Photoelectron-beam generation up to short threshold

We report on the behaviour of electron beams extracted from metal cathodes illuminated by a powerful XeCl-laser beam. The target materials used were Zn, Y and Ge. During the electron extraction a plasma was created on the target surface. At high laser energies and high accelerating voltages the plasma introduces an impedance into the cathode-anode region. Owing to the plasma formation short-circuits occur in the accelerating region thus limiting the maximum output current. Under our experimental conditions, a laser energy of 27 mJ and 3 mm of distance cathode-anode, a maximum output corrent density of about 4.9 A/cm² was obtained with the Zn cathode. The yttrium target may provide a higher current density but so far it could not be demonstrated due to plasma formation which short-circuits the anode-cathode region. The experimental results obtained with Ge targets are interesting for obtaining short electron pulses.     

Wavelength-dispersive spectral analysis and quantification of monazites by electron-probe microanalysis

The problem of determining the chemical composition of monazite grains through electron probe microanalysis is studied, by using a scanning electron microscope with a wavelength dispersive spectrometer. A careful qualitative analysis is performed with the purpose of determining all the elements present in the samples, the lines to be used in the quantifications trying to minimize interferences, the angular positions and the acquisition times for the measurement of peak and background intensities and the crystals to be used. Particular emphasis is devoted to the analysis of Th, U and Pb, which are used to determine the age of the rock by means of the U-Th-Pb method, commonly used in geochronology. Quantitative determinations of the chemical composition of monazite grains are performed, optimizing the experimental conditions on the basis of the qualitative analysis. The determinations were made under two different criteria of quantification of oxygen, and the dissimilar results obtained are discussed.     

Investigation of the interface in silica-encapsulated liposomes by combining solid state NMR and first principles calculations

In the context of nanomedicine, liposils (liposomes and silica) have a strong potential for drug storage and release schemes: such materials combine the intrinsic properties of liposome (encapsulation) and silica (increased rigidity, protective coating, pH degradability). In this work, an original approach combining solid state NMR, molecular dynamics, first principles geometry optimization, and NMR parameters calculation allows the building of a precise representation of the organic/inorganic interface in liposils. {(1)H-(29)Si}(1)H and {(1)H-(31)P}(1)H Double Cross-Polarization (CP) MAS NMR experiments were implemented in order to explore the proton chemical environments around the silica and the phospholipids, respectively. Using VASP (Vienna Ab Initio Simulation Package), DFT calculations including molecular dynamics, and geometry optimization lead to the determination of energetically favorable configurations of a DPPC (dipalmitoylphosphatidylcholine) headgroup adsorbed onto a hydroxylated silica surface that corresponds to a realistic model of an amorphous silica slab. These data combined with first principles NMR parameters calculations by GIPAW (Gauge Included Projected Augmented Wave) show that the phosphate moieties are not directly interacting with silanols. The stabilization of the interface is achieved through the presence of water molecules located in-between the head groups of the phospholipids and the silica surface forming an interfacial H-bonded water layer. A detailed study of the (31)P chemical shift anisotropy (CSA) parameters allows us to interpret the local dynamics of DPPC in liposils. Finally, the VASP/solid state NMR/GIPAW combined approach can be extended to a large variety of organic-inorganic hybrid interfaces.     

Chemical composition and possible in vitro antigermination activity of three Hypericum essential oils

The essential oils of Hypericum perforatum, H. perfoliatum and H. hircinum, growing in Southern Italy, were analyzed by GC and GC/MS. In the three oils, 111 compounds in all were identified: 53 for the oil of H. hircinum (93.7% of the total oil), 55 for H. perforatum (96.5% of the total oil) and 63 for H. perfoliatum (98.7% of the total oil). The major fraction of the essential oils of H. perforatum and H. hircinum was represented by sesquiterpene hydrocarbons, while the monoterpene alpha-pinene, and the phenol thymol were the most abundant compounds in the essential oil of H. perfoliatum. The oils were evaluated for their potential in vitro phytotoxic activity against germination and early radicle elongation of Raphanus sativus and Lepidium sativum. The germination of this latter was significantly inhibited by the essential oil of H. hircinum, at the highest doses tested, whereas radicle elongation of garden cress was significantly inhibited by the essential oils of H. perfoliatum and H. hircinum. The radicle elongation of radish was inhibited by the essential oil of H. hircinum to a major extent and by H. perforatum and perfoliatum in a minor measure.     

Development and characterisation of an XeCl excimer laser-generated soft-X-ray plasma source and its applications

Summary A large-aperture, long-pulse XeCl excimer laser has been used to generate a soft-X-ray plasma source. Several laser optical configurations have been employed to optimise X-ray emission, including positive-branch unstable resonators and injection by seeding the gain region of the laser with a small commercial excimer laser, resulting in power densities in the range 10¹²–10¹⁴ W cm⁻². The characteristics of the plasma source for each different laser configuration and for different target materials are investigated. The most suitable source conditions (spectral energy distribution, time duration, etc.) for specific applications are discussed. Supported by the Royal Society, UK. Supported by INFM, Italy.     

Flame atomic absorption analysis of gold in jewelry samples

Summary A new procedure is proposed for the determination of gold by flame atomic absorption in jewelry samples. The method is based on the acid digestion of 10 mg of sample in a microwave oven and on the on-line dilution, using a three-way valve. The dynamic range of the technique is extended to 100 mg/l gold solutions and using a weighted bracketing method for the treatment of the results, accuracy errors lower than 2% can be found in the analysis of real samples as compared with those obtained by fire assay. The method is very fast and implies only little damage to the jewelry samples.     

Study of a positive corona discharge in argon at different pressures

The corona discharge in argon at atmospheric pressure has been studied by means of a 2D model. The reduced characteristic derived from the experimental data has been described by linear regressions for the different pressures and the two studied inter-electrode distances thus confirming the validity of Townsend's approximation also in case of point to plane configuration and argon as process gas. The model validated this hypothesis which has been attributed to the minor influence of space charge in the ionization zone. Its effect is, on the other hand, more significant in the drift zone where the electric field is greatly enhanced, leading, for higher currents, to the formation of a spark gap. Electron and ion distributions allow the influence of structural (electrode configurations and distance) and operative (pressure and discharge current) parameters to be evaluated including the current loss due to diffusion through different confining boundaries.     

Graphs with prescribed Levi form trace

Abstract We prove existence and uniqueness of a viscosity solution of the Dirichlet problem related to the prescribed Levi mean curvature equation, under suitable assumptions on the boundary data and on the Levi curvature of the domain. We also show that such a solution is Lipschitz continuous by proving that it is the uniform limit of a sequence of classical solutions of elliptic problems and by building Lipschitz continuous barriers. Keywords: Levi mean curvature, Quasilinear degenerate elliptic PDE’s, Viscosity solutions, Comparison principle, Global Lipschitz estimates     

Averaging, Conley index continuation and recurrent dynamics in almost-periodic parabolic equations

We study a non-autonomous parabolic equation with almost-periodic, rapidly oscillating principal part and nonlinear interactions. We associate to the equation a skew-product semiflow and, for a special class of nonlinearities, we define the Conley index of isolated compact invariant sets. As the frequency of the oscillations tends to infinity, we prove that every isolated compact invariant set of the averaged autonomous equation can be continued to an isolated compact invariant set of the skew-product semiflow associated to the non-autonomous equation. Finally, we illustrate some examples in which the Conley index can be explicitly computed and can be exploited to detect the existence of recurrent dynamics in the equation.     

Energy loss measurements and electronic properties of thianthren

Summary The dielectric and electronic properties of thianthren (C₆H₄(S)₂H₄C₆) are determined by means of optical reflectivity, absorption measurements and reflection electron energy loss spectroscopy. The experimental results are interpreted on the basis of a Complete Neglect of Differential Overlap (CNDO) calculation used in three different parametrization schemes. Emphasis is laid on the discussion of the problems which generally affect the analysis of electron energy loss spectra, and a procedure to obtain the complex dielectric function from electron energy loss measurements performed in the reflection mode is suggested. Work partially supported by the MURST through the GNSM.