The X-ray fluorescence cross-section for bromide and iodide compounds

The K_α and K_β X-ray fluorescence cross-sections for the bromide and the iodide compounds were measured by a high-resolution Si(Li) X-ray detector. The vacancies were produced by heavily filtered ²⁴¹Am gamma rays. We found that K_α and K_β X-ray fluorescence cross-sections are changed by chemical effect for different Br and I compounds. Experimental results were compared with the calculated values of Br and I elements. Received 15 May 2001 and Received in final form 8 July 2001     

High-precision measurement of the 14O half-life using a mass-separated radioactive beam

A high-precision measurement of the ¹⁴O half-life has been performed using a mass-separated radioactive beam in combination with a germanium detector set-up. This is the first ¹⁴O half-life measurement with a contamination-free source. The final result of 70.560 ± 0.049 seconds is in agreement with the generally adopted mean value. Received: 5 April 2001 / Accepted: 6 August 2001     

Soft X-ray magnetic circular dichroism in Fe and Fe 0.50 Co 0.48 V 0.02 films: quantitative analysis of transmission

Magnetic circular dichroism (MCD) in X-ray absorption has been measured at the L _{2, 3} edges of Fe in ex-situ grown Fe and Fe _0.50 Co _0.48 V _0.02 films by means of the transmission method. A new approach is developed for fitting the observed transmittance, which describes the resonance lineshapes as (generalized) Fano profiles. Analytical integration of each single resonance allows a more reliable determination of the orbital and spin magnetic moments based on the MCD sum rules. The results are consistent with an increase of the Fe spin and orbital magnetic moment in Fe-Co alloys as obtained by other experiments and band structure calculations. Received 15 August 2000 and Received in final form 11 June 2001     

The electronic structure of the doped one-dimensional transition metal oxide Y 2 - x Ca x BaNiO 5 studied using X-ray absorption

A strong anisotropic distribution of the holes in Ni 3 d and O 2 p orbitals is observed in the polarization dependent O 1 s and Ni 2 p _3/2 X-ray absorption spectroscopy of the linear-chain nickelate Y_2-xCa_xBaNiO₅ (x = 0, 0.05, 0.1, 0.2), which demonstrates the one-dimensional nature of the electronic state in these compounds. The holes introduced by Ca-doping occupy both O 2 p and Ni 3 d orbitals along the NiO₅ chains. By comparing the experimental Ni 2 p _3/2 absorption spectra of Y_2-xCa_xBaNiO₅ to those from charge transfer multiplet calculations we can derive the orbital character of the additional holes to be of ∼60% O2 p and ∼40% Ni 3 d. Received 7 January 2002     

The use of high energy laser-plasma sources in soft X-ray contact microscopy of living biological samples

In this paper the results of an experiment on soft X-ray contact microscopy using a laser-plasma source are presented. A resolution of 50 nm has been achieved imaging pig sperm cells, while other specimens, such as algae and yeast cells, showed internal details, proving the technique to be a powerful tool for biological investigations. Original biological information has been obtained and the conditions for optimal image formation have been studied. Received 5 June 2002 Published online 24 September 2002 RID="a" ID="a"e-mail: batani@mib.infn.it     

Dynamics and correlations in magnetic colloidal systems studied by X-ray photon correlation spectroscopy

We have studied the static and dynamic behaviour of magnetic colloidal systems (ferrofluids) by Small-Angle X-ray Scattering (SAXS) and X-ray Photon Correlation Spectroscopy (XPCS). The main features of the novel XPCS technique will be illustrated by data taken on a model system, colloidal silica spheres. We will then present the results obtained on an optically opaque suspension of magnetic colloids (maghemite) in the wave vector range from 10^-3 to 10^-2?^-1. Translational diffusion in zero field and anisotropic diffusion under external magnetic fields will be discussed. Received 9 August 1999 and Received in final form 26 July 2000     

Vanadium valency and hybridization in V-doped hafnia investigated by electron energy loss spectroscopy

The valency of vanadium, and thus indirectly the oxygen stoichiometry, of V-doped hafnia synthesized under different atmospheres have been investigated on a nanometer scale by means of electron energy loss spectroscopy (EELS). The EELS V L_2,3 spectra are compared with the results of crystal field multiplet calculations and experiments on reference vanadium oxides. The EELS spectra indicate that V-doped hafnia prepared under reducing (H₂) and neutral (Ar) atmosphere are unambiguously substituted with trivalent vanadium atoms leading to the creation of oxygen vacancies in the structure. On the contrary, stoichiometric (Hf, V)O₂ compound (i.e. V⁴⁺) is more likely to be stabilized under oxidative (air) atmospheres. We also show that the amount of hybridization alters for the different compounds studied but may in part be analyzed by high spatially resolved EELS. The crystal field multiplet calculations particularly indicate that a simple reduction of the Slater integrals gives a good account of the spectral modification induced by hybridization for the case of tetravalent vanadium atoms. Received 17 November 2000 and Received in final form 17 April 2001     

Target-ion source unit ionization efficiency measurement by a method of stable ion beam implantation

At the IRIS facility a rather precise method of the target-ion source unit ionization efficiency measurement has been developed. The method exploits an off-line mass-separator for the implantation of the ion beams of selected stable isotopes into a tantalum foil placed inside a Faraday cup in the focal plane of the mass-separator. After the implantation of the required amount of the investigated species, tantalum foil has been inserted into the volume of the target-ion source unit prepared for the on-line utilization at the IRIS on-line separator. The first tests have ensured the ionization efficiency values (90±10)% for Rb and (85±10)% for Cs in the empty combined target-ion source unit, which was used as a reference one. For the target-ion source unit with UC target material inside the measured value of the ionization efficiency of Rb was (52±20)%.     

Electronic structure of LiMnO : X-ray emission and photoelectron spectra and band structure calculations

The electronic structure of LiMnO₂ and Li₂MnO₃ was studied by means of X-ray photoelectron and soft X-ray emission spectroscopy. For LiMnO₂, LSDA and LSDA+U calculations were carried out. The LSDA+U calculations are in rather good agreement with the measured valence-band structure as well as with the magnetic and electrical properties of LiMnO₂. It is shown that the band gap in LiMnO₂ is determined by the charge-transfer effect. Received 15 March 1999 and Received in final form 14 July 1999     

Chromophores of the green fluorescent protein studied in the gas phase

Absorption of gas-phase biomolecules has been studied at the heavy-ion storage ring ELISA. Here we discuss the absorption characteristics of the chromophores of the Green Fluorescent Protein (GFP). The gas-phase absorption maximum of the deprotonated chromophore (anion form) is at 479 nm. This is almost identical to one of the two absorption maxima of the protein, being at 477 nm, which is ascribed to a deprotonated chromophore in the protein. The protonated chromophore (cation form) has a maximum at 406 nm in the gas phase. We compare the gas-phase results with absorption profiles of GFP and chromophores in liquids, and argue that the absorption characteristics of GFP are mainly ascribed to intrinsic chemical properties of the chromophore. Evidently, the special β-can structure of GFP provides shielding of the chromophore from the surroundings without significantly changing the electronic structure of the chromophore through interactions with amino acid side chains. Received 28 December 2001 Published online 13 September 2002     

Pure and Sb-doped SnO 2 nanoparticles studied by photoelectron spectroscopy

We describe photoemission results from pure and Sb-doped SnO₂ nanoparticles deposited on gold substrates. Photoelectron spectra with synchrotron radiation were recorded for Sn 3d, Sb 3d and O 1s core levels and valence bands in the 500-1200 eV energy range. For pure SnO₂ nanoparticles the surface is terminated by an oxygen rich layer with no obvious surface environment for Sn. When doped n-type with 9.1% or 16.7% Sb, dopant atoms are concentrated near the surface of the nanoparticles. The valence state of the dopant atoms is predominantly Sb^V. Plasmon satellite features are also observed in core level photoemission spectra and their intensity relative to the main peak increases with increasing photon energy. Received 30 November 2000     

Structure of metal nanowires in nanoporous alumina membranes studied by EXAFS and X-ray diffraction

The structure of nanowires of different metals grown within nanoporous alumina membranes has been studied by EXAFS, WAXS and high energy X-ray diffraction. Nanowires of gold, silver, copper and iron adopt the lattice structure and bond distances of the bulk metals. Cobalt nanowires on the other hand were composed of a mixture of hcp phase, stable at room temperature, and fcc phase, which in bulk cobalt is normally stable only at high temperatures, in a ratio depending on the pore size. The nanowires are non-continuous but are made of nanocrystallites whose shape and size was found to depend strongly on the metal. All the metals except gold showed the presence of a preferred orientation which was slight in the case of Ag and Cu but much stronger in the case of iron and cobalt nanowires. Received 30 November 2000     

Beta decay of the MT=-1 nucleus 58Zn studied by selective laser ionization

Beta decay of ⁵⁸Zn has been studied for the first time. A new laser ion-source concept has been used to produce mass-separated sources for beta and gamma spectroscopy. The half-life of ⁵⁸Zn was determined to be 86(18) ms. Comparisons are made with previous data from charge-exchange reactions. Our Gamow–Teller strength to the 1⁺ state at 1051 keV excitation in ⁵⁸Cu agrees well with the value extracted from a recent (³He, t) study. Extensive shell-model calculations are presented. Received: 5 June 1998 / Revised version: 24 July 1998     

Investigation of S-H bonds in biologically important compounds by sulfur K-edge X-ray absorption spectroscopy

X-ray Absorption Near Edge Structure (XANES) spectroscopy, often provides a direct correlation between observed resonances in the spectrum and molecular bonds in the sample. This can be used as a fingerprint for the presence of a given molecular environment of the absorber atom in a sample. As the white line is found at similar energy positions for S-C and S-H bonds, this approach is impossible when both types of bond are present simultaneously, as often in biological systems. To develop a criterium for the presence of S-H bonds in such samples, reduced glutathione, reduced coenzyme A, cysteine and their corresponding oxidized forms were investigated using sulfur K-edge XANES, revealing a unique feature at 2 475.8 eV in the respective difference spectra. To correlate this structure to S-H bonds, H₂S and H₂S₂ were measured, whose difference spectrum also shows a structure at this energy position, whereas it is not present throughout a variety of C-S-C/C-S-S-C environments. Theoretical investigations suggest its correlation to a Rydberg transition occurring in the case of a S-H bond. Using this criterium, the presence of S-H bonds is in the purple sulfur bacterium Allochromatium vinosum during oxidation of intracellular accumulated sulfur, is proved, as expected from biological considerations. Received 1st February 2002 / Received in final form 10 June 2002 Published online 13 September 2002     

Damage processes in Fe 3 O 4 magnetic insulator irradiated by swift heavy ions. Experimental results and modelisation

The behavior of the magnetic properties of magnetite Fe₃O₄ irradiated by swift heavy ions is investigated by magnetization measurements. Although there is no induced structural phase transformation, both coercive field and saturation magnetization are sensitive to ion irradiation and exhibit different behaviors depending on the ion fluence range. In the low fluence regime, the coercive field increases, which is evidence for a strong pinning of magnetic domain boundaries by the induced defects. The magnetization shows a decrease in the saturation value and tends to reorient perpendicularly to the ion track axis. At high fluence, the initial magnetic properties of the sample are nearly restored. The changes in the magnitude and the direction of magnetization are interpreted by magnetostrictive effects related to the stress induced by irradiation. A phenomenological model is applied to reproduce the fluence evolution of the saturation magnetization, assuming relaxation of the stress induced around the core of defects of the tracks by overlapping effects at high fluence. The results are compared to those obtained in the case of yttrium iron garnet Y₃Fe₅O₁₂. Received 18 April 2001 and Received in final form 24 July 2001     

Densification and nanocrystallisation of sol-gel ZrO 2 thin films studied by surface plasmon polariton-assisted Raman spectroscopy

Very thin ZrO ₂ films (few nanometers) have been prepared by sol-gel process. These films were deposited onto a stack of a thin silver layer evaporated on a glass substrate for Surface Plasmons Resonance (SPR) experiments. The first aim of this work is to study the high densification of the sol-gel films followed by the refractive index and thickness accurate measurements at each step of the annealing procedure, using an optical set-up based on SPR. Secondly, SPR excitation coupled with micro-Raman experiment has also been performed to determine the thin films structure depending on layer thickness. Finally, Conventional Transmission Electron Microscopy (CTEM) and High Resolution (HRTEM) studies have been conducted to check and complete Raman spectroscopy results. A discussion compares the optical results and the Transmission Electron Microscopy observations and shows that ultra thin layers structure is strongly depends on films thickness. Received 14 May 2001 and Received in final form 2 January 2002     

Two-color three-photon resonant excitation spectrum of strontium in the autoionization region

We report new studies of the odd parity autoionizing Rydberg series of strontium attached to the 4d( ² D _{3/2, 5/2}) ionic limits possessing J = 1-3 based on the two-color three photon resonant excitation technique in conjunction with an atomic beam apparatus. Using the 4d ²³ P ₀ intermediate levels, we have been able to record the autoionizing Rydberg series of J = 1 whereas, from the 4d ²³ P ₂ intermediate level the series of Rydberg levels possessing J = 1, 2 and 3 have been observed. The level assignments and the line shapes simulations of the autoionizing resonances have been made using the multichannel quantum defect theory. Received 21 November 2001 / Received in final form 2 May 2002 Published online 19 July 2002     

Measurement of isotope shifts,fine and hyperfine structure splittings of the lithium D lines

The lithium D lines were studied using a diode laser that was frequency modulated by an electro-optic modulator, to excite an atomic beam. The transmission of part of the laser beam through an etalon was monitored to correct for the nonlinearity of the laser scan. The results for the ^6,7Li 2 S _1/2 and 2 P _1/2 hyperfine splittings agree very well with the best existing data while those for the D1 isotope shift and ^6,7Li fine structure splittings disagree significantly from data obtained by a previous laser atomic beam experiment. Our result for the D1 isotope shift is very close to the latest value computed using Hylleraas variational theory. Received 8 April 2002 / Received in final form 26 June 2002 Published online 21 January 2003 RID="a" ID="a"e-mail: wvw@yorku.ca     

Study of oxygen-C 60 compound formation by NEXAFS and RIXS

The interaction of oxygen with C₆₀ molecules was studied on a C₆₀ film which had been exposed simultaneously to oxygen and UV-light for 190 hours, producing an approximately C₆₀O₁ stoichiometry in the bulk of the sample. C K-edge and O K-edge NEXAFS (using total fluorescence yield detection) and resonant X-ray inelastic scattering (RIXS) spectra from the sample film were measured and the C K-edge data were compared to the spectra from pristine C₆₀ as reference. The C K-edge absorption and emission spectral profiles of the oxygen-doped sample are similar to those of the C₆₀ reference, suggesting that cage breaking of C₆₀ under these conditions, if any, is negligible. However, the redistribution of intensities in the spectra indicates changes in the occupancies of different molecular orbitals, possibly due to changes in electron density upon reaction. Similarities of the O K-edge soft X-ray emission (SXES) spectra to several small oxygen-containing molecules is being discussed in terms of bonding models. Received 4 December 2000