Showing papers on "Ion published in 1990"

Photoelectron spectroscopy of hydrated electron cluster anions, (H2O)−n=2–69

Abstract: Etude de la variation de l'energie de photodetachement vertical en fonction de n, en vue de distinguer les agregats a electron de surface ou a electron interne solvate

On the use of atmospheric pressure plasmas as electromagnetic reflectors and absorbers

Abstract: Tenuous man-made plasmas in the Earth's atmosphere from sea level to 100 km are discussed. An ionization source generates a tenuous plasma with an electron number density n/sub e/ that is high near the source and diminishes with distance from the source. After the source shuts off, n/sub e/ decreases as a function of time as electrons recombine with positive ions or attach to negative ions. The electromagnetic properties that are essential to an understanding of these plasmas, which can be modeled as cold collisional plasmas, is discussed. Gas and plasma characteristics, such as momentum-transfer collision rate, plasma lifetime, recombination kinetics, and the effect of noble gases, are presented. Typical collision rates and plasma lifetimes at atmospheric pressure are quantified. Applications for a plasma with a gradient are discussed. They include a high-altitude plasma that can reflect or absorb from HF to VHF and a broadband atmospheric pressure absorber. The generation and use of plasma, including electron impact ionization with a high-energy electron-beam source and UV photoionization of an alkali vapor or an organic vapor such as tetrakis (dimethylamino)ethylene (TMAE), is described. The power required to sustain a plasma is quantified, and properties such as maximum absorption and bandwidth are discussed. Tradeoffs among maximum absorption, absorption bandwidth, duty ratio, and applied power permit optimization of absorption primarily at VHF. >

Electrospray ionization mass spectrometer with new features

Abstract: An electrospray ion source is designed for ready and simple plugging into commercial mass analyzers for mass spectrometric analysis of organic molecules. The electrospray is carried out is the ambient air and the ions and other charged species enter the mass analyuzer through a long metal capillary tube and three stages of differential pumping. The use of the long tube allows (a) convenient injection of the ions into the mass analyzer (b) optimization of the spray by direct visualization in the air (c) efficient and controlled heat transfer to the droplets and (d) efficient pumping of the region between the capillary exit and the skinner. Desolvation of the solvated ions is carried out using a combination of controlled heat transfer to the charged droplets during the transit through the tube and collisional activation in a region of reduced pressure. Desolvation with this system does not involve use of a strong countercurrent flow of heated gas. The system also may be used to obtain the collisional activated fragmentation spectra of molecule ions. The use of a metal capillary tube avoids complications from charging that arise from the use of dielectric capilliary tubes.

Softening of the H2+ molecular bond in intense laser fields.

Abstract: We report a new kind of multiphoton dissociation in intense laser fields. ${\mathrm{H}}_{2}^{+}$ molecular ions were formed in above-threshold ionization of ${\mathrm{H}}_{2}$ gas by intense 532-nm, 100-ps laser pulses. Observations suggest that multiphoton couplings soften the molecular bond, resulting in dissociation at laser intensities over 50 TW/${\mathrm{cm}}^{2}$. The ion-dissociation spectra have multiple peaks caused by multiphoton transitions during dissociation of ${\mathrm{H}}_{2}^{+}$.

Efficiencies of photoinduced electron-transfer reactions: role of the Marcus inverted region in return electron transfer within geminate radical-ion pairs

Abstract: In photoinduced electron-transfer processes the primary step is conversion of the electronic energy of an excited state into chemical energy retained in the form of a redox (geminate radical-ion) pair (A + D A'-/D'+). In polar solvents, separation of the geminate pair occurs with formation of free radical ions in solution. The quantum yields of product formation, from reactions of either the free ions, or of the geminate pair, are often low, however, due to the return electron transfer reaction (A'-/D'+ - A + D), an energy-wasting step that competes with the useful reactions of the ion pair. The present study was undertaken to investigate the parameters controlling the rates of these return electron transfer reactions. Quantum yields of free radical ion formation were measured for ion pairs formed upon electron-transfer quenching of the first excited singlet states of cyanoanthracenes by simple aromatic hydrocarbon donors in aceonitrile at room temperature. The free-ion yields are determined by the competition between the rates of separation and return electron transfer. By assuming a constant rate of separation, the rates of the return electron transfer process are obtained. These highly exothermic return electron transfer reactions (-AG,, = 2-3 eV) were found to be strongly dependent on the reaction exothermicity. The electron-transfer rates showed a marked decrease (ea. 2 orders of magnitude in this AG, range) with increasing exothermicity. This effect represents a clear example of the Marcus "inverted region". Semiquantum mechanical electron-transfer theories were used to analyze the data quantitatively. The electron-transfer rates were found also to depend upon the degree of charge delocalization within the ions of the pair, which is attributed to variations in the solvent reorganization energy and electronic coupling matrix element. Accordingly, mostly on the basis of redox potentials, one can vary the quantum yield of free-ion formation from a few percent to values approaching unity. Use of a strong donor with a strong acceptor to induce reactions based on electron transfer is likely to be inefficient because of the fast return electron transfer in the resulting low-energy ion pair. A system with the smallest possible driving force for the initial charge-separation reaction results in a high-energy, and therefore long-lived ion pair, which allows the desired processes to occur more efficiently. The use of an indirect path based on secondary electron transfer, a concept called "cosensitization", results in efficient radical-ion formation even when the direct path results in a very low quantum yield.

Apatite coating on ceramics, metals and polymers utilizing a biological process

Abstract: A novel method of apatite coating is presented. The main characteristics of the method are that the apatite layer obtained consists of bone-like apatite and can be coated on various substrates including ceramics, metals and organic polymers. Coating is carried out in a simulated body fluid, the ion concentrations, temperature and pH of which are adjusted to almost equal to those of human blood plasma, using a plate of CaO, SiO2-based glass as a source of nucleating agent of apatite on the surfaces of substrates. The apatite layer obtained, formed in a similar environment to that in the body, is thus expected to show higher bone-bonding ability than that formed by conventional methods, and this method is applicable to various materials having different mechanical properties.

Cross Sections and Related Data for Electron Collisions with Hydrogen Molecules and Molecular Ions

Abstract: Data are compiled and evaluated for collision processes of excitation, dissociation, ionization, attachment, and recombination of hydrogen molecules and molecular ions (H+2, H+3) by electron impact as well as for properties of their collision products.

Grain‐Boundary Chemistry of Barium Titanate and Strontium Titanate: I, High‐Temperature Equilibrium Space Charge

Abstract: Direct observations using scanning transmission electron microscopy (STEM) of the grain-boundary chemistry of selectively doped SrTiO3 and BaTiO3 show the predominant solute segregation in both systems to be that of acceptors (negative effective charge). Appreciable donor segregation is not observed even at lattice concentrations as high as 10 mol%. Donor and acceptor codoped materials show segregation of the acceptor only. The results are consistent with a grain-boundary space-charge distribution consisting of a positive boundary and negative space charge. All grain boundaries examined also show an excess of Ti relative to the A-site cations, suggesting that the positive boundary charge is at least partially accommodated by an excess of Ti ions. The sign and magnitude of the electrostatic potential appear to be remarkably insensitive to changes in lattice defect structure with solute doping. Grain-boundary chemistry appears dominated by space-charge segregation, in contrast with the predictions of recent atomistic simulations which neglect the space-charge potential.

Long-lived nonmetallic silver clusters in aqueous solution: preparation and photolysis

Abstract: Radiolytic reduction of Ag{sup +} ions in aqueous solution and reduction by sodium borohydride in the presence of polyphosphate leads to long-lived nonmetallic silver clusters besides particles of metallic silver (having a narrow plasmon absorption band at 380 nm) and quasi-metallic silver (having a blue-shifted and broadened plasmon absorption band). The best conditions for the preparation of cluster solutions are described, and a mechanism of cluster formation is discussed. The Ag{sub 4}{sup 2+} cluster formed in the first stages of Ag{sup +} ion reduction lives for many hours even in the presence of air. It is oxidized by H{sub 2}O{sub 2} and K{sub 2}S{sub 2}O{sub 8}. Its great stability is explained on the basis of the oscillatory behavior of the standard redox potential of silver microelectrodes at very small agglomeration numbers as described previously. Larger clusters with sharp absorption bands at 300, 330, and 345 nm (absorption coefficients of more than 10{sup 4} M{sup {minus}1} cm{sup {minus}1}) are formed in the later stages of Ag{sup +} reduction. They are very sensitive toward air and thermally less stable than Ag{sub 4}{sup 2+}. The absorptions of these clusters are tentatively attributed to CTTS transitions.

Production of hollow atoms by the excitation of highly charged ions in interaction with a metallic surface.

Abstract: The capture of many electrons by ${\mathrm{Ar}}^{17+}$ ions, at low velocity, near a metallic surface, has been studied. Multiexcited bound states with many electrons in the outermost shells (hollow atoms) have been observed. The surrounding of an ionic excited core by many outermost electrons greatly decreases the lifetimes of the states. This characteristic decrease explains the main striking features of the relaxation of the ions.

Detection of extracellular calcium gradients with a calcium-specific vibrating electrode

Abstract: We have developed a vibrating calcium-specific electrode to measure minute extracellular calcium gradients and thus infer the patterns of calcium currents that cross the surface of various cells and tissues. Low-resistance calcium electrodes (routinely approximately 500 M omega) are vibrated by means of orthogonally stacked piezoelectrical pushers, driven by a damped square wave at an optimal frequency of 0.5 Hz. Phase-sensitive detection of the electrode signal is performed with either analogue or digital electronics. The resulting data are superimposed on a video image of the preparation that is being measured. Depending on the background calcium concentration, this new device can readily and reliably measure steady extracellular differences of calcium concentration which are as small as 0.01% with spatial and temporal resolutions of a few microns and a few seconds, respectively. The digital version can attain a noise level of less than 1 microV. In exploratory studies, we have used this device to map and measure the patterns of calcium currents that cross the surface of growing fucoid eggs and tobacco pollen, moving amebae and Dictyostelium slugs, recently fertilized ascidian eggs, as well as nurse cells of Sarcophaga follicles. This approach should be easily extendable to other specific ion currents.

Self-consistent model of a direct-current glow discharge: Treatment of fast electrons.

Abstract: Mathematical models of dc glow discharges sustained by electrons emitted by the cathode and accelerated into the cathode fall must take into account the highly nonequilibrium nature of these fast electrons. However, the electric field profile through the discharge is determined mainly by the distribution of ions and slow electrons. In this paper we explore three methods to account for fast, nonequilibrium electrons: the single-beam method, the multibeam method, and particle (Monte Carlo) simulations. Ions and cold electrons are treated using equations of change assuming collisionally dominated motion (i.e., drift and diffusion), and the self-consistent electric field is determined by solving these equations simultaneously with Poisson's equation. Creation rates for ions and slow electrons are obtained from the fast-electron models. Simulation results indicate that, although the single-beam model is qualitatively correct, it is hampered by its sensitivity to assumptions in the numerical approach, and its tendency to predict negative voltage-current characteristics at low pressures and high voltages, which are not evident in results from the higher-order multibeam model. Although an improvement over the single-beam model, comparison with experimental optical-emission measurements reveals that the multibeam model predicts excitation profiles that extend too far into the discharge. Accurate comparisons are possible with particle simulations, which incorporate angular scattering of fast electrons.

Ion and Solvent Transport in Ion‐Exchange Membranes I . A Macrohomogeneous Mathematical Model

Abstract: A mathematical model for the simulation of ion and solvent transport within an ion‐exchange membrane is developed and analyzed. A Nernst‐Planck equation is employed for the description of ion transport by diffusion, migration, and convection. Solvent transport driven by pressure and electric‐potential gradients is described by an equation of motion. The physicochemical parameters used in this analysis are experimentally easy to obtain. The set of equations used in the simulation are presented in a dimensionless form so that important dimensionless parameters can be evaluated. In particular, an electrokinetic Peclet number is identified; large values of this dimensionless quantity, for example, would indicate that convection is the dominant mode of mass transport.

Bioactivity of CaO·SiO2-based glasses:in vitro evaluation

Abstract: In order to fundamentally study compositional dependence of bioactivity of glasses, both the surface structural changes of P2O5-free CaO · SiO2 glass due to exposure to a simulated body fluid and the effects of adding a third component, such as Na2O, MgO, B2O3, Fe2O3, P2O5 and CaF2, were investigated. An acellular aqueous solution which had almost equal ion concentrations to those of the human blood plasma was used as the simulated body fluid. The surface structure changes were examined by electronprobe X-ray microanalysis, thin-film X-ray diffraction (XRD) and Fourier transform infrared (FTIR) reflection spectroscopy. It was found that even P2O5-free CaO · SiO2 glass forms an apatite layer on its surface in the simulated body fluid, and that the rate of formation of the surface apatite layer is increased with the addition of Na2O and P2O5 while it decreased with the addition of MgO, B2O3, CaF2 and Fe2O3. This indicates that even P2O5-free CaO · SiO2 glass can bond to living bone, forming the surface apatite layer in the body and that its bioactivity is increased with the addition of Na2O and P2O5 while it is decreased with MgO, B2O3, CaF2 and Fe2O3. It is speculated that a glass of the composition CaO · SiO2 100, Fe2O3 3 in weight ratio does not bond to living bone.

Chemical reactions and physical property modifications induced by keV ion beams in polymers

Abstract: Factors influencing the chemical modification of polymers under ion bombardment are discussed. The chemical state and the electronic structure of ion-bombarded polymers are studied by XPS and REELS techniques. Ion bombardment of polymers is shown to induce well defined reactions involving the gradual modification of the original chemical structure within well defined fluence ranges, characteristic for each type of chemical reaction. The rate and the relative yield of the bombardment induced chemical reactions are shown to depend on the primary chemical structure of the bombarded polymers as well as on the total ion deposited energy, fluence and energy deposition mechanism. Such a dependence is discussed for some ion induced reactions in polyvynilpyridine (PVPY), polyethersulphone (PES). It is shown that polymer samples bombarded at very high fluence (10 15 −10 16 ions/cm 2 ) still keep “memory” of their primary stoichiometry and chemical structure showing spectroscopic characteristics similar to those of hydrogenated amorphous carbons. A number of cases are discussed in which the physical and electronic structure of the “carbonaceous” materials obtained depend upon the energy deposition mechanisms.

Crystal and electronic structures of (BEDT-TTF)2(MHg(SCN)4)(M=K and NH4).

Abstract: The crystal structures of (BEDT–TTF)2[MHg(SCN)4] (M=K at 298 and 104 K and M=NH4 at 298 K) were determined by X-ray analyses. A donor layer and an anion sheet stack alternately along the b-axis. The packing pattern of the donor is close to α-phase, while an anion sheet constructs a thick two-dimensional polymer plane parallel to (010). The linkage of (Remark: Graphics omitted.) (or NH4) (Remark: Graphics omitted.) in an anion sheet is very unique. The band structure calculation indicates both a closed and an open Fermi surfaces, resembling that of κ-(BEDT–TTF)2[Cu(NCS)2].

Ion bombardment in rf plasmas

Abstract: Ion bombardment energy and angle distributions have been measured in an argon plasma. The measured ion angle distribution at 10 mTorr shows that 30% of the ions have incident angles greater than 10° from the surface normal. However, ions with large incident angles have much lower energies than those incident perpendicular to the surface. At 500 mTorr a very large fraction of the ions have large incident angles, and the average energies of these ions are relatively independent of incident angle. Monte Carlo simulations of the sheath kinetics predict the trends shown in the experimental data for ion energy and angle distributions. Fine structure in the ion energy distribution was observed below 50 mTorr and is shown to be caused by charge‐exchange collisions in the sheath. The average ion energy in a symmetric parallel plate system is linearly related to the voltage applied across the electrodes for measured plasma pressures up to 500 mTorr.