Showing papers on "Ion published in 1991"
TL;DR: A physical mechanism and proposed theory permits a quantitative explanation for the main characteristics of experimentally observed effects of weak magnetic fields with biological systems.
Abstract: A physical mechanism is suggested for a resonant interaction of weak magnetic fields with biological systems. An ion inside a Ca(2+)-binding protein is approximated by a charged oscillator. A shift in the probability of ion transition between different vibrational energy levels occurs when a combination of static and alternating magnetic fields is applied. This in turn affects the interaction of the ion with the surrounding ligands. The effect reaches its maximum when the frequency of the alternating field is equal to the cyclotron frequency of this ion or to some of its harmonics or sub-harmonics. A resonant response of the biosystem to the magnetic field results. The proposed theory permits a quantitative explanation for the main characteristics of experimentally observed effects.
553 citations
TL;DR: In this article, a 1.053-μm, 1-psec Nd:glass laser was used to study the ionization of noble gases in the tunneling regime.
Abstract: Laser ionization of noble gases was studied with a 1.053-μm, 1-psec Nd:glass laser. A systematic scan of intensities from mid-1013 W/cm2 to mid-1016 W/cm2 was performed, resulting in the production of charge states as high as Xe12+. Ionization occurs exclusively in the tunneling regime. We compare experimental ion production rates with those predicted by several different theories. Agreement between experimental ion-production curves and theoretical predictions is good for two theoretical models: (1) an elaboration of the Keldysh tunneling model, developed by Ammosov et al. [ Sov. Phys. JETP64, 1191 ( 1986)] and (2) a much more primitive model, based on Coulomb-barrier suppression, in which tunneling and other quantum-mechanical effects are ignored completely. The success of the more primitive model suggests that a new term, barrier-suppression ionization, rather than tunneling or multiphoton ionization, may be the most appropriate at this wavelength and in this range of intensities.
416 citations
TL;DR: The electron momentum spectroscopy (EMS) as mentioned in this paper is a technique that uses electron-impact ionization reactions near the Bethe ridge at total energies of the order of 1000 eV or higher.
Abstract: Unique information about the motion and correlation of valence electrons in atoms, molecules and their ions is obtained from electron-impact ionization reactions near the Bethe ridge at total energies of the order of 1000 eV or higher. This is electron momentum spectroscopy. The history, theory and practice of the field are discussed and its value is shown by numerous examples.
416 citations
TL;DR: In this paper, the acidities of 35 phenoxyl radical cations were estimated from pK HA, E ox (A − ), and E ox(HA) values over a range of greater than 40 kcal/mol.
Abstract: Oxidation potentials for 35 phenoxide ions and 3 naphthoxide ions have been combined with their pK HA values to estimated homolytic bond dissociation energies (BDEs) for the O-H bonds in phenols. Comparison with literature values shows that there is remarkably good agreement ΔBDE values determined by different methods. A good correlation of E ox (A − ) values for p-GC 6 H 4 O − phenoxide ions with σ + constants was observed over a range of greater than 40 kcal/mol. The acidities of 35 phenoxyl radical cations have been estimated from pK HA , E ox (A − ), and E ox (HA) values. A good correlation of E ox (HA) vs pK HA •+ was observed from m-GC 6 H 4 OH •+ radical cations, but the points for para donors were found to deviate from the line
401 citations
TL;DR: In this article, a simple on-column concentration technique in high-performance capillary electrophoresis (HPCE) is reported, where a short plug of water before sample introduction is injected to provide a high electric field strength from the beginning of the injection.
394 citations
TL;DR: In this paper, a new mass selective method was developed for the determination of the optical spectra of molecular ions and for the production of state selected ions, which can be used for mass selection.
Abstract: The developement of a new mass selective method is reported for the determination of the optical spectra of molecular ions and for the production of state selected ions.(AIP)
372 citations
TL;DR: A theoretical analysis of the neutralization dynamics above the surface, prior to impact, based on the classical over-the-barrier model, finds satisfactory agreement with recent data for Auger yields and describes the transient formation of hollow'' atoms.
Abstract: The neutralization of highly charged ions during interaction with metallic surfaces is accompanied by the ejection of a large number of secondary electrons Recent experiments demonstrate two main contributions to this electron ejection process: one from the region below the surface and the second from the above-surface portion of the trajectory We present a theoretical analysis of the neutralization dynamics above the surface, prior to impact, based on the classical over-the-barrier model The theory incorporates resonant multielectron capture of conduction electrons, resonant loss into unoccupied states of the conduction band, and intra-atomic Auger deexcitation The effective barrier potential includes quantum corrections to the classical image potential The effect of below-barrier (``tunneling'') transfer is investigated The solution of a coupled system of rate equations allows the approximate determination of the n-shell populations, the projectile charge state, and the total number of Auger electrons The calculation describes the transient formation of ``hollow'' atoms We find satisfactory agreement with recent data for K Auger yields by Meyer et al [Phys Rev Lett 67, 723 (1991)]
358 citations
TL;DR: In this article, the Hirshman and Sigmar moment approach was used to derive explicit expressions for the poloidal and toroidal rotation speeds of the primary ion and impurity species.
Abstract: Explicit expressions for the neoclassical poloidal and toroidal rotation speeds of primary ion and impurity species are derived via the Hirshman and Sigmar moment approach. The rotation speeds of the primary ion can be significantly different from those of impurities in various interesting cases. The rapid increase of impurity poloidal rotation in the edge region of H‐mode discharges in tokamaks can be explained by a rapid steepening of the primary ion pressure gradient. Depending on ion collisionality, the poloidal rotation speed of the primary ions at the edge can be quite small and the flow direction may be opposite to that of the impurities. This may cast considerable doubts on current L to H bifurcation models based on primary ion poloidal rotation only. Also, the difference between the toroidal rotation velocities of primary ions and impurities is not negligible in various cases. In Ohmic plasmas, the parallel electric field induces a large impurity toroidal rotation close to the magnetic axis, which seems to agree with experimental observations. In the ion banana and plateau regime, there can be non‐negligible disparities between primary ion and impurity toroidal rotation velocities due to the ion density and temperature gradients. Detailed analytic expressions for the primary ion and impurity rotation speeds are presented, and the methodology for generalization to the case of several impurity species is also presented for future numerical evaluation.
329 citations
TL;DR: In this paper, thin films of titanium oxysulfides which could be used as positive electrode materials in microbatteries were analysed by XPS and the results showed the existence of a new type of titanium.
328 citations
TL;DR: In this article, a review of recent research on the theory and computer simulations of electromagnetic ion/ion instabilities and their consequences in space plasmas is presented. But the authors do not consider the effects of these instabilities on the dynamics of the space physics regimes which display enhanced fluctuations due to them.
Abstract: This paper reviews recent research on the theory and computer simulations of electromagnetic ion/ion instabilities and their consequences in space plasmas. Ion/ion instabilities are growing modes in a collisionless plasma driven unstable by the relative streaming velocity v(0) of two distinct ion components such that v(0) is parallel or antiparallel to the uniform background magnetic field. The space physics regimes which display enhanced fluctuations due to these instabilities include the solar wind, the terrestrial foreshock, the plasma sheet boundary layer, and distant cometary environments.
328 citations
TL;DR: Improved estimates of the nonrelativistic stationary nucleus correlation energies of the ground-state atomic ions with three to ten electrons and up to 20 are derived by combining experimental data and improved calculations with relativistic complete-valence-space Hartree-Fock energies.
Abstract: Improved estimates of the nonrelativistic stationary nucleus correlation energies of the ground-state atomic ions with three to ten electrons and Z up to 20 are derived by combining experimental data and improved ab initio calculations. Unlike previous work in this area, we focus on the correlation contribution to individual ionization energies, computed by comparing experimental data with relativistic complete-valence-space Hartree-Fock energies.
TL;DR: In this article, the results of experimental investigations of the photodoping of amorphous chalcogenides by metals, and in particular by silver, are reviewed, and the results in the literature are analysed, giving answers to such fundamental questions as the following: where is the actinic light which is efficient for photodissolution absorbed? Which is the diffusion-limiting step? What is the state of the diffusing species (neutral atoms or ions)?
Abstract: The present paper reviews the results of experimental investigations of the photodoping of amorphous chalcogenides by metals, and in particular by silver. The kinetics of the photodissolution of the metal are described as also is the influence on the photodissolution rate of such factors as the wavelength and intensity of light, the composition and temperature of the semiconductor, and an external electric field. A separate section describes lateral diffusion. The properties of photodoped semiconductors (electrical, optical, photoelectric, etc.) are presented. The results in the literature are analysed, giving answers to such fundamental questions as the following. Where is the actinic light which is efficient for the photodissolution absorbed? Which is the diffusion-limiting step? What is the state of the diffusing species (neutral atoms or ions)? The dissolution of silver induced by electron or ion beams and the photodissolution of group II metals are mentioned. The experimental results are fol...
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01 Nov 1991
TL;DR: In this article, a slow particle-induced electron emission from solid surfaces under ion bombardment is described. But it is not shown that the electron emission is due to the pre-equilibrium of charge distributions.
Abstract: Kinetic electron emission from solid surfaces under ion bombardment.- Kinetic electron emission from ion penetration of thin foils in relation to the pre-equilibrium of charge distributions.- Slow particle-induced electron emission from solid surfaces.
TL;DR: In this paper, the production of ammonia from dinitrogen and water vapor at mild conditions of temperature and pressure on Fe{sup 3+}-doped titanium dioxide powders under UV radiation has been studied in a continuous photoreactor working in the gas-solid regime.
Abstract: The production of ammonia from dinitrogen and water vapor at mild conditions of temperature and pressure on Fe{sup 3+}-doped titanium dioxide powders under UV radiation has been studied in a continuous photoreactor working in the gas-solid regime. A net activity decline was observed after a few hours of irradiation; this decline did not depend on the reactor temperature, the powder composition, or the amounts of produced ammonia. The highest activity is found when no excess iron is segregated at the surface; overall turnover numbers for dinitrogen reduction as high as six electrons per iron atom can then be reached before powder deactivation, showing the catalytic character of the participation of Fe in this process. An IR investigation of the active and spent specimens revealed that the irradiation determines the almost complete disappearance of the OH groups from the powder surface. Furthermore, an ESR study of all the powders showed that bulk Fe{sup 3+} ions are better electron traps than Ti{sup 4+} ions so that, when a UV photon generates a hole-electron pair, the electron can be stabilized on the iron ions.
TL;DR: In this article, the velocity distributions of polypeptide molecule ions (molecular mass 1000 − 15600 u) produced by matrix assisted ultraviolet laser desorption were measured using a modified time-of-flight mass spectrometer.
TL;DR: In this article, a discussion of experimental and theoretical evidence of the two-centre electron emission (TCEE) is presented. But the use of heavy ions or antiprotons as projectiles allows to unravel these effects by monitoring the two centre potential.
Abstract: When an atom is ionized by ion impact, the electron is ejected into a final continuum state of a two-centre potential due to the Coulomb fields of the projectile and ionized atom. The related effects on the electron yield or energy and angular distributions are referred to as two-centre electron emission (TCEE). The present report is devoted to a discussion of experimental and theoretical evidence of this TCEE. The use of heavy ions or antiprotons as projectiles allows to unravel these effects by monitoring the two centre potential. On the theoretical side, the continuum distorted wave-eikonal initial state theory (CDW-EIS) accounts for the TCEE thus allowing a detailed interpretation of the experimental findings.
TL;DR: In this paper, the crystal structure of Cd5,Zn2-metallothionein from rat liver has been refined at 2.0Aresolution of a R-value of 0.176 for all observed data.
TL;DR: In this paper, the Coulomb forces over a charged particle by other charged particles are derived for the sums over Coulomb force exerted on a charge by other charge particles, the central cell system being repeated to infinity by periodic boundary conditions.
Abstract: Formulae are derived for the sums over Coulomb forces exerted on a charged particle by other charged particles, the central cell system being repeated to infinity by periodic boundary conditions. Such sums are needed in molecular dynamics simulations involving either ions or neutral molecules represented as bound conglomerates of charges, and in astrophysical simulations of gravitating masses. The derived sums are rapidly convergent, being expressed in terms of Bessel functions Kr(z), which decrease exponentially with z. The force expressions are integrated analytically to give the potential function, which may be used in Monte Carlo simulations. The geometries considered are: (i) systems confined between two parallel walls, and (ii) unconfined three-dimensional systems.
TL;DR: In this paper, several techniques for extending the mass/charge range of the quadrupole ion trap were presented, including the use of smaller electrodes, operation at lower radio frequencies, and resonance ion ejection using a voltage of appropriate frequency, applied symmetrically across the end-cap electrodes during the mass scan.
TL;DR: The reactivity of the third-row transition-metal ions, both bare and with simple ligands, may be explained in terms of electronic structure considerations, such as promotion energy, exchange energy, and the relative and absolute sizes of the valence s and d orbitals as discussed by the authors.
Abstract: Methane is spontaneously dehydrogenated in the gas phase by many metal ions of the 5d transition series. In most cases, the MCH{sub 2}{sup +} produced undergoes further reactions, leading eventually to products such as WC{sub 8}H{sub 16}{sup +}. The reactivity of the third-row transition-metal ions, both bare and with simple ligands, may be explained in terms of electronic structure considerations. Promotion energy, exchange energy, and the relative and absolute sizes of the valence s and d orbitals all appear to be important.
TL;DR: The first direct experimental evidence for clathrate structures of (H2O)nH+ (n=20,21) is reported based on a technique allowing the number of nonhydrogen-bonded surface hydrogens to be counted.
Abstract: First direct experimental evidence for clathrate structures of (H2O)nH+ (n=20,21) is reported based on a technique allowing the number of nonhydrogen‐bonded surface hydrogens to be counted. Neutral clusters (H2O)n⋅((CH3)3N)m, prepared in a pulsed nozzle supersonic expansion, are ionized by multiphoton ionization and investigated with a reflectron time‐of‐flight mass spectrometry technique. The magic numbers (n,m) in the ion intensity distributions of (H2O)n⋅((CH3)3N)m⋅H+ studied under various experimental conditions are well correlated to the stable hydrogen‐bonding structures. For the mixed cluster ion (H2O)20⋅((CH3)3N)m⋅H+, the intensity distribution displays an abrupt intensity drop after the magic number at (20,11), while for (H2O)21⋅((CH3)3N)m⋅H+ the magic number appears at (21,10). The findings provide experimental evidence for a stable clathrate structure of (H2O)20H+, with the proton residing on the surface, while for (H2O)21H+, the H3O+ ion is encaged inside the clathrate structure of (H2O)20; th...
TL;DR: In this paper, the authors used a modified LAMMA 1000 reflecting time-of-flight (TOF) mass spectrometer to study the fragmentation of protein and peptide ions generated by matrix-assisted laser desorption.
Abstract: Fragmentation of protein and peptide ions generated by matrix-assisted laser desorption has been investigated using a modified LAMMA 1000 reflecting time-of-flight (TOF) mass spectrometer. Whereas fragmentation of covalent bonds prior to ion acceleration (i.e., within several ns after the laser pulse) in general is not observed using the matrix technique, extensive fragmentation on a longer time scale can be studied in our instrument. The high mass resolution (M/ΔM≈1200–1800 for insulin and peptides) permits the investigation of even small mass losses from parent molecular ions (occuring in the first section of the field-free drift region) by measuring flight time differences of daughter ions acquired during passage through a two-stage reflection. The kind and extent of this metastable decay has been found to depend strongly on the substance under investigation. Typical fragmentations are loss of ammonia and parts of the amino acid side-chains. The large abundances of peaks due to such metastable fragmentation, observed for most of the peptides and proteins investigated, may, at least in part, explain peak broadening (and, hence, poor mass resolution) typical in matrix-assisted laser-desorption TOF mass spectra.
TL;DR: In this paper, a newly designed flow tube source, in which the nascent ions undergo on the order of 105 collisions with the helium flow gas, is used to produce thermalized N+4 ions.
TL;DR: A resonant enhancement of the O 2{ital p} valence-band photoemission intensity is observed for both stoichiometric and reduced TiO{sub 2} surfaces as the photon energy is swept through the Ti 3 {ital p}{r arrow}3{ital d} optical transition energy, which indicates a strong hybridization between the oxygen and titanium ions.
Abstract: We have performed a resonant photoemission study on both stoichiometric and reduced TiO{sub 2} (110) surfaces using synchrotron radiation. A resonant enhancement of the O 2{ital p} valence-band photoemission intensity is observed for both stoichiometric and reduced TiO{sub 2} surfaces as the photon energy is swept through the Ti 3{ital p}{r arrow}3{ital d} optical transition energy, which indicates a strong hybridization between the oxygen and titanium ions. Compared to the stoichiometric surface, the amplitude of the resonance is smaller for the reduced surface. In addition, the reduced surface has higher emission intensity from the O 2{ital p} bonding orbitals and lower emission intensity from the nonbonding orbitals. For both stoichiometric and reduced surfaces, the intensity of emission from the O 2{ital p} bonding orbitals resonates more than that from the O 2{ital p} nonbonding orbitals, and the resonance profile for the nonbonding orbitals maximizes at a higher photon energy than that for the bonding orbitals. The resonant enhancement of the O 2{ital p} bonding orbitals is consistent with theoretical calculations, which predict that the O 2{ital p}--Ti 3{ital d} hybridization involves primarily those O 2{ital p} orbitals. The energy dependence of the resonant enhancement for the O 2{ital p} nonbondingmore » orbitals suggests that it results predominantly from resonant processes involving Ti 3{ital p}{r arrow}4{ital s} excitation at the surface.« less
TL;DR: The ionization potentials of several large carbon clusters Cn (n ≥ 48), including the enhanced abundance (‘‘magic number’’) clusters C50, C60, and C70, have been determined by Fourier transform ion cyclotron resonance (FTICR) mass spectrometric charge transfer bracketing experiments.
Abstract: The ionization potentials (IPs) of several large carbon clusters Cn (n≥48), including the enhanced abundance (‘‘magic number’’) clusters C50, C60, and C70, have been determined by Fourier transform ion cyclotron resonance (FTICR) mass spectrometric charge transfer bracketing experiments. The IPs of C50, C60, and C70 were bracketed by the same two charge transfer compounds, leading to a common value of 7.61±0.11 eV. The IPs of even numbered clusters adjacent to these magic number clusters were found to be lower by as much as 0.5 eV and all clusters between C50 and C200 were determined to have IPs greater than 6.20 eV. The reaction rates of C+60 and C+70 with metallocenes were anomalously slow in comparison to the other large carbon cluster ions. IP and reactivity results suggest that C50, C60, and C70 may indeed have different or more stable structures than neighboring clusters, which supports the hypothesis of closed‐shell, spherical species. The implications of these results for the mechanism of C+n formation by direct laser vaporization are also discussed.