Search results for "Absorption edge"
showing 10 items of 95 documents
Carbon coatings for soft-x-ray reflectivity enhancement
2007
In X-ray astronomical telescopes, the focalization of the radiation is achieved by means of grazing incidence Wolter I (parabola + hyperbola) optics in total reflection regime. In general, high density materials (e.g. Au, Pt, Ir, W) are used as reflecting coatings, in order to increase as much as possible the cut-off angles and energies for total reflection. However these materials present an important reduction of the reflectivity between 0.2 and 5 keV, due to the photoabsorption, and this phenomenon is particularly enhanced in correspondence of the M absorption edges (between 2 and 3.5 keV). In general, this determines a strong decrease of the telescope effective area. To overcome the pro…
A Complex Environment around Circinus X-1
2007
We present the results of an archival 54 ks long Chandra observation of the peculiar source Cir X-1 during the phase passage 0.223-0.261. We focus on the study of detected emission and absorption features using the HETGS. A comparative analysis of X-ray spectra, selected at different flux levels of the source, allows us to distinguish between a very hard state, at a low count rate, and a brighter, softer, highly absorbed spectrum during episodes of flaring activity. The spectrum of the hard state clearly shows emission lines of highly ionized elements, while, during the flaring state, the spectrum also shows strong resonant absorption lines. The most intense and interesting feature in this …
Resonant X-ray scattering in biological structure research
2007
The use of anomalous X-ray scattering of light elements like sulfur and phosphorus is of particular interest in biological structure research. These elements serve as native labels in proteins, nucleic acids and membranes. Their medium scattering power is drastically changed at their K absorption edges at wavelengths between 5 and 6 A where X-ray absorption excludes the use of open air diffractometers. The construction of a new diffractometer tunable to wavelengths between 1.2 and 7 A is presented. First results of anomalous scattering from sulfur in bacteriorhodopsin near the K absorption edge have been obtained recently. Their possible impact on crystallography will be considered. A compa…
Irradiation effects on the absorption edge of silica glass
2007
Abstract Vacuum ultraviolet absorption experiments were carried out on a variety of specimens of amorphous silica β-irradiated at different doses from ∼103 to 5 × 106 kGy. Changes in the width of the absorption (Urbach) edge were investigated. These changes strongly depend on the kind of silica considered: in particular the Urbach energy of silica of industrial manufacture increases in the irradiated samples, whereas in sol–gel silica it is poorly influenced by the irradiation. The fictive temperature of the different materials before and after irradiation was also monitored. The changes of the Urbach energy and of the fictive temperature are tentatively discussed considering the disorder d…
Interpretation of absorption edges by resonant electronic spectroscopy: experiment and theory
2004
Abstract Resonant electronic spectroscopy consists in measuring a non-radiative decay process (Auger or autoionization process) excited with photon energies around an absorption edge. The resonant spectra carry information both on the nature of the electronic transitions near the absorption edge by scanning the very first empty orbitals above the Fermi level (through the absorption process), and, on the other hand, on the atomic electronic configuration through the lineshape of the observed decay process. In this paper, after a quick review of the pioneering works in this field, we show that resonant measurements and their theoretical modeling can be used to precisely interpret complex abso…
Electronic exchanges between adsorbed Ni atoms and TiO2(110) surface evidenced by resonant photoemission
2011
Abstract Nickel was deposited on stoichiometric TiO2(1 1 0) surface in the 0.02–2.1 equivalent monolayer (eqML) range and analyzed by means of photoemission and resonant photoemission. In the case of very low coverage (lower than 0.1 eqML), deposited nickel reacts with the surface through an electronic transfer from nickel atoms towards titanium ions. This exchange caused the filling of unoccupied Ti3d states leading to the increase of a peak in the TiO2 band gap. These states can be better characterized through resonant photoemission experiments at the Ti 3p → 3d absorption edge: for very low coverage, these states in the TiO2 band gap have resonant behavior of Ti3d electrons rather than N…
Evaluation of multiple-scattering contribution in extended X-ray absorption fine structure for MO4 and MO6 clusters
1994
Abstract We present a theoretical ab initio evaluation of the multiple-scattering contribution in the extended X-ray absorption fine structure for MO4 and MO6 clusters with M = Mg, Ca, Mn, Zn, Se, Sr, Mo, Ag, Te, Ba, Nd, Tb, W, Au or Bi. The dependence of the multiple-scattering signal on the absorber type, the photoelectron angular momentum and the local distortion is discussed. It is shown that the multiple-scattering contribution is significant in the photoelectron wave-vector range up to 6–7 A−1 and strongly depends on both the path geometry and the atomic species involved in the scattering process.
Anomale röntgenstreuung zur erforschung makromolekularer strukturen
1982
The possibilities of anomalous X-ray scattering for the investigation of semi- or noncrystalline macromolecular structures are discussed on the grounds of the first successful measurements of anomalous scattering of iron in dissolved hemoglobin. As anomalous scattering of this kind of material is limited to the near vicinity of the X-ray absorption edges, the use of synchrotron radiation appears to be obligatory. The anomalous dispersion of the atomic form factors slightly increases with the atomic number. Anomalous dispersion is nearly doubled when going from the K-absorption edge to the LIII-absorption edge and from there to the Mv- absorption edge. Therefore, the use of anomalous dispers…
The electron beam induced short lived absorption in PLZT optical ceramics
2013
Abstract The short lived absorption induced by a pulsed electron beam in the led–lanthanum–zirconate–titanate (PLZT) optical ceramics was studied. The measured absorption spectrum covers 1.1–2.9 eV energy range and consists of several strongly overlapping peaks. The rise of the absorption is delayed with respect to the excitation pulse, due to charge carrier migration before trapping at centers responsible for the absorption. The formation rate of absorption centers and decay rate of absorption depend on the photon energy. The kinetics of short lived absorption varies over spectrum and can be approximated with a stretched exponent. The stretched exponent parameters indicate that the local d…
Absorption edge in silica glass
2005
Measurements of optical absorption in the v-UV range in a variety of silica glass are used to determine the width of the absorption edge (Urbach energy). Measured values range from 60 meV up to 180 meV. So high a variability over silica types is tentatively ascribed to the different disorder degree, which characterizes different materials.