Extended soft X-ray emission spectroscopy: quantitative assessment of emission intensities

Soft X‐ray emission spectroscopy (SXES) in the energy range between 150 eV and 1500 eV has typical attenuation lengths between tens and a few hundred nanometres. In this work the transmission of soft X‐rays in synchrotron‐based SXES has been quantitatively analysed using specially prepared layer samples. The possibility of extending the standard qualitative analysis of SXES by exploiting the information underlying the emission intensity was examined for thin layer structures. Three different experiment series were accomplished with model layer systems based on different sulfur‐containing substrates: (i) MoS₂, (ii) CuInS₂, (iii) Cu(In,Ga)(S,Se)₂. The absorption of the S L_2,3 emission line by ZnO cover layers of up to 80 nm thickness was monitored and compared with theoretical expectations. By comparison with a reference intensity recorded from a bare substrate, the attenuation of the S L_2,3 emission could be used to accurately determine the ZnO overlayer thickness up to a critical thickness, depending on the set‐up and the net S L_2,3 emission intensity. The results from these local energy‐resolved spot measurements were compared with spatially resolved scans of the integral S L_2,3 emission intensity over areas of several mm². In the scan images the attenuation of the S L_2,3 emission intensity clearly reflects the local ZnO layer thickness. From the attenuation the ZnO layer thicknesses were calculated and compared with ellipsometric measurements and were found to be in excellent agreement. These results demonstrate the benefits of a quantitative analysis of SXES, making it an even more powerful tool for examining buried interfaces and for monitoring lateral inhomogeneities.     

Kβ X-ray emission from He-like ions

Relativistic calculations on the energies and electric dipole rates of K_β X-rays from 1s3p(¹P₁,³P₁)-1s² (¹S₀) transitions for He-like ions in the range Z=14–54 are carried out using multi-configuration Dirac–Fock (MCDF) wave functions in the active space interaction approach. The contributions from Breit interaction and quantum electrodynamics have also been included in the calculation. An attempt has been made to find a scaling expression for Breit energy in terms of Zα .The scaled Breit energies are in good agreement with the earlier accurate relativistic results and this ensures the reliability of our scaling procedure. The behavior of MCDF wavefunctions for a given J in the non-relativistic limit has also been studied. The calculated K_β X-ray energies and rates agree well with other available experimental and theoretical values.     

Characterization of the high—voltage pulsed discharge plasma of ammonia by emission spectroscopy

We have used optical emission spectroscopy to characterize the high-voltage pulsed discharge of ammonia.Ammonia was highly dissociated in the discharge at low pressures.More atomic nitrogen was generated as compared to the discharge of nitrogen gas at the same pressure of 0.8kPa.We discuss the elimination of the oxygen impurity in the ammonia discharge,and we estimate the time-dependent atomic excitation temperature and the electron density from the measured spectra.     

X-ray absorption spectroscopy in biology – present and future

From May 10 to 12, the GISAXS Workshop 2005 was hosted at HASYLAB/DESY in Hamburg, Germany. GISAXS stands for Grazing Incidence Small Angle X-ray Scattering, a surface-sensitive technique to observe structures on large length scales up to several hundred nanometers. Organized by R. Gehrke and S. V. Roth from HASYLAB and P. Müller-Buschbaum from University of Technology Munich, the workshop attracted more than 50 participants from all over the world, originating a very stimulating atmosphere with invited talks, a contributed poster session and practical training, including real data acquisition at the beamline BW4.     

X-ray emission via Kα resonance complexes in gold ions

The excitation energies from ground state for 63 fine structure levels of the terms belonging to 1 s², 1snl, 2lnl′ (n = 2,3), 3l3l′ (l ≠ ≠ l′) configurations of Au⁷⁷⁺, as well as oscillator strengths for electric-dipole-allowed and intercombination transitions among the fine-structure levels were theoretically evaluated. The important relativistic effects in intermediate coupling are incorporated by means of the Breit–Pauli Hamiltonian. Extensive comparisons are performed with several approximations and previous data sets in order to size up contributing effects and to estimate accuracy ratings. The atomic excitation is obtained by electron impact in close coupling approach. For optically allowed transitions, the electron-impact collision strengths at collision energy up to 80 keV are reported. The results are relevant to the laboratory and astrophysical plasmas.     

Prospects for emission Mössbauer spectroscopy in chemical investigations

Emission Mössbauer spectroscopy is a radiochemical method for investigating materials and the consequences of nuclear transformations taking place in them. Isotopes are traditionally used as structural probes, and the sensitivity of the method is 2–3 orders of magnitude higher than that of absorption Mössbauer spectroscopy. The elements of Mössbauer isotopes with parent nuclei that undergo electron capture or a converted isomeric transition (i.e., lead to high Auger ionization) are the best-studied elements. The electron processes that accompany ionization and their effect on the state of daughter Mössbauer atoms in qualitatively different compounds, from elementary oxides, superconductors, insulators and magnetics to sophisticated bioorganic complexes, are considered.     

A deep view in cultural heritage—confocal micro X-ray spectroscopy for depth resolved elemental analysis

Quantitative X-ray fluorescence (XRF) and particle induced X-ray emission (PIXE) techniques have been developed mostly for the elemental analysis of homogeneous bulk or very simple layered materials. Further on, the microprobe version of both techniques is applied for 2D elemental mapping of surface heterogeneities. At typical XRF/PIXE fixed geometries and exciting energies (15–25 keV and 2–3 MeV, respectively), the analytical signal (characteristic X-ray radiation) emanates from a variable but rather extended depth within the analyzed material, according to the exciting probe energy, set-up geometry, specimen matrix composition and analyte. Consequently, the in-depth resolution offered by XRF and PIXE techniques is rather limited for the characterization of materials with micrometer-scale stratigraphy or 3D heterogeneous structures. This difficulty has been over-passed to some extent in the case of an X-ray or charged particle microprobe by creating the so-called confocal geometry. The field of view of the X-ray spectrometer is spatially restricted by a polycapillary X-ray lens within a sensitive microvolume formed by the two inter-sectioned focal regions. The precise scanning of the analyzed specimen through the confocal microvolume results in depth-sensitive measurements, whereas the additional 2D scanning microprobe possibilities render to element-specific 3D spatial resolution (3D micro-XRF and 3D micro-PIXE). These developments have contributed since 2003 to a variety of fields of applications in environmental, material and life sciences. In contrast to other elemental imaging methods, no size restriction of the objects investigated and the non-destructive character of analysis have been found indispensable for cultural heritage (CH) related applications. The review presents a summary of the experimental set-up developments at synchrotron radiation beamlines, particle accelerators and desktop spectrometers that have driven methodological developments and applications of confocal X-ray microscopy including depth profiling speciation studies by means of confocal X-ray absorption near edge structure (XANES) spectroscopy. The solid mathematical formulation developed for the quantitative in-depth elemental analysis of stratified materials is exemplified and depth profile reconstruction techniques are discussed. Selected CH applications related to the characterization of painted layers from paintings and decorated artifacts (enamels, glasses and ceramics), but also from the study of corrosion and patina layers in glass and metals, respectively, are presented. The analytical capabilities, limitations and future perspectives of the two variants of the confocal micro X-ray spectroscopy, 3D micro-XRF and 3D micro-PIXE, with respect to CH applications are critically assessed and discussed.     

Prospects of “water-window” X-ray emission from subpicosecond laser plasmas

Soft-X-radiation in the “water-window” region (23.3–43.6 ?) mainly from carbon laser plasmas generated by subpicosecond (700 fs) 0.248-μm laser pulses is studied as a function of angle of incidence and intensity (up to 10¹⁸ W/cm²) for p-polarized laser light. Furthermore, comparison is made between plasmas generated from massive and foil targets. Numerical calculations are performed using a hydrocode coupled to X-ray line and continuum emission calculations including radiation transport. The optimized conditions to achieve maximum water-window X-ray emissivity and, in particular, carbon Lyman-α line emission are investigated. In addition, analytical scalings are presented. These theoretical results are essentially confirmed by previous experiments. It is found that at optimized conditions, picosecond or subpicosecond laser plasma X-ray sources with a power of the order of 1–10 GW in a spectral window of 1 ? could be developed. Received: 6 August 1998 / Final version: 6 August 1999 / Published online: 30 November 1999     

Relation between 2p X-ray photoelectron and Kα X-ray emission spectra of manganese and iron oxides

The high resolution Mn and Fe Kα X-ray emission spectra (XES), and Mn and Fe 2p X-ray photoelectron spectra (XPS) for manganese and iron oxides were measured. The spectra were compared with those of [MnO₄]⁻, [Fe(CN)₆]⁴⁻ and [Fe(CN)₆]³⁻ ions. As the electronic structure of the latter compounds do not change with electron hole creation in the core levels, satellite peaks due to charge transfer are not observed in the 2p XPS spectra, and the peak profiles of metal 2p XPS and Kα XES are governed by the exchange splitting between 2p and valence electrons. The metal 2p XPS spectra of the oxides had satellite peaks, but the XES spectra had no satellites. FWHMs of the metal 2p_3/2 main peaks of the compounds being low spin states are smaller than those of metal Kα₁ XES spectra. However, FWHMs of Mn 2p_3/2 of the manganese oxide were nearly equal to those of Mn Kα₁ XES spectra, and those of Fe 2p_3/2 XPS spectra of the iron oxides are greater than those of Fe Kα₁ XES spectra.     

X-ray Lα emission and LIII absorption spectra of copper compounds

The X-ray Lα _{1, 2} emission spectra and L_III absorption spectra in Cu, Cu₂O, CuCl, Cu₂S, CuO, CuCl₂, CuS, CuF₂ and Cu(en)₂Cl₂ are investigated. It is shown that the emission spectra of divalent copper compounds are considerably distorted due to a sharp absorption peak near the L_III edge. The nature of this peak is discussed and a relation is made with satellite peaks in X-ray photoelectron spectra.     

Gamma-X-ray coincidence Mössbauer emission spectroscopy on57Co/CoO

The time integral Mössbauer emission spectrum of a⁵⁷Co/Co_1–x O source (x 10^–5) at RT consists of two single Lorentzian lines of an Fe²⁺ (76%) charge state and an Fe³⁺ (24%) aliovalent charge state. The spectrum measured by -X-ray coincidence spectrpscopy shows the same fraction of the aliovalent charge state, contrary to the expectation derived from the competing acceptor model as successfully applied by Tejada and Parak [1], who could explain the dependence of the formation of aliovalent charge states after the nuclear transformation on the stoichiometric parameterx. The consequences of this unexpected behaviour for the competing acceptor model are discussed.     

Infrared emission spectroscopy of a new Σ-Σ system of TiCl

The spectra of TiCl have been reinvestigated in the 4200-8500 cm⁻¹ region using the 1-m Fourier transform spectrometer associated with the National Solar Observatory at Kitt Peak. The molecules were excited in a microwave discharge lamp operated with 3.0 Torr of He and a trace of TiCl₄ vapor, and the spectra were recorded at a resolution of 0.01 cm⁻¹. Three new bands with origins near 6938.9, 6900.2, and 6861.7 cm⁻¹ have been assigned as the 0-0, 1-1, and 2-2 bands of a new - transition. This assignment is supported by our recent ab initio calculations on TiCl and ZrCl [J. Chem. Phys. 114 (2001) 3977]. A rotational analysis of these bands has been carried out and spectroscopic constants have been extracted for the states.     

The interpretation of X-ray diffraction data from thin epitaxial lead-tin telluride crystals—I

Calculations are made of the expected strength of X-ray diffraction peaks from (100) PbTe and Pb_1−xSn_xTe crystals. The calculations show that in practice the form of the 800 diffraction peak can be related to the epitaxial layer thickness of heterostructures and also indicates the degree of tin diffusion across the heterojunction. The calculations are illustrated in graphical form.The growth system used for the structures examined is shown to produce tin diffusion across a stratum of crystal comparable in thickness to the thickness of the deposited PbTe surface layers.     

Thermoluminescence and ESR phenomena in X-ray irradiated mixed alkaline-alkaline earth metal sulfates—I

Abstract

Thermoluminescence (TL) and ESR phenomena induced by X-ray irradiation of the mixed samples of K₂SO₄ and alkaline earth metal sulfate (MgSO₄ or BaSO₄) were investigated in terms of the reactivities and the crystallogrpahic properties. A high intensive TL resulted from the diffusion of a small amount of Mg²⁺ into K₂SO₄ crystals. The amount formed of K₂Mg₂(SO₄)₃, langbeinite, compound had a maximum at the stoichiometric composition. From the ESR measurements, SO^? ₃ radicals were found to be easily formed in the langebeinite by excitation. Both the TL and the ESR phenomena were hardly observed in the K₂SO₄–BaSO₄ system due to little diffusion of Ba²⁺ with much larger radius than Mg²⁺. The results were mainly discussed on the basis of the radii of the component cations in these systems. In addition, the fundamental data as to the application to the ESR dosimetry were obtained.     

In situ high-resolution X-ray photoelectron spectroscopy – Fundamental insights in surface reactions

Since the advent of third generation synchrotron light sources optimized for providing soft X-rays up to 2 keV, X-ray photoelectron spectroscopy (XPS) has been developed to be an outstanding tool to study surface properties and surface reactions at an unprecedented level. The high resolution allows identifying various surface species, and for small molecules even the vibrational fine structure can be resolved in the XP spectra. The high photon flux reduces the required measuring time per spectrum to the domain of a few seconds or even less, which enables to follow surface processes in situ. Moreover, it also provides access to very small coverages down to below 0.1% of a monolayer, enabling the investigation of minority species or processes at defect sites. The photon energy can be adjusted according to the requirement of a particular experiment, i.e., to maximize or minimize the surface sensitivity or the photoionization cross-section of the substrate or the adsorbate. For a few instruments worldwide, a next step forward was taken by combining in situ high-resolution spectrometers with supersonic molecular beams. These beams allow to control and vary the kinetic and internal energies of the incident molecules and provide a local pressure of up to ~10^?5 mbar, which can be switched on and off in a controllable way, thus offering a well-defined time structure to study adsorption or reaction processes.Herein, we will review some specific scientific aspects which can be addressed by in situ XPS in order to demonstrate the power and potential of the method: In particular, the following topics will be addressed: (1) The sensitivity of the binding energy to adsorption sites will be analyzed, using CO on metals as example. From measurements at different temperatures, the binding energy difference between different sites can be derived, and exchange processes between different adsorbate species at step edges can be followed. (2) The vibrational fine structure of adsorbed small hydrocarbon species on metal surfaces will be analyzed in detail. We will first introduce the linear coupling model, then discuss the properties of adsorbed methyl and of a number of other small hydrocarbons, and show that the vibrational signature can be used as fingerprint for identifying surface species. (3) It is demonstrated that the binding energy of equivalent atoms in a molecule can be differentially changed by adsorption to a substrate; this sensitivity to the local environment will be discussed for adsorbed ethylene, benzene and graphene. (4) By temperature programmed XPS, the thermal evolution of adsorbed species can be followed in great detail, allowing for the identification of reaction intermediates and the determination of their stabilities. (5) The investigation of reaction kinetics by isothermal XPS measurements will be discussed; here results for the oxidation of sulfur and of CO will be presented and the corresponding activation energies of the rate limiting steps will be determined.     

Oxidation state characterization in Cr oxides by means of Cr-Kβ emission spectroscopy

High-resolution Kβ spectra of Cr oxide were measured using a non-conventional spectrometer. Theoretical spectra were obtained using the DV-Xα method in order to interpret the Kβ spectrum structures. Kβ spectrum structures were analyzed and spectral parameters show a great sensitivity to the oxidation state and to the Cr-O distance. High-purity samples of CrO₂ were obtained by means of thermal treatment at 513 °C under oxygen pressure of 200 bar. X-ray diffraction patterns show a typical rutile structure, without spurious phases. The CrO₂ data allowed to confirm the linear dependency of the Kβ_1,3 and Kβ_2,5 energy positions with the oxidation state. The energy of the Kβ_2,5 line relative to the Kβ_1,3 line seems to be a suitable parameter for characterization of the oxidation state. The relative Kβ″ transition probability per Cr-O falls exponentially with Cr-O increasing distance. This behaviour was not found in the literature for Cr oxides.     

Mössbauer emission spectroscopy of grain boundaries in poly- and nanocrystalline niobium

Grain boundaries in polycrystalline Nb, prepared by cold rolling with subsequent recrystallization annealing, and nanocrystalline Nb, obtained by high-pressure torsion, have been studied by ^119m Sn Mössbauer spectroscopy and the data obtained have been compared.     

Mössbauer emission spectroscopy of pure and doped MgO:57Co

Mössbauer emission studies on ⁵⁷Co embedded in pure and doped MgO have been carried out. The appearance of anomalous ⁵⁷Fe+³ in the Mössbauer spectra is related to the concentration and ionization degree of the electron acceptor levels existing in the energy gap. The temperature dependence of isomer shift of both ferrous and ferric lines agrees well with the Debye model.