agreement with former and present optical studies and electron energy loss data, show for the first time, that the bulk and surface plasmon losses are well detectable, until the third order, in the XP spectra of high quality NiO single crystals.

The preparation method and the sensing properties (sensitivity and selectivity to interfering gases) towards carbon monoxide of the new ternary compound Sn1-xFexOy deposited in the form of thin films, are presented in this paper. The metal of the VIIIB group is introduced with concentrations in the range 0

The electronic structure of a Bi2CuO4 single crystal is studied by means of Electron Energy Loss Spectroscopy (EELS), at two primary beam energies (200 eV and 2 keV). The valence and shallow core electron excitation regions are investigated and the detected spectral features assigned, also exploiting the two different probing depths. Beside these results, the accidental loss of oxygen induced by a relatively long exposure of the sample to UV and X radiation, with the subsequent recovery of the oxygen stoichiometry, by exposing the crystal to oxygen atmosphere, allow to identify unambiguously some features in the EEL spectrum of this compound. It is proposed that the loss at 3.6 eV, seen in some cuprates, is not detected in Bi2CuO4 because it corresponds to an excitation involving the apical oxygens, not present in this compound.

After the discovery of copper oxide based high temperature superconductors an extraordinary number of studies based on high energy spectroscopy measurements have been published. In this work we attempt to summarise, to the best of our knowledge, the more significant works concerning the Cu2p X-ray photoelectron core-lines. In particular, a critical study of the work done up to now shows that the most important question arises from a correct interpretation of the correlation mechanisms that determine the components and the weights of the copper configurations d8, d9 and d10L in the XPS spectra.

Interpretations are given for the core-level binding energies, BE's, of the surface and bulk atoms of metals; in particular, the origin of possible differences, or shifts, between these BE's is considered. The mechanisms which are responsible for the BE shifts are identified and characterized through theoretical analyses of the surface electronic structure. The dominant mechanisms for the BE shifts are those which lead to the initial-state changes in the orbital energies and, hence, the Koopmans' Theorem BE's. This is particularly important because it means that the shifts can be directly related to the chemistry and physics of the systems. The surface electronic structure is described with molecular-orbital wavefunctions for cluster models of the surface; this theoretical approach is particularly well suited for the study of local chemical interactions. The different mechanisms act in a cancelling fashion which leads to rather small net shifts between the BE's of bulk and surface atoms. The mechanisms which explain the surface and bulk core-level BE shifts are rather general and may provide a basis for understanding other cases of core-level BE shifts.

The electronic structure of Al 70 Ni 15 Co 15 decagonal quasi-crystal has been studied by X-ray photoelectron spectroscopy (XPS). The disappearance of the Ni2p(3d 9 ) satellite and the decreased intensity of the 3 F state in the NiL 3 M 45 M 45 Auger spectra indicate a strong d—sp hybridization accompanied by the filling of the Ni3d bands. The filling of the Ni3d bands produces closed-shell type XPS spectra of the Ni core lines, while Co core line XPS spectra show that a significant amount of d holes are still localized on Co. This picture suggests that the magnetic properties are mainly determined by Co. The dispersion of the 3d holes is also responsible for the drop of the 3d states near E F but the angle integrated density of states does not exhibit a pseudo-gap. Finally, the correlation energy, U A , has been calculated from the kinetic energy of the 1 G term of the NiL 3 M 45 M 45 Auger emission.

A new impurity-cluster configuration-interaction model that treats charge transfer and exchange correlations on the same footing is applied to interpret the 3s core-level X-ray photoelectron spectra of NiO, CoO, K 2 NiF 4 and K 2 CoF 4 . The interplay between charge transfer and exchange correlations in determing the final state satellite structures is clarified and an estimate of the exchange integrals between the 3s hole and the 3d electrons is given. The results are consistent with the electronegativity scale, as regards charge transfer energy, and with the trend suggested by the Van Vleck rule, as concerns the exchange integrals.

An X-ray photoelectron spectroscopy study of yttria-stabilised zirconia (YSZ) single crystals is reported. It is shown that the shape and the binding energy positions of zirconium and yttrium core levels are independent of the yttria stoichiometry when the content of this oxide ranges from 12 to 24 mol.% of Y2O3. These results could indicate that the local crystallographic order around the Zr4+ and Y3+ ions does not depend on the Zr/Y ratio.

This paper reports an X-ray photoemission study (XPS) of the core levels and the valence band of vanadia-titania catalysts and their precursor oxides. The XPS spectra of the vanadium core levels indicate that surface vanadium atoms are present with different oxidation states. Furthermore, it is demonstrated, for the first time, that the electronic states observed in the XPS valence-band spectra between the Fermi edge and the O 2p bands have mainly a V3d character.

X-ray photoemission spectroscopy (XPS) and ab initio molecular orbital cluster model wavefunctions are used in a combined study of the electronic structure of CuO. High-resolution XPS spectra for single-crystal CuO identify and clarify several features of the core and valence level spectra. The molecular orbital cluster wavefunctions show that there is a significant covalent contribution to the ionic CuO bond. For the copper core hole final states, the states where the core hole is screened by charge transfer from O2p to Cud lie at lower binding energy than the states where charge transfer does not screen the core hole. The multiplet splitting of the higher binding energy 2p-hole states is large (ca. 3 eV).

Cluster model wavefunctions for the alkaline-earth oxides are analyzed to determine the origin of the trend of the chemical shifts of the binding energies, BEs, of the metal core levels. Two cancelling initial states mechanisms in the oxides are responsible for the trend. One is the increase of the BEs for the dication over the neutral atom. The second is the decrease of the free dication BEs due to the Madelung potential of the ionic oxides. The trend given by the sum of these two contributions to the chemical shifts is in full agreement with experiment. These mechanisms are electrostatic and do not involve chemical bonding effects.

This paper shows that in copper oxide based compounds where Cu ions are in a square planar geometry with four coordination oxygen ions, i.e. CuO and Nd2CuO4, the Cu2p 3 2 (3d)10L) X-ray photoelectron core lines are narrower than in similar compounds with Cu ions in a pyramidal or octahedral geometry with five or six neighboring oxygens, i.e. Bi2Sr2CaCu2O8 and La2CuO4. To explain this difference two different charge transfer mechanisms from the oxygen p orbitals to the Cu3d orbitals are proposed. For square planar geometry (CuO4) only the px,y → dx2 - y2 charge transfer is allowed, whereas for octahedral or pyramidal geometry the additional pz → dz2 transition could take place. These two different final state configurations will produce different ionization potentials for the Cu2p 3 2 core levels.

We report off-normal angle-resolved photoemission data from NiO(001), which is complementary to the earlier normal-emission result. On the one hand, the energy position of the bands along this direction and the dispersion of the lowest oxygen band agree with the band calculation very well. On the other hand, the existence of the valence band satellite and the absence of the energy band in the gap region do not agree with the band calculation. For the first time, interesting movement of the Ni d7 satellite has been observed as a function of the emission angle and the photon energy, which is most likely a result of the satellite dispersion.

Li/Ni ratios in LixNi1 − xO films were determined by simultaneous use of the PIXE (particle-induced X-ray emission) and PIGE (proton-induced gamma-ray emission) techniques using the 3.5 MeV proton beam provided by the CISE tandem Van de Graaff accelerator. The general features of the experimental setup are presented and discussed along with the results on the dependence of Li/Ni ratios on the e-beam evaporation and thermal treatments. Preliminary results obtained by XRD (X-ray diffraction) are also discussed. The combined use of PIXE and PIGE proves to be a unique opportunity for nondestructive measurement in thin films and, through the correlation with structural data, it allows a careful optimization of the deposition process and thermal treatment parameters to be obtained.

In this paper an X-ray photoelectron Spectroscopy (XPS) study of the interaction between oxygen and Pt clusters on Teflon and on SiO 2 supports is presented. Unambiguous experimental data give direct evidence of the oxidation of Pt clusters. PtO x appears to form spontaneously in conditions where, in contrast, a continuous Pt surface does not oxidize. It is also shown that the oxidation rate depends on the coverage and on the nature of the substrate. These results are interpreted in terms of the relative occupations of the Pt s and d orbitals, the effective coordination number of the Pt cluster atoms and the different shape of the clusters on Teflon or on SiO 2 supports.

We report about very high-quality epitaxial NiO films grown on a Fe(0 0 1) passivated surface. X-ray magnetic linear dichroism at the Ni L 2 absorption edge has been applied to investigate the magnetic anisotropy in the NiO layer. We find that very thin NiO films (7–20 atomic layers) develop an in-plane uniaxial magnetic anisotropy, whose axis is found to be perpendicular to the Fe substrate magnetization M . This behaviour is in contrast with what observed in the specular Fe/NiO(0 0 1) interface, where the coupling is parallel.

X-ray photoemission valence band spectra of CuGeO3, Bi2CuO4, and CuO single crystals are presented. Impurity cluster calculations, that well reproduce the correlated satellites and the low-lying binding energy states, clearly indicate a strong dependence of the Zhang–Rice singlets behaviour on the Cu–O bond ionicity.

EPR susceptibility and XPS measurements were performed on sintered BaCuO 2 . A structured sharp EPR signal superimposed on a broad band was observed. It is shown that the structured line results from the superposition of two signals associated with divalent copper ions lying in two different distorted octahedral sites. These ions are probably placed on the outermost layer of the sintered grains.

Yttria stabilized zirconia is widely used as an electrolyte in electrochemical devices such as high temperature solid oxide fuel cells (HTSOFC) (1) and oxygen sensors (2). In order to reduce the ohmic resistance it is interesting to obtain electrolyte layers as thin as possible. Gas tight films of stabilized zirconia were prepared by chemical vapor deposition, plasma spraying and tape casting and successfully used in HTSOFC prototypes (1). As far as we know, however, the basic properties of these films, and in particular the correlation among preparation conditions, structure, microstructure and electrical properties, have not been extensively investigated. This fact has stimu lated the present work on plasma sprayed zirconia based electrolytes (3–5)

This paper describes a technique in which a laser light vibration sensor based on a Michelson interferometer is used. With a 5 mW laser the instrument will make measurements on a moving target at ranges greater than 200 m without using retro-reflective materials. Careful optimization of the electro-optic design reduces the effects of environmental disturbances and allows vibration amplitude resolution of 0·2 μm with a flat response in the bandwidth 0·1–150 Hz. Field tests and actual measurements of the radial and tangential displacements of an arch dam are shown.

Sputtered discontinuous thin films of gold were prepared at various substrate temperatures. The absorption spectra of these films were found to depend on the film preparation. It is suggested that changes in the lattice parameters and/or the crystal structure could be responsible for this behaviour.

A new optical absorption band in small metal clusters is predicted and studied. It might be due to resonance of free electrons confined inside the metal clusters. The model attempts to explain some anomalous absorption peaks in the infrared absorption spectra of fine metal particles.

Plasma frequency shift induced by particle size in metal clusters is explained as due to the interatomic distances of the crystal lattice. In particular the model explains the observed drastic changes in the plasma frequency not predicted by the previous theories.

We have investigated electric and magnetic properties of YBa2Cu3O7−x Ag2O superconducting compounds and the correlation of these parameters with their microstructures. Silver oxides addition reduces the presence of carbonates in the sintered samples and improves the reaction process when added before calcination. The normal state resistivity is greatly reduced whereas transport and intragrain critical currents are not significantly enhanced.

We report X-ray photoemission data from YBa 2 Cu 3 O 7 showing that the measured composition and the electronic structure depend significantly upon the photo-electron escape depth. For large take-off angles the valence band spectrum near the Fermi edge is well structured and shows a number of features, arising from Cu O hybrid states. Their intensities are drastically reduced in the energy distribution curves measured at grazing angles. The results are interpreted assuming that the surface is mainly composed by Ba O planes. This interpretation is shown to be consistent with electronic structure calculations for the bulk and the Ba O surface.

The room temperature absorption, in the threshold region of single crystals of K 2 CoF 4 , Rb 2 CoF 4 , K 2 NiF 4 was measured. Synchrotron radiation was used as continuum light source. The threshold is attributed to electron orbital promotion transitions 3 d n a 3 d n −1 (4 s , 4 p ) 1 of 3 d metal ions. Crystals reflectivity was measured in the 8–35 eV region. It is shown that all the optical structures are associated only with transitions of the alkali and halide electrons.