Search

"Los defoliadores del pino se consideran una plaga forestal importante, que es subestimada por los daños que ocasionan la presencia de otras, como los descortezadores y plantas parásitas. Para el control de plagas, existen plantas bioactivas que pertenecen a la familia Solanaceae, dentro de las cuales se encuentra Datura metel L. por lo que el presente trabajo determinó la mortalidad en larvas de cuarto instar de N. autumnalis S. y establecer el nivel de toxicidad de los extractos metanólicos de hoja, tallo y raíz de D. metel, sobre poblaciones de Pinus leiophylla Schl. y Cham. Para ello, se evaluó la toxicidad de las hojas, tallo y raíz de los extractos metanólicos de D. metel (Solanales: Solanaceae), sobre larvas de cuarto instar de defoliador del pino N. autumnalis a diferentes concentraciones (1, 10, 50 y 100 mg/L) y se tomaron lecturas de mortalidad en intervalos de 2 h (0 a 24 h), a partir de la exposición de las larvas a los extractos metanólicos. El mayor efecto tóxico fue del extracto de raíz (86,0 % de mortalidad, CL50 de 3,68 mg/L), seguido por las hojas (83,3 %, CL50 4,98 mg/L), y tallo (80,0 %, CL50 de 12,79 mg/L), lo que es de gran utilidad para el manejo de la plaga forestal. Los extractos crudos de hojas, tallos y raíces de D. metel contienen alcaloides, los cuales interrumpen el ciclo de vida de las larvas de cuarto instar de N. autumnalis, por lo que pueden utilizarse como biocidas."

Arctostaphylos pungens HBK is a dominant species with increasing abundance and distribution in chaparral ecosystems as a result of range management and, possibly, changes in climate. The value of this species for carbon (C) sequestration is unknown, and the standard 50% C out of total tree biomass is used as an approximate value. In this study, we aim to determine the C concentration of the primary components of A. pungens. The total C expressed as a percentage of biomass was determined with a Solids TOC Analyzer. We found the C concentration to vary among components. Leaves exhibited the highest C concentration (51.70%). Roots (46.11%), stems (47.30%), fruits (47.89%) and twigs (48.40%) had similar C concentration. These results provided superior estimates of C concentration, and reliable information concerning the potential use of A. pungens in climate-change mitigation programs.

Durum wheat (Triticum turgidum L. Desf.) is a species well adapted to the Mediterranean environments where salt stress due to seawater intrusion is an increasing problem. The purpose of this study was to deep insight into the relationships among physiological, productive and qualitative aspects under salinity, being these aspects still poorly investigated in durum wheat. In 2004–2005 crop season 10 durum wheat genotypes of different origin and breeding time were grown in a naturally-lit polycarbonate greenhouse under three irrigation water salinity levels (0.9, 6.0 and 12.0 dS m−1). A complete randomized block design with three replications was adopted. The osmotic damage was evaluated by estimating relative water content (RWC), leaf water potential (Φw) and osmotic potential at full turgor (Φπ100). The toxic damage to the plants was evaluated by measuring Na+ accumulation and Na+/K+ ratio in the leaves. Differences in yield performance were evaluated by assessing the main yield components and some qualitative traits, carotenoid pigment and protein content and Sedimentation test in sodium dodecyl sulphate. A significant effect of genotype, salt stress and of their interaction on all the characters was observed. Durum wheat genotypes generally showed a moderate tolerance to salt stress. The genotype performance was dependent on stress level and RWC maintenance. Osmotic adjustment and low sodium accumulation were found to play a key role in salt tolerance. An improvement in the grain quality characters on increasing salinity level, consistently with a yield decrease, was observed.

SUMMARY The use of fuels derived from vegetable oils represents a potential opportunity to meet EU Commission guidelines promoting renewable energy utilization. This is particularly true for Mediterranean countries where farmers are well familiarized with oil crops. Sunflower is one of the most adapted crops to dry conditions typical of Mediterranean environments. The oleic acid content in sunflower oil seems to be a suitable character for biofuel production with regard to fuel oxidative stability. For the same reason, the high oleic varieties are considered particularly promising. In this study the effect of climatic conditions and agronomic practices on oleic acid content of sunflower seeds is investigated with respect to the Southern Italy regions. Effects of high temperature, water deficit, irrigation water quality, sowing time and genotype on oil yield and quality are evaluated. Finally, the potential of sunflower cultivation for biofuel production in the light of the recent European agricultural policy is discussed.

We present here a set of Auger and photoemission results obtained on three different rare earths: lanthanum, samarium and thulium taken at the 3d absorption threshold. The de-excitation process appears to be different depending of the 4f occupation number. This can be understood considering the localized character of the excited 4f electron. Constant initial state results are also shown.

We present here an EXAFS (extended x-ray absorption fine structure) experiment performed at the K edge of the sulfur atom of OCS isolated in an argon matrix. The aim was to elucidate the structure of the trapping site of the molecule. We show that the argon atoms are located at a mean distance of 3.78 A from the sulfur atom, with a root mean square variation of the S–Ar distances of about 0.08 A. Our results indicate that the host atoms occupy a statistical set of positions around the molecule, rather than well-defined sites.

Resonant silicon Auger KLL and 2s and 2p photoemission spectra of a porous silicon sample have been studied when excited by photons in the energy domain of the 1s edge in pure silicon and silicon oxide. Characteristic features of a resonant process could be detected. In particular, the constant initial state spectrum of the 2p state of silica behaves similarly to that encountered in systems which present a well-defined atomic level. This is due to the existence of a well-localized molecular orbital built in the SiO 4 unit. The use of high-energy photons, which generate high-energy electrons, allows these photoemission experiments to be quite bulk sensitive.

A reliable estimate of the density of states having in-plane and out-of-plane symmetry in the O K-edge and the -edge polarized spectra is important for checking the validity of several theoretical models which predict an active role for these states in the mechanism of superconductivity. In the present contribution, we report on our fluorescence-yield O K-edge and -edge polarized spectra obtained for a well characterized and oriented overdoped Tl(2212) thin film as well as our simultaneous fluorescence- and total-yield measurements on an optimally doped Tl(2212) thin film. Our results show that a significant number of out-of-plane and holes also exist in thallium cuprates, more so in the overdoped case, and that use of the bulk-sensitive fluorescence technique is better for obtaining a reliable estimate of the density and symmetry of these holes. The spectra are compared with those reported earlier for other cuprate systems and are interpreted in terms of the available band-structure calculations. No direct correlation between superconductivity and the presence or absence of out-of-plane covalent and doping hole densities appears plausible, so theories based on the existence of a large density of such holes appear to be inconsistent with the experiment.

Besides the now well-known self-absorption effect, several phenomena related to the multiplet structure of the intermediate state may occur which render x-ray fluorescence different from the true absorption in 3d transition metals at the L edge and at the M{sub 4,5} edges of rare earths. Special selection rules of the radiative de-excitation process play an important role there. We have measured the absorption coefficient of thin films of lanthanum, samarium, and thulium deposited on an aluminum foil, at room temperature, through the simultaneous detection of the transmission, total electron yield, and 150-eV bandwidth fluorescence yield. The latter result shows differences as compared to the other two, and exhibits polarization effects depending upon the angle between incident and outgoing photons. The resonant x-ray fluorescence spectrum is calculated using an atomic model, and then integrated over the emitted energy, to predict the fluorescence yield spectrum. Very good agreement is obtained between the theory and experiment. {copyright} {ital 1997} {ital The American Physical Society}

Beside the now well known self-absorption effect, several phenomena related to the multiplet structure of the intermediate state the occur which will render the X-ray fluorescence different from the true absorption result on 3d transition metals at the L edge and on the M4,5 edges of rare earths. Special selection rules of the radiative de-excitation process may play an important role there. We have measured the absorption coefficient of a thin film of thulium deposited onto an aluminum foil, at room temperature, through the simultaneous detection of the transmission, 160 eV bandwidth fluorescence yield and total electron yield. While transmission and electron yield results are very similar, as expected for a very thin sample, the fluorescence yield spectrum is definitely different. Theoretically, the resonant X-ray fluorescence spectrum was calculated using an atomic model, and then integrated over the emitted energy, to predict the fluorescence yield spectrum. A very good agreement is obtained between the theory and experiment.

Auger spectra of pure silicon. in-situ oxidized silicon and porous silicon have been recorded at different photon energies across the K absorption edge of silicon and silica. The results show an intense resonant effect with, between the normal Auger lines of pure silicon and silica, a new contribution whose intensity and energy position depends on the incident photon energy. We discuss these results.

The Al site structure of serum transferrin and lactoferrin is investigated using X-ray absorption near edge structure (XANES) spectroscopy. Al K-edge spectra in the mono- and dialuminum forms of the proteins have been recorded for the first time. Our results show that the aluminium ion is hexa-coordinated in an octahedral-like symmetry and that the monoaluminum form, where only the C-terminal binding site is saturated, has an increased structural distortion around the metal site.

Tcoptimization of as-synthesized Tl 2212 thin films can be obtained through post-annealing treatments at low temperatures (180 and 220°C). In this paper, the optimization process was controlled by ac magnetic susceptibility measurements for theTc's and by polarized X-ray absorption spectroscopy measurements for the doping hole densities. The latter technique allows a direct insight into the hole density only in the (a,b) plane of the Tl 2212 structure. All the studied thin films are overdoped with respect to the optimum of 0.12 doping holes per copper defined previously.Tcincreases up to 6 K were observed after post-annealing treatments under an argon atmosphere and were correlated with significant reduction of the doping hole density. In contrast to what has been observed on sintered samples, a post-annealing treatment at 220°C resulted in an insufficient doping hole density, below the optimum, probably resulting from the small sample thickness.

The system Si/GaAs(110) was studied by surface extended x‐ray absorption fine structure at the silicon K edge, starting from a 0.5 monolayer (ML) coverage up to 3 ML. At very low coverages, the first adsorption site of the silicon has been determined to be above the center of the triangle formed by two arsenic and one gallium surface atoms. The growth mode of silicon on this surface obeys a modified version of the Stranski–Krastanov model: the formation of three‐dimensional clusters appears at 2.5 ML and involves all the deposited silicon atoms.

G4XANES, a software package calculating X-ray Absorption Near Edge Structure (XANES) spectra in the frame of the one-electron full multiple scattering formalism, is presented. The G4XANES package performs one-electron K,L-XANES “ab initio” simulations on different kinds of compounds (small molecules, crystals, free clusters, metal sites in biological compounds) by using a minimal set of input data. Polarized spectra as a function of the size of the scattering cluster are calculated, allowing to extract information on both the electronic and atomic structure around the selected absorbing atom. Recently reported spin-dependent XANES spectra have been also successfully treated. A description of the package, as well as its applications for such important classes of materials as high temperature superconductors, magnetic materials and metalloproteins, are presented.

Thin films of a layered thallium cuprate have been studied by XAS at Cu L 3 -edge in normal incidence in order to estimate the doping hole densities n h in the [CuO 2 ] ∞ planes of the Tl(2212) structure. When going from as-synthesized samples to post-annealed ones under Ar, a decrease of the doping hole densities has been observed which corresponds to an increase of T c' s . In order to evaluate the surface effects on the results, the absorption has been recorded simultaneously in total electron yield (TY) and fluorescence yield (FY) modes. After full correction of the self-absorption of the soft X-ray fluorescence photon, the estimated doping hole densities appeared to be in good agreement whatever the detection method.

The polarized E\ensuremath{\perp}c oxygen K-edge x-ray-absorption near-edge structure (XANES) spectrum of ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ (Bi 2:2:1:2) has been calculated by a multiple-scattering approach and compared with experimental data. The symmetry of the conduction bands over a range of 15 eV above the Fermi level has been studied by a combined analysis of five different polarized x-ray-absorption spectra at the following edges: O K edge, Cu K edge, and Cu ${\mathit{L}}_{3}$ edge. Information on the partial density of states of the conduction bands, ${\mathit{scrD}}_{\mathit{X}(\mathit{l},}$${\mathit{m}}_{\mathit{l}}$), projected on the atomic site X (Cu and O) with selected orbital angular momentum l, and its projection ${\mathit{m}}_{\mathit{l}}$ along the c axis (${\mathit{scrD}}_{\mathrm{Cu}(2,0)}$, ${\mathit{scrD}}_{\mathrm{C}\mathrm{u}(2,\ifmmode\pm\else\textpm\fi{}1)}$, and ${\mathit{scrD}}_{\mathrm{C}\mathrm{u}(2,\ifmmode\pm\else\textpm\fi{}2)}$ for the Cu site; ${\mathit{scrD}}_{\mathrm{O}(1,0)}$ and ${\mathit{scrD}}_{\mathrm{O}(1,\ifmmode\pm\else\textpm\fi{}1)}$ for the oxygen site) have been obtained. The symmetry of the itinerant states induced by doping at the Fermi level is deduced to be a mixture of 3d holes with ${\mathit{m}}_{\mathit{l}}$=\ifmmode\pm\else\textpm\fi{}2 and ${\mathit{m}}_{\mathit{l}}$=0 orbital angular momenta and holes on planar oxygen in the molecular-orbital combination of local ${\mathit{b}}_{1}$ and ${\mathit{a}}_{1}$ symmetry [the ligand holes L(${\mathit{b}}_{1}$) and L(${\mathit{a}}_{1}$)].

The linearly polarized Cu {ital L}{sub 3}-edge x-ray-absorption near-edge structure (XANES) of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} has been measured and the spectra are interpreted by the full multiple-scattering approach in real space. The polarized spectra over a range of 20 eV can be predicted in terms of the one-electron dipole ({Delta}{ital l}=+1) transition Cu 2{ital p}{r arrow}{epsilon}{ital d}, probing the unoccupied {ital d}-like ({ital l}=2) density of states projected on the Cu site with orbital angular momentum {ital m}{sub {ital l}}=0, 1 in the {bold E}( ){ital z} spectra, and the {ital m}{sub {ital l}}=2, 1, and 0 in the {bold E}{perpendicular}{ital c} spectra. The oscillator strength for the dipole allowed transitions ({Delta}{ital l}={minus}1) Cu 2{ital p}{r arrow}{var epsilon}{ital s} is shown to be a factor of 100 weaker than the 2{ital p}{r arrow}3{ital d} transitions. The Coulomb interaction in the final state between the Cu 2{ital p} core hole and the excited Cu 3{ital d} electron is found to be 5.5 eV forming a bound state below the continuum threshold, the well-known Cu {ital L}{sub 3} white line. On the contrary, the core hole induces a nearly rigid redshift about 1 eV of the high-energy conduction bands.

The linearly polarized Cu K-edge x-ray-absorption near-edge structure (XANES) of ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ (Bi 2:2:1:2) has been measured, and the spectra are interpreted by the multiple-scattering approach in real space. The polarized spectra can be predicted in terms of a one-electron dipole (\ensuremath{\Delta}l=+1) transition Cu 1s\ensuremath{\rightarrow}\ensuremath{\epsilon}p, probing the unoccupied p-like (l=1) density of states projected on the Cu site with orbital angular momentum ${\mathit{m}}_{\mathit{l}}$=0 in the E\ensuremath{\parallel}z spectra, and the ${\mathit{m}}_{\mathit{l}}$=1 in the E\ensuremath{\perp}c spectra. Therefore we show that the electronic structure of the high-energy conduction bands, beyond the Cu 3d band, are well described in terms of the one-electron approximation. Final-state effects induced by the core hole have been studied and calculated. A satellite 7 eV above the main K-XANES peak in both polarizations is shown to be a multielectron shake-up excitation, and it is enhanced going into the insulating phase. Structural information on the copper site in ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ and in ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{YCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ has been extracted from XANES spectra, providing evidence for the decrease of the Cu-O distance in the metallic phase.

The symmetry of the conduction bands over a range of 15 eV above the Fermi level in La2CuO4 has been studied by the joint analysis of the polarized X-ray absorption spectra at several edges: O K-edge, Cu K-edge, and Cu L3-edge. The partial density of states of the conduction bands DX (l, ml) projected on the atomic site X (Cu and O) with selected orbital angular momentum l, and its projection ml along the c-axis have been obtained. The oxygen K-edge X-ray absorption near edge structure (XANES) spectrum has been calculated by multiple scattering approach. A good agreement between the one-electron calculations in the fully relaxed final state potential and the experimental spectra for the final states in the high energy conduction bands has been found.

The polarized Cu K-edge X-ray Absorption Near Edge Structure (XANES) of La2CuO4 has been interpreted by the multiple scattering approach. The size of the cluster of neighbouring atoms having good agreement with the XANES experimental data is determined by 45 atoms surrounding the absorbing Cu. The polarized spectra can be predicted in term of a one-electron dipole (Δl=+1) transition Cu 1s→ϵp, probing the unoccupied p-like (l=1) density of states projected on a Cu site with orbital angular momentum ml=0 in the E∥z spectra, and the ml=1 for the E⊤c spectra. Thus we show that the electronic structure of the high energy conduction bands, beyond the Cu 3d band, of La2CuO4 are well described in terms of the one-electron approximation. It is shown that XANES spectra are consistent with the contraction of the Cu-apex oxygen distance with doping. Final state effects induced by the core hole have been indentified: (i) the core transitions take place in the fully relaxed potential, (ii) the satellite at 7 eV above the main K-XANES peak in both polarizations is assigned to a multielectron shake up excitation. Finally the shoulder on the rising absorption edge, present in many Cu compounds and usually assigned to a shake down multi-electron excitation, is shown to be due to a one-electron transition to a state delocalized over a large cluster.

The reflectance of highly oriented Nd2−xCexCuO4−y single crystals has been measured from 100 to 20000 cm−1 for x = 0 and 0.15 and for different values of y. Sample structures and critical temperatures have been determined by different techniques. The energies of six phonon modes in the a−b plane have been determined in Nd2CuO4. The dependence of the optical conductivity \ gs(ω) on x and y has been studied. A mid-infrared band has been determined in the x = 0.15 samples.

The Al site structure of serum transferrin and lactoferrin is investigated using X-ray absorption near edge structure (XANES) spectroscopy. Al K-edge spectra in the mono- and dialuminum forms of the proteins have been recorded for the first time. Our results show that the aluminium ion is hexa-coordinated in an octahedral-like symmetry and that the monoaluminum form, where only the C-terminal binding site is saturated, has an increased structural distortion around the metal site.

To understand the role of out-of-plane copper orbitals, polarization dependent measurements of Cu ${\mathit{L}}_{3}$-absorption edge on well characterized Tl(2212) thin films have been performed. The density of unoccupied states having 3${\mathit{d}}_{{\mathit{z}}^{2}\mathrm{\ensuremath{-}}{\mathit{r}}^{2}}$ character is found to depend on the amount of doping suggesting that out-of-plane copper orbitals essentially play a role of hole reservoir. The energy shift in the positions of white lines of E\ensuremath{\parallel}(a,b) and E\ensuremath{\parallel}c spectra is also found to be doping dependent. The probable reason for the observed shift is discussed. \textcopyright{} 1996 The American Physical Society.

The Ag/MgO(100) interface has been studied by EXAFS. At room temperature, in agreement with previous investigations, silver at low coverage is shown to form a two-dimensional layer at the MgO(100) surface. EXAFS data demonstrate that the lattice parameter of the silver overlayer is expanded so as to fit the MgO parameter to give rise to an interface having ${(100)}_{\mathrm{AG}}$\ensuremath{\parallel}${(100)}_{\mathrm{MGO}}$ and ${[100]}_{\mathrm{AG}}$\ensuremath{\parallel}${[100]}_{\mathrm{MGO}}$ orientation. Interface Ag atoms are atop the oxygen atoms of the MgO(100) surface at a distance of 2.53\ifmmode\pm\else\textpm\fi{}0.05 \AA{}.

The orbital angular momentum ml of the Cu 3d holes in high Tc superconductors has been measured by Cu L3 x-ray absorption spectroscopy. The symmetry of the itinerant states $$3d^9 \underline L$$ (1) induced by doping at the Fermi level is deduced to be a mixture of 3d holes with ml=±2 and ml=0 orbital angular momentum and oxygen O 2px,y holes in the molecular orbital combination of local bl and a1 symmetry, $$\underline L \left( {b_1 } \right)$$ (1) and $$\underline L \left( {a_1 } \right)$$ (1) . The coupling of the itinerant charges with lattice dynamics and doping induced structural changes, point toward a polaron associated with the 3d3z2-r2 electronic states.

The goal of these experiments is to make use at the same time of the sensitivity of the M IV–V XAS to the valence of rare earth atoms and of the linear X-ray dichroism on magnetic systems in order to understand the magnetism of small clusters. First experimental results on large species are reported here.

The application of fluorescence yield (FY) in the soft energy domain, and the problem of self-absorption corrections in the presence of polarization dependent white lines are discussed here. A comparison of X-ray FY and total electron yield (EY) X-ray absorption spectra at the Bi2Sr2CaCu2O8 CuL3 edge is presented.

The change of the orbital angular momentum M l of the Cu 3d holes going from the insulating to the metallic phase has been studied in several families of high Tc superconductors. The symmetry of the 3d9 L states in the metallic phase has been studied by quantitative analysis of the variation of polarized Cu L3 x-ray absorption spectra. At a doping level nh∼15% we have found 10% of Zhang-Rice singlets 3d x 2 -y 2 L (b 1 ) and 5% of 3d 3z 2 -r 2 L (a 1 ) states. Therefore the percentage of the 3d3z2−r2 L states on the total number of the 3d9 L states is about 30% i.e. much larger than the probability of single hole states 3d3z2−r2 in the insulating phase. The EXAFS and XANES studies of the Cu site structure and dynamics in Bi2Sr2Ca1−xYxCu2O8 system point toward the coupling of the charge carriers with distortions of the Cu sites driven by the M l=0 character of the Cu 3d holes that can be called a 3dz2−r2 polaron.

By using x-ray absorption spectroscopy different configurations of the Cu site structure in the CuO2 plane of the Bi 2:2:1:2 high Tc superconductor are found. The different Cu sites are characterized by short 2.3 Å and long 2.45 Å Cu-O(apical) distances. The linear arrays of different Cu sites forming domains with a corrugated-iron-foil shape is proposed to be a key feature of superconducting domains in the CuO2 plane. The wavelength λ of the modulation is close to the superconducting coherence length ξ and kF·λ∼2π. The ordering of the distorted Cu sites is suggested to be evidence for ordering of polarons driven by the pseudo Jahn Teller electron lattice interaction.

The variation of the Cu K-edge x-ray absorption near edge structure (XANES) determined by the structural changes that accompany the loss of oxygen going from the high Tc superconductor YBa2Cu3O7 to the antiferromagnetic insulator YBa2Cu3O6 has been calculated by multiple scattering theory. The elongation of the Cu(2)-O(4) distance (between the Cu(2) in the CuO2 plane and its apical oxygen) induces the shift of the main peak in polarized XANES spectra E//c toward lower energy. The formation of the oxygen vacancies induces changes in the E ⊥ c spectra for the Cu(1) ion in the linear chains. The results are in agreement with the experimental changes of the Cu XANES spectra induced by oxygen vacancies.

Experimental evidence for the increasing probability of the Cu 3d z 2 holes with increasing T c has been found in BiCaSrCuO superconductors by high resolution Cu L 3 XAS. The weight of the Cu 3d z 2 holes increases by about 5% going from the normal phase at 300K to the superconducting phase at 10K. The energy splitting Δ z between the d z 2 and d x 2 −y 2 hole states decreases by increasing T c by doping within each class of high T c superconductors. The range of values of Δ z increases going from one class of high T c superconductors to another class with higher T c . These results give a positive experimental test for pairing mechanisms involving the Cu 3d z 2 holes and in particular for pairing mediated by virtual d-d excitations.