Synchrotron Radiation Absorption Research Articles

Sulfur K-edge XANES study of local electronic structure in ternary monosulfide solid solution [(Fe, Co, Ni) 0.923 S

Synchrotron radiation S K-edge XANES spectra and unit-cell parameters are used to investigate the local electronic structure of non-stoichiometric binary and ternary Fe-Co-Ni monosulfide solid solution (mss; M0.923S, M = Fe, Co, Ni) quenched from 800 °C and low pressure. The prominent absorption edge feature of the XANES spectra represents transition of S 1s core level electrons to unoccupied S 3p σ* antibonding orbitals hybridized with empty metal 3d(eg) orbitals. There is a progressive increase in area of the edge peak from Fe0.923S to Ni0.923S and Co0.923S, which correlates with progressive decrease in c and a parameters for the NiAs-type subcell and increase in metallic character, and reflects increase in the number and availability of empty eg β orbitals and covalence of metal-S bonds. More generally, the area of the edge peak exhibits an inverse linear correlation with a, c and unit-cell volume of binary and ternary mss. This inverse linear correlation is attributed to progressive increase in covalence and M-S-M bonding interaction in the c-axis direction, through metal-S [M 3d(eg) - S 3p (or 3d)] π bonding. However, the area of the edge peak does not correlate very well with the average number of 3d electrons per metal atom in these solid solutions, showing that the absorption of synchrotron radiation reflects the local electronic structure of individual absorber atoms (i.e. the SM6 cluster), and is not a group (crystal energy band) effect.

Collective electron excitations in 3D graphite clusters

Group-theoretical analysis and subsequent quantum-chemical calculations based on the molecular orbital method applied to a cyclic model of 3D semimetallic graphite lead to a multiplet of spectroscopic combinations of Slater determinants. The transition energies ΔE between terms of the multiplet are interpreted as the energies of collective electron mesoscopic excitations ℏω in the entire set of electron states characterizing the metal-type conductivity of a cluster. The estimate ℏω∼0.2ΔE(N0/1000)2/3 is obtained for a cluster consisting of N0 primitive cells. Depending on the thermal processing, N0=(0.3–20)×106 in pyrolytic graphite, and accordingly ℏω∼(10–150) eV. In the case when the energy cannot be determined accurately, methods permitting the variation of an excitation over a wide range (such as the spectroscopy of synchrotron radiation absorption and the characteristic energy losses of charged particles) appear to be the most promising.

Density of phonon states in iron at high pressure

The lattice dynamics of the hexagonal close-packed (hcp) phase of iron was studied with nuclear inelastic absorption of synchrotron radiation at pressures from 20 to 42 gigapascals. A variety of thermodynamic parameters were derived from the measured density of phonon states for hcp iron, such as Debye temperatures, Gruneisen parameter, mean sound velocities, and the lattice contribution to entropy and specific heat. The results are of geophysical interest, because hcp iron is considered to be a major component of Earth's inner core.

DOS: Evaluation of phonon density of states from nuclear resonant inelastic absorption

Theoretical background and specific features of the calculation of the phonon density of states from energy spectra of nuclear resonant inelastic absorption of synchrotron radiation are presented. Double Fourier transformation is used to deconvolute data and an instrumental function and to eliminate the multiphonon contributions. A computer program is developed and an example of its work is shown.

Phonon density of states in epitaxial FE/CR(0 0 1) superlattices

Incoherent nuclear resonant absorption of synchrotron radiation at the 14.413 keV nuclear resonance of 57Fe was employed to measure directly the Fe-projected (partial) phonon density of states (DOS) in epitaxial FeCr(0 0 1) superlattices and in an 57Fe0.03Cr0.97(0 0 1) alloy film MBE-grown on MgO(0 0 1). The measurements were performed at 300 K with 2.3 meV energy resolution around 14.413 keV. At the interfaces, longitudinal vibrations of Fe atoms are suppressed, and a strong resonance phonon mode appears near 23 meV.

Temperature dependent phonon density of states of the invar alloy Fe 72Pt 28

Inelastic nuclear resonant absorption of synchrotron radiation was applied to determine the phonon density of states of the invar alloy Fe 72Pt 28. Using a double crystal monochromator with asymmetric Si (975) reflections we arrived at an energy resolution of 0.8 meV. Anharmonic behaviour was observed in the energy regime of the TA [110] phonon branch and the highest energy optical branch. The data may be used to determine the lattice thermal expansion, the vibrational entropy and the vibrational contribution to the capacity of heat.

Inelastic nuclear resonant absorption of synchrotron radiation in thin films and multilayers

Inelastic nuclear resonant absorption of synchrotron radiation is an efficient and unique method for the direct measurement of vibrational density of states (VDOS) of thin films and interfaces that contain Mossbauer isotopes. This is demonstrated for the 57Fe nuclear resonance in the case of amorphous and crystalline Tb–Fe alloy thin films and buried Fe/Cr interfaces in epitaxial α-Fe(0 0 1)/Cr(0 0 1) superlattices.

Temperature dependence of nuclear inelastic absorption of synchrotron radiation in alpha -57Fe.

Energy spectra of nuclear inelastic absorption of synchrotron radiation in $\ensuremath{\alpha}$-$^{57}\mathrm{Fe}$ were measured with 4.4-meV energy resolution in a temperature range 24-400 K. The temperature dependence of the recoil fraction and of the multiphonon contribution was determined. The density of phonon states was derived at each temperature and showed a 4% relative frequency change within the studied temperature range.

Angular distribution of photoelectrons in resonant photoionization of polarized atoms

The angular distribution of photoelectrons produced by excitation of polarized atoms to autoionizing states is analysed. In particular, core-excited autoionizing states produced in absorption of synchrotron radiation by laser-pumped excited atoms are considered. A general expression for the angular distribution of arbitrarily oriented laser and synchrotron beams is obtained and further reduced to a simpler form for a particular geometry used in experiments. A case of an isolated autoionizing state (resonance) interacting with several continua is considered and parametrization of the observable parameters in the region of the resonance is discussed. Peculiarities of the circular dichroism in the region of autoionizing resonances are analysed. Formulae suitable for particular resonances with J<or=5/2 are presented and all parameters necessary for analysis of experimental angular distributions are tabulated. As examples, excitations of autoionizing 2p53s3p and 2s3p2 states in Na and 3p53d4s4p states in Ca are considered. Theoretical angular distributions are compared with recent experimental data.

Radiation stability of SiO2-antireflective film coated SiN and SiC x-ray mask membranes

Radiation stability of SiO2 antireflective film coated SiN and SiC x-ray mask membranes was studied. Although the transmission at 633 nm of the SiO2-coated SiN was reduced by about 8% after synchrotron radiation (SR) exposure with an absorber dose of 100 MJ/cm3, the oscillation of the transmission due to interference was considerably suppressed even after the SR absorption. Furthermore, the optical transmission spectrum of the SiO2-coated SiC was nearly equal to that of uncoated SiC after the SR absorption of 100 MJ/cm3. The pattern displacement induced by the SR absorption of 10 MJ/cm3 for the SiO2-coated SiN and for the SiO2-coated SiC was σx=6 nm, σy=7 nm and σx=8 nm, σy=6 nm on the 25-mm-square area, respectively, the values of which were within the reproducibility of the displacement measurement. Electron spin resonance analysis indicated that no significant difference of spin density between the SiO2-coated and uncoated SiN membranes was recognized before and after the SR absorption.

Improvement in Radiation Stability of SiN X-Ray Mask Membranes

The synchrotron radiation (SR) stability of silicon nitride (SiN) X-ray mask membranes was successfully improved by low-pressure chemical vapor deposition at a growth temperature of 1000°C. The transmission of the SiN membrane formed at 1000°C exceeded 90% at about 633 nm even after SR absorption with a dose of about 100 MJ/cm3, although those of membranes formed at 800 and 900°C decreased to below 85% after the same SR absorption. In addition, SR-induced pattern displacements for the SiN membrane formed at 1000°C were suppressed to σx=11 nm and σy=10 nm with a dose of 20 MJ/cm3. The N/Si atomic ratio increased, and the H and O concentrations in the SiN decreased with the increase of the growth temperature. The increase ratio of the spin density before and after the SR absorption decreased with the increase of the growth temperature.

Reconstruction of the 3-D Atomic Structure of CoSi2 (111) by Photoelectron Holography

ABSTRACTWe report the use of photoelectron holography for the study of the atomic geometry of the (111) surface of single crystal CoSi2. We employ as a reference wave the Co 3p core electron emitted with a kinetic energy of 695 eV by absorption of synchrotron radiation. Remarkably sharp images of the nearest Si neighbors of the Co emitter were found using a computer reconstruction algorithm which compensates for the anisotropies of both reference and object waves. Comparison with other reconstruction schemes such as the Helmholtz-Kirchoff (HK) algorithm and the scattered-wave included Fourier transform (SWIFT), demonstrates the necessity of compensating for the reference-wave anisotropy.

Study of radiation stability in SiN x-ray mask membranes for synchrotron radiation lithography

The radiation stability in silicon nitride (SiN) membranes for synchrotron radiation (SR) lithography has been investigated in a helium ambient, using scanning-mirror exposure method. It was found that the displacement along the y-axis of an SiN membrane is larger than that along the x-axis. A mirror was oscillated along the y-axis in order to obtain a 30 mm square irradiation, thereby producing the variation in the SR spectrum absorption difference in the membrane, especially in the wavelength region below the silicon absorption edge (0.67 nm in wavelength), although an SR power per unit area was given on a resist almost uniformly. As a result, it is considered that this SR spectrum absorption difference causes larger displacements along the y-a axis of the SiN membrane.

Emission and absorption of synchrotron radiation in tokamak plasmas

Emission and absorption of high harmonics radiation in a Tokamak plasma is investigated. The interplay of spatial inhomogeneities of the plasma parameters and toroidal variation of the magnetic field is discussed for horizontal and vertical propagation.

Finite element analysis of a SiC mirror receiving synchrotron radiation

The finite element method is used to answer some fundamental questions regarding maximum temperature, thermal stresses, and thermal distortions of a SiC mirror under thermal load. Absorption of synchrotron radiation, generated from sources such as insertion devices and bending magnets, produces the thermal load. Results of several analyses are presented to show typical behavior of a mirror with simple geometry under different conditions and designs. Both steady state and transient cases with radiation cooling alone and with a combination of radiation and convection cooling are considered.

Molecular spectroscopy and dynamics of core and valence excited states by electron scattering and synchrotron radiation

This paper reviews i) the spectroscopy of core and valence excited states in CO 2, OCS and CS 2 molecules, chosen as model systems. The analysis is based on excitation spectra obtained with X-ray synchrotron radiation absorption and with electron impact energy loss spectroscopy. Then, we discuss the relative energies of unoccupied valence orbitals ii) the dynamics of electronically excited states i.e. dissociation versus ionization, in OCS and HBr, chosen as illustrative examples. We use recent results obtained with various complementary experimental methods, such as fluorescence excitation spectroscopy of neutral fragments, photoionization and (threshold) photoelectron spectroscopy combined with VUV synchrotron radiation. We show that dissociation may compete efficiently with ionization or more over preceeds it.

High-resolution analytical electron microscopy (HRAEM) study of LaCo5 intermetallic catalysts

Intermetal1ic compounds, such as LaCo5, have been used as active and selective catalysts in chemical reaction involving (CO,H2) mixtures. The general understanding regarding these catalysts is that the observed activation is associated with a partial transformation of the intermetallic component into a metal-support system. The macroscopic study of LaCo3 by use of synchrotron radiation absorption measurements (EXAFS and XANES data analysis) largely support this hypothesis. In this work, the microscopic structure of these catalysts has been studied by HRAEM.

Threshold electron impact excitation of Cl2

A high resolution electron impact threshold spectroscopy technique was used to examine the excitation of Cl2 in the 2–14 eV region. This study complements previous photon absorption and emission measurements, because it is capable of detecting transitions which are optically forbidden. In the region up to 7.5 eV, broad dissociative structures are correlated with optically active valence states, although relative intensities in the threshold spectrum differ considerably and indicate a substantial contribution from the optically forbidden transitions. At 7.46 eV a series of 5 equidistant sharp peaks is detected and interpreted as arising from the2 π g Feshbach resonance, which differs from the ground state positive ion Cl 2 + by a pair of Rydberg electrons: (4sσ)2. The decay channels responsible for the appearance of the resonance in a threshold spectrum are discussed and it is suggested that they include several valence states of the (2431) and (2341) configurations, whose potential energy curves cross the Cl 2 − ,2πg curve in the region of energy at which the resonance state is formed. At higher incident electron energies and up to ionisation, Rydberg states predominate, starting with (2430) 4s3,1 π g states detected for the first time. The absence of broad peaks above 8 eV and the irregular appearance of Rydberg bands is consistent with the strong Rydberg-valence configuration mixing proposed by Peyerimhoff and Buenker. Where our resolution permits comparison, good general agreement is found with recent synchrotron radiation absorption measurements of optically allowed transitions.

Radiation hydrodynamics of high-luminosity accretion into black holes

To extend Shapiro's (1973a, b) calculations of black hole accretion to the regimes of interstellar gas densities and of black hole masses for which emergent luminosities are expected to be high, the radiation hydrodynamics of spherically symmetric gas flows in static isotropic metrics is discussed. Since for the more luminous objects the optical depth of the accretion volume becomes large, particular attention has to be paid to radiative transfer through non-Euclidean spaces, and a method for solving the full transfer problem is presented. The method is applied to accretion into black holes of mass between 10M⊙ and 105M⊙, under the conservative assumption that all other heat sources, like dissipation of magnetic or turbulent energy, can be neglected in comparison to the compressional work term,p dV. In the interstellar gas parameter range of interest, the radiation field is then dominated by emission and absorption of synchrotron radiation from inner zones of the flow. Temperature stratifications, luminosities and emergent spectra resulting from these processes are calculated.

Simple Calculation ofLII,IIIAbsorption Spectra of Na, Al, and Si

The shape of soft x-ray absorption spectra extending 100 eV above the ${L}_{\mathrm{II},\mathrm{III}}$ x-ray threshold is calculated for Na, Al, and Si using a simple orthogonal plane-wave final state including backscattering from near neighbors, and hydrogenic-core wave functions. Comparison with synchrotron-radiation absorption and newly measured electron energy-loss spectra shows good agreement between the present simple calculation and experimental data.