Soft X-ray Photoabsorption Research Articles

Development of Depth-resolved Soft X-ray Photoabsorption Spectroscopy

Development of Depth-resolved Soft X-ray Photoabsorption Spectroscopy

Soft x-ray photoabsorption spectra of photoionized CH4 and CO2 plasmas

In this work we report soft x-ray (SXR) photoabsorption measurements in neutral methane and carbon dioxide molecules and their corresponding photoionized plasmas. The SXR radiation was generated by a table-top laser produced plasma source based on a double stream gas puff target. At low SXR intensities only features related to neutral molecules are present in the absorption spectrum. On the other hand, as the radiation intensity increases, we observe new absorption features in the low photon energy side of the spectrum. In that case fragments such as neutral and ionized molecules, atoms and atomic ions are found to contribute to the absorption spectrum of the plasmas. To our knowledge these are the first measurements where this laser plasma based SXR source is used to both create and probe a molecular plasma. Emphasis is placed on identifying the fragment species and the corresponding transitions.

Sulfur speciation in growth layers of shell cross section of the long-lived bivalve Margaritifera laevis using synchrotron spectromicroscopy analysis

To reliably utilize the information on growth and environment stored within the hard carbonate shells of long-lived bivalves, it is necessary to accurately count the annual growth lines from their shell cross-sections. In this study, we investigated the relationship between the distribution of minor elements and visual lines in the shell cross-sections of the long-lived mussel Margaritifera laevis collected from the Chitose River in Hokkaido, Japan. The distribution of minor elements, such as sulfur (S), phosphorus (P), strontium (Sr), magnesium (Mg), and sodium (Na), in a cross-section of the umbo was determined by X-ray fluorescence analysis using synchrotron soft X-ray radiation. The chemical properties of S and P were analyzed in-situ using soft X-ray photoabsorption spectroscopy, and the chemical form dependence of S distribution was determined by speciation mapping. In addition, the annual and disturbance lines were visualized by staining shell cross-sections. Our results showed that the distribution of reduced S (i.e., SC and SH) corresponded to the annual lines, whereas the distribution of oxidized S (i.e., SO42−) corresponded to the disturbance lines. Overall, this study revealed that speciation analysis of S in shell cross-sections can be used to determine the origin of visual lines in chronological studies.

Near edge X-ray absorption mass spectrometry on coronene.

We have investigated the photoionization and photodissociation of free coronene cations C24H12 (+) upon soft X-ray photoabsorption in the carbon K-edge region by means of a time-of-flight mass spectrometry approach. Core excitation into an unoccupied molecular orbital (below threshold) and core ionization into the continuum both leave a C 1s vacancy, that is subsequently filled in an Auger-type process. The resulting coronene dications and trications are internally excited and cool down predominantly by means of hydrogen emission. Density functional theory was employed to determine the dissociation energies for subsequent neutral hydrogen loss. A statistical cascade model incorporating these dissociation energies agrees well with the experimentally observed dehydrogenation. For double ionization, i.e., formation of intermediate C24H12 (3+⋆)trications, the experimental data hint at loss of H(+) ions. This asymmetric fission channel is associated with hot intermediates, whereas colder intermediates predominantly decay via neutral H loss.

Electron yield soft X-ray photoabsorption spectroscopy under normal ambient-pressure conditions.

Ambient-pressure soft X-ray photoabsorption spectroscopy (XAS) was demonstrated to be applicable to the chemical analysis of hydrated transition-metal compounds. For this purpose, even under ambient-pressure conditions, electron yield detection XAS (EY-XAS), based on a simple drain-current set-up, was used to overcome a weakness in fluorescence yield detection XAS (FY-XAS), which does not give a pure soft XAS. The feasibility of EY-XAS was investigated and it was clarified that the EY-XAS under ambient-pressure conditions corresponds to the mixed data of the total EY and conversion EY spectra. Normal ambient-pressure EY-XAS analysis was applied to anhydrous (CoCl2) and to hydrated (CoCl2·6H2O) cobalt chloride at the Co L23-edge. The present measurements demonstrated the ability to unambiguously distinguish the different chemical states of cobalt ions, relying upon spectral differences that indicate octahedral/quasi-octahedral structural changes as a result of hydration/dehydration reactions.

The adsorption of adenine on mineral surfaces: Iron pyrite and silicon dioxide

The adsorption of the nucleobase adenine on surfaces of the minerals iron pyrite and silica has been studied by photoemission and soft X-ray photoabsorption spectroscopy. Pyrite samples were prepared by fracture under nitrogen followed by transfer to ultrahigh vacuum, or by cleavage in vacuum. By comparing data with multilayer spectra, adenine was found to chemisorb on pyrite, with small changes in the valence band spectrum, and stronger changes in the NEXAFS spectrum. The molecules were bonded with the molecular plane at a steep angle to the surface plane. On silica the molecule was found to adsorb at a reduced angle to the surface. The C and N 1s photoemission spectra on this surface suggest chemisorption, although the nitrogen NEXAFS spectra are similar to multilayer spectra.

Electronic and magnetic states of Ce 4f electrons in CeRh 3B 2

CeRh 3B 2 is a ferromagnet with exceptionally high Curie temperature ( T C = 115 K) among the Ce compounds with no transition metal magnetic element. Magnetic circular dichroism of soft X-ray photoabsorption (XMCD) has revealed that the orbital angular momentum of Ce 4f electron is quenched to some extent. In addition, preferential occupation of the Ce 4f orbital pointing to the c-axis has been confirmed by the soft X-ray photoabsorption measurements with different geometries. On the other hand, resonant photoemission around the Ce 3d → 4f photoabsorption edge has been adopted in order to disentangle the Ce 4f and Rh 4d components in the valence band. The Rh 4d spectrum is qualitatively consistent with the band structure calculation. Although the Ce 4f spectrum has a peak near the Fermi level, the spectral shape is qualitatively different from that of the typical Kondo materials and is rather similar to that of itinerant Ce 4f systems.

Magnetic X-ray circular dichroism on in situ grown 3d magnetic thin films on surfaces.

Epitaxic thin and ultrathin films on surfaces allow crystallographic phases that do not occur naturally in the bulk to be stabilized. They also offer new possibilities for an improved understanding of soft X-ray photoabsorption in magnetic systems. Data collected using the Elliptically Polarizing Undulator at BL 5.2 of the Stanford Synchrotron Radiation Laboratory are presented herein. Fe, Co and Ni films were prepared on Cu(100) surfaces. L2,3-edge spectra were recorded with circular and linear light. Fresnel diffractometry was used to quantify the degree of transverse beam coherence. A quantitative analysis of the spectral features indicates a correlation of the spectral intensities and the transverse beam coherence. Resonant reflectivity spectra for Co ultrathin films that exhibit strong dichroism are presented. The reflectivity data indicate that interference effects of the reflected beams at the two interfaces are of importance, even for ultrathin films.

UV, VUV and soft X-ray photoabsorption of dimethyl ether by dipole (e,e) spectroscopies

Absolute UV and VUV photoabsorption oscillator strengths (cross-sections) for the valence shell discrete and continuum regions of dimethyl ether (CH 3OCH 3, DME) have been measured from 5 to 32 eV using high resolution (HR) (∼0.05 eV f.w.h.m.) dipole (e,e) spectroscopy. A wide-range spectrum, spanning the UV, VUV and soft X-ray regions, from 5 to 200 eV has also been obtained at low resolution (LR) (∼1 eV f.w.h.m.), and this has been used to determine the absolute oscillator strength scale by employing valence shell Thomas–Reiche–Kuhn (i.e., S(0)) sum-rule normalization. The presently reported HR and LR absolute photoabsorption oscillator strengths are compared with previously published data from direct photoabsorption measurements in those limited energy regions where such data are available. Evaluation of the S(−2) sum using the presently reported absolute differential photoabsorption oscillator strength data gives a static dipole polarizability for dimethyl ether in excellent agreement (within 0.5%) with previously reported polarizability values. Other dipole sums S( u), ( u=−1,−3,−4,−5,−6,−8,−10), and logarithmic dipole sums L( u), ( u=−1 to −6), are also determined from the presently reported absolute differential photoabsorption oscillator strength data using dipole sum rules.

Microscopic chemical state identification of a silicon-carbide fiber by soft x-ray photoabsorption spectroscopy

Chemical states of a 140 μm silicon carbide fiber are determined by soft x-ray photo-absorption spectroscopy, using a magnetic projection photoelectron microscope, performed on the surface obtained by fracturing. The spectrum of the carbon core of the fiber has prominent π* and σ* resonance peaks and is very similar to that measured on highly oriented pyrolitic graphite, indicating a graphitic nature of the carbon core. The spectrum of the silicon carbide coating of the fiber reveals similar features to those of single-crystal silicon carbide, indicating that this coating is in the crystalline or polycrystalline phase.

Absolute UV and soft X-ray photoabsorption of acetylene by high resolution dipole (e,e) spectroscopy

Abstract Absolute photoabsorption oscillator strengths (cross sections) for the discrete electronic transitions of acetylene from 5.5 to 11.2 eV have been measured using high resolution ≈0.05 eV FWHM) dipole (e,e) spectroscopy. This experimental resolution enabled absolute oscillator strengths for most of the individual vibronic transitions to be obtained. A broader range spectrum, obtained from 5.5 to 200 eV at a resolution of ≈1 eV FWHM, was used to establish to absolute oscillator strength scale via TRK sum-rule normalization. The absolute oscillator strengths for the Rydberg transitions are in many cases significantly higher than previously reported values obtained using direct optical photoabsorption methods.

Absolute UV and Soft X-ray photoabsorption of ethylene by high resolution dipole (e, e) spectroscopy

Abstract Absolute photoabsorption oscillator strengths (cross sections) for the valence shell discrete region of ethylene from 6 to 10.7 eV have been measured using high resolution (∼ 0.05 eV fwhm) dipole (e, e) spectroscopy. A long-range spectrum was also obtained from 6 to 200 eV at low resolution ( ∼ 1 eV fwhm), from which the absolute oscillator strength scale was obtained via valence-shell TRK sum-rule normalization. The presently determined absolute oscillator strengths were found to be more consistent with the (exact) theoretical value of the total S (0) TRK sum-rule than previously published data. In addition, the electric dipole polarizability of ethylene determined from the total S (−2) sum-rule incorporating the presently reported oscillator strengths is in very good agreement with experimentally measured polarizability values.

2p absorption spectra of atomic copper using the soft X-ray absorption and total photoion yield methods

The soft X-ray photoabsorption and total photoion yield spectra of atomic Cu have been determined in the range of the L-edges. At the L2 and L3 thresholds there are clearly visible jumps in the absorption cross section. Superimposed on this step-like increase are weak but in part surprisingly sharp structures. The atomic data are compared both to Hartree-Fock calculations and the soft X-ray absorption of copper metal.

Ionization current detection of soft X-ray photoabsorption: Sulfur and chlorine k-shell spectra of SO 2F 2, SO 2FCl and SO 2Cl 2

An apparatus for recording inner-shell excitation spectra by detecting currents produced by the X-ray ionization of gases is described. The equivalence of ion current and true photoabsorption (log( I 0/ I)) detection for the case of S 1s spectroscopy of molecules is demonstrated. The ion current detection has been used to record the S 1s and Cl 1s spectra of SO 2FCl and SO 2Cl 2 and the S 1s spectrum of SO 2F 2. The spectral features are analysed in terms of excitations to σ* orbitals characteristic of the local bonding to the K-shell excited atom. The correlation between SX bond lengths and the positions of S 1s→σ*(SX) excitations relative to the S 1s ionization threshold is discussed and related to corresponding correlations for OX bonds.

The X-Ray Opacity of the Interstellar Medium

In order to interpret soft X-ray spectra of cosmic X-ray sources, it is necessary to know the photoabsorption cross-section of the intervening interstellar material. Current models suggest that the interstellar medium contains two phases which make a substantial contribution to the X-ray opacity: cool, relatively dense clouds that exist in pressure equilibrium with hot, tenuous intercloud regions. We have computed the soft X-ray photoabsorption cross-section of each of these two phases. The calculations are based on a model of the interstellar medium which includes chemical evolution of the galaxy, the formation of molecules and grains, and the ionization structure of each phase.