Showing papers on "Ion published in 1997"

Velocity map imaging of ions and electrons using electrostatic lenses: Application in photoelectron and photofragment ion imaging of molecular oxygen

Abstract: The application of electrostatic lenses is demonstrated to give a substantial improvement of the two-dimensional (2D) ion/electron imaging technique. This combination of ion lens optics and 2D detection makes “velocity map imaging” possible, i.e., all particles with the same initial velocity vector are mapped onto the same point on the detector. Whereas the more common application of grid electrodes leads to transmission reduction, severe trajectory deflections and blurring due to the non-point source geometry, these problems are avoided with open lens electrodes. A three-plate assembly with aperture electrodes has been tested and its properties are compared with those of grid electrodes. The photodissociation processes occurring in molecular oxygen following the two-photon 3dπ(3Σ1g −)(v=2, N=2)←X(3Σg −) Rydberg excitation around 225 nm are presented here to show the improvement in spatial resolution in the ion and electron images. Simulated trajectory calculations show good agreement with experiment and ...

Steric Effects in Electrolytes: A Modified Poisson-Boltzmann Equation

Abstract: The adsorption of large ions from solution to a charged surface is investigated theoretically. A generalized Poisson-Boltzmann equation which takes into account the finite size of the ions is presented. We obtain analytical expressions for the electrostatic potential and ion concentrations at the surface, leading to a modified Grahame equation. At high surface charge densities the ionic concentration saturates to its maximum value. Our results are in agreement with recent experiments.

Design and synthesis of metal ion-recognition-induced conjugated polymers: An approach to metal ion sensory materials

Abstract: The synthesis and metal ion responsive properties of two 2,2‘-bipyridyl-phenylene-vinylene-based polymers is reported. These polymers are designed to be partially conjugated in their metal-free state and fully conjugated when exposed to metal ions so that the ion-induced conjugation enhancement can be transduced into a measurable signal. It is found that these polymers exhibit highly ionochromic effects with a wide variety of transition and main group metal ions excluding metal ions of the alkali and alkaline earth groups. For instance, both absorption and fluorescence emission bands of the polymers upon exposure to metal ions can be red-shifted up to 120 nm, depending on the metal ions present and the polymers used.

High-energy ions produced in explosions of superheated atomic clusters

Abstract: Efficient conversion of electromagnetic energy to particle energy is of fundamental importance in many areas of physics A promising avenue for producing matter with unprecedented energy densities is by heating atomic clusters, an intermediate form of matter between molecules and solids1, with high-intensity, ultra-short light pulses2–4 Studies of noble-gas clusters heated with high-intensity (>1016Wcm–2) laser pulses indicate that a highly ionized, very high temperature micro-plasma is produced The explosion of these superheated clusters ejects ions with substantial kinetic energy3–5 Here we report the direct measurement of the ion energy distributions resulting from these explosions We find, in the case of laser-heated xenon clusters, that such explosions produce xenon ions with kinetic energies up to 1 MeV This energy is four orders of magnitude higher than that achieved in the Coulomb explosion of small molecules6, indicating a fundamental difference in the nature of intense laser–matter interactions between molecules and clusters Moreover, it demonstrates that access to an extremely high temperature state of matter is now possible with small-scale lasers

A Predictive Model for Matrix and Analyte Effects in Electrospray Ionization of Singly-Charged Ionic Analytes

Abstract: In electrospray ionization (ESI), droplets with a surface excess charge are created. The rate of production of surface excess charge is a constant and is equal to the rate of ion production. The ions appearing in the mass spectrum are postulated to be those that formed the surface excess charge at the time of droplet formation (or their collision products). An equilibrium model based on competition among the ions in the solution for the limited number of excess charge sites has been developed. This model accurately predicts the response curves of singly-charged ionic analytes as a function of the concentration of electrolyte and other analytes and provides an explanation for the selective effectiveness of ESI. At low concentrations of total analyte (micromolar and less), the response curves are linear, indifferent to the presence of other low concentration analytes, and suppressed by electrolyte concentrations in excess of the minimum required. At higher analyte concentrations, the response becomes independent of analyte concentration but highly affected by the presence of other analytes.

Recoil-ion momentum spectroscopy

Abstract: High-resolution recoil-ion momentum spectroscopy (RIMS) is a novel technique to determine the charge state and the complete final momentum vector of a recoiling target ion emerging from an ionizing collision of an atom with any kind of radiation. It offers a unique combination of superior momentum resolution in all three spatial directions of with a large detection solid angle of . Recently, low-energy electron analysers based on rigorously new concepts and reaching similar specifications were successfully integrated into RIM spectrometers yielding so-called `reaction microscopes'. Exploiting these techniques, a large variety of atomic reactions for ion, electron, photon and antiproton impact have been explored in unprecedented detail and completeness. Among them kinematically complete experiments on electron capture, single and double ionization in ion - atom collisions at projectile energies between 5 keV and 1.4 GeV have been carried out. Double photoionization of He has been investigated at energies close to the threshold up to . At the contributions to double ionization after photoabsorption and Compton scattering were separated kinematically for the first time. These and many other results will be reviewed in this paper. In addition, the experimental technique is described in some detail and emphasis is given to envisaging the rich future potential of the method in various fields of atomic collision physics with atoms, molecules and clusters.

Influence of ion energy and substrate temperature on the optical and electronic properties of tetrahedral amorphous carbon (ta-C) films

Abstract: The properties of amorphous carbon (a-C) deposited using a filtered cathodic vacuum arc as a function of the ion energy and substrate temperature are reported. The sp3 fraction was found to strongly depend on the ion energy, giving a highly sp3 bonded a-C denoted as tetrahedral amorphous carbon (ta-C) at ion energies around 100 eV. The optical band gap was found to follow similar trends to other diamondlike carbon films, varying almost linearly with sp2 fraction. The dependence of the electronic properties are discussed in terms of models of the electronic structure of a-C. The structure of ta-C was also strongly dependent on the deposition temperature, changing sharply to sp2 above a transition temperature, T1, of ≈200 °C. Furthermore, T1 was found to decrease with increasing ion energy. Most film properties, such as compressive stress and plasmon energy, were correlated to the sp3 fraction. However, the optical and electrical properties were found to undergo a more gradual transition with the deposition temperature which we attribute to the medium range order of sp2 sites. We attribute the variation in film properties with the deposition temperature to diffusion of interstitials to the surface above T1 due to thermal activation, leading to the relaxation of density in context of a growth model.

High-resolution ion mobility measurements

Abstract: Gas phase ion mobility measurements can resolve structural isomers for polyatomic ions and provide information about their geometries. A new experimental apparatus for performing high-resolution ion mobility measurements is described. The apparatus consists of a pulsed laser vaporization/desorption source coupled through an ion gate to a 63-cm-long drift tube. The ion gate is a critical component that prevents the diffusion of neutral species from the source into the drift tube. Ions travel along the drift tube under the influence of a uniform electric field. At the end of the drift tube some of the ions exit through a small aperture. They are focused into a quadrupole mass spectrometer, where they are mass analyzed, and then detected by an off-axis collision dynode and by dual microchannel plates. The apparatus is operated with a drift voltage of up to 14 000 V and a helium buffer gas pressure of around 500 Torr. The resolving power for ion mobility measurements is over an order of magnitude higher than has been achieved using conventional injected-ion drift tube techniques. Examples of the application of the new apparatus in resolving isomers of laser desorbed metallofullerenes, in studying silicon clusters generated by laser vaporization, and in following the isomerization of small nanocrystalline (NaCl)nCl− clusters as a function of temperature, are presented.

High Intensity Laser Absorption by Gases of Atomic Clusters

Abstract: We have measured the energy absorption efficiency of high intensity, picosecond laser pulses in low density gases composed of large atomic clusters. We find that, though the average density of the resulting plasmas is low, the energy absorption can be very high $(g95%)$, indicating that substantial laser energy is deposited per particle in the plasma. Ion energy measurements confirm that this efficient energy deposition results in plasmas with very high (multi-keV) ion temperatures.

The Cluster Ion Spectrometry (CIS) Experiment

Abstract: The Cluster Ion Spectrometry (CIS) experiment is a comprehensive ionic plasma spec-trometry package on-board the four Cluster spacecraft capable of obtaining full three-dimensional ion distributions with good time resolution (one spacecraft spin) with mass per charge composition determination. The requirements to cover the scientific objectives cannot be met with a single instrument. The CIS package therefore consists of two different instruments, a Hot Ion Analyser (HIA) and a time-of-flight ion Composition and Distribution Function analyser (CODIF), plus a sophisticated dual-processor-based instrument-control and Data-Processing System (DPS), which permits extensive on-board data-processing. Both analysers use symmetric optics resulting in continuous, uniform, and well-characterised phase space coverage. CODIF measures the distributions of the major ions (H+, He+, He++, and O+) with energies from -0 to 40 keV/e with medium (22.5°) angular resolution and two different sensitivities. HIA does not offer mass resolution but, also having two different sensitivities, increases the dynamic range, and has an angular resolution capability (5.6° × 5.6°) adequate for ion-beam and solar-wind measurements.

Spectroscopy of metal ion complexes: gas-phase models for solvation.

Abstract: Weakly bound metal ion complexes are produced in molecular beams and studied with mass-selected laser photodissociation spectroscopy. The metal ions Mg+ and Ca+ are the focus of these studies because they have a single valence electron and strong atomic resonance lines in convenient wavelength regions. Weakly bound complexes of these ions with rare-gas atoms and small molecules are prepared with laser vaporization in a pulsed nozzle cluster source. The vibrationally and rotationally resolved electronic spectra obtained for these complexes help to determine the complexes' structures and bonding energetics. Observations from these studies have provided many new insights into the fundamental interactions in electrostatic bonding.

Disulfide-Intact and -Reduced Lysozyme in the Gas Phase: Conformations and Pathways of Folding and Unfolding

Abstract: The conformations of gaseous lysozyme ions (+5 through +18) produced by electrospray ionization have been studied in the gas phase using ion mobility mass spectrometry techniques. When solutions containing the disulfide-intact and disulfide-reduced lysozyme are electrosprayed, the gas-phase ions that are produced have distinctly different collision cross sections. Disulfide-intact ions favor two conformer types: a highly folded conformer with a cross section near that calculated for the crystal structure and a partially unfolded conformer that is formed when the ions are injected into the drift tube at high injection voltages. Ions formed from the disulfide-reduced solution have collision cross sections that are much larger than any observed for the disulfide-intact protein, showing that these ions are largely unfolded. Gas-phase proton-transfer reactions in the ion source can be used to favor lower charge states for both solutions. When protons are removed from disulfide-intact lysozyme ions, highly fol...

Photoelectron and photofragment velocity map imaging of state-selected molecular oxygen dissociation/ionization dynamics

Abstract: A substantial improvement in the photofragment imaging technique is illustrated in a study of molecular oxygen photodynamics. In this method, labeled velocity map imaging, electrostatic ion lenses are shown to allow mapping of all particles with the same initial velocity vector onto the same point on a 2D detector, irrespective of their position of creation in the ionization volume. This leads to a dramatic increase in image resolution. Velocity map imaging of photoelectrons from molecular ionization is also demonstrated and applied along with O+ imaging to identify the processes leading to O+ formation when using (2+1) resonantly enhanced multiphoton ionization (REMPI) detection for O2. Oxygen molecules prepared in the (v=2, N=2) level of the 3dπ(3Σ1g−) Rydberg state by two-photon excitation at 11.02 eV are excited by a third photon to an energy near v=24 of ground-state O2+ (equivalent to one-photon excitation at 75 nm). All energetically accessible excited oxygen atoms and an extensive range of vibrati...

Ion-induced effective surface diffusion in ion sputtering

Abstract: Ion bombardment is known to enhance surface diffusion and affect the surface morphology. Here we demonstrate that preferential erosion during ion sputtering can lead to a physical phenomenon reminiscent of surface diffusion, what we call effective surface diffusion (ESD), that does not imply mass transport along the surface and is independent of the temperature. We calculate the ion-induced ESD constant and its dependence on the ion energy, flux and angle of incidence, showing that sputtering can both enhance and suppress surface diffusion. The influence of ion-induced ESD on ripple formation and roughening of ion-sputtered surfaces is discussed and summarized in a morphological phase diagram.

Electron-Ion Recombination Rate Coefficients, Photoionization Cross Sections, and Ionization Fractions for Astrophysically Abundant Elements. II. Oxygen Ions

Abstract: We present a comprehensive and self-consistent set of new atomic data for ionization balance in radi- atively and collisionally ionized astrophysical plasmas. Complex resonant phenomena resulting in rapid energy variation in the cross sections for photoionization and recombination require accurate and large- scale calculations, as reported. Another new development is the consideration of the uni-ed nature of the recombination process in an ab initio manner, via resonances embedded in the electron-ion continua, which have been heretofore considered as separate processes of radiative recombination and dielectronic recombination. A single set of total electron-ion recombination rate coefficients is thereby obtained as a function of electron temperature. The present calculations also meet the hitherto neglected, but theoreti- cally essential, criterion of self-consistency between the rates for the inverse processes of photoionization and recombination, ensured by describing all atomic processes with an identical set of eigenfunction expansion within the close-coupling approximation using the R-matrix method. Photoionization cross sections and total electron-ion recombination rate coefficients for the carbon and nitrogen isonuclear sequences, C IEC VI and N IEN VII, are presented. Ionization fractions in coronal equilibrium are also computed. The present photoionization cross sections have been calculated using more extensive eigen- function expansions than those in the Opacity Project. In addition to the total photoionization and recombination data, state-speci-c cross sections are also obtained for a large number of excited states for non-LTE models. Complete data sets are available electronically. Subject headings: atomic data E atomic processes E plasmas

Soft-Landing of Polyatomic Ions at Fluorinated Self-Assembled Monolayer Surfaces

Abstract: A method of preparing modified surfaces, referred to as soft-landing, is described in which intact polyatomic ions are deposited from the gas phase into a monolayer fluorocarbon surface at room temperature. The ions are trapped in the fluorocarbon matrix for many hours. They are released, intact, upon sputtering at low or high energy or by thermal desorption, and their molecular compositions are confirmed by isotopic labeling and high-resolution mass measurements. The method is demonstrated for various silyl and pyridinium cations. Capture at the surface is favored when the ions bear bulky substituents that facilitate steric trapping in the matrix.

Kinetic Analysis of Non-Thermal Plasmas Used for Pollution Control

Abstract: Non-thermal plasma techniques are being developed for the treatment of many gas-phase pollutants. In these methods electrical energy from electron beams or electrical discharges is directed selectively into the production of electrons, ions and radicals, or into molecular excitations that will result in the efficient destruction of the unwanted species. The processes by which this can be achieved are described. Kinetic analysis of the deposition of energy into contaminated air is illustrated by studies of the decomposition of nitrogen oxides, methylene chloride, carbon tetrachloride and methanol.

Kinetic model of the ring current-atmosphere interactions

Abstract: Our numerical model of the ring current-atmosphere coupling (RAM) is further developed in order to include wave-particle interaction processes. The model calculates the time evolution of the phase space distribution function in the region from 2 RE to 6.5 RE, considering losses due to charge exchange, Coulomb collisions, and plasma wave scattering along ion drift paths. The spatial regions of ion cyclotron wave instability are determined by calculating the convective growth rates for electromagnetic ion cyclotron (EMIC) waves, integrating them along wave paths, and selecting regions of maximum wave amplification. The source regions are located on the duskside in agreement with the predominant occurrence of EMIC waves. A spectral power density of 1 nT2/Hz is adopted within the unstable regions. According to quasi-linear theory, the fluctuating fields are regarded as imposed on the system, and the losses due to wave-particle interactions are described with diffusive processes. The effects of the presence of heavy ion components on the quasi-linear diffusion coefficients are also considered. Resonance with ion cyclotron waves reduce the anisotropy of the proton population and the unstable regions disappear with time. Global patterns of precipitating ion fluxes are obtained and compared with observations.

Spectroscopy and structure of solvated alkali-metal ions

Abstract: Vibrational pre-dissociation spectroscopy combined with mass spectrometry has been used to obtain the infrared spectra of mass-selected cluster ions. The onset of new spectroscopic features, as a function of solvent number, has been shown to correspond to specific structural changes such as the filling of solvent shells and the formation of hydrogen bonds. These small finite systems offer a number of advantages over traditional solution-based measurements and perhaps are the most useful way to test current theoretical calculations and models. Whereas thermodynamic properties, derived from high-pressure mass spectrometry measurements, give some indication of solvent shell size, vibrational spectra suggest a more complex picture. When combined with recent infrared studies of size-selected neutral clusters, considerable detail of the solvent structure about the ion can be obtained. The solvation of the alkali-metal ions Na and Cs by a number of solvents, including water, will be discussed. Future directions ...

Ion sizes and finite-size corrections for ionic-solvation free energies

Abstract: Free energies of ionic solvation calculated from computer simulations exhibit a strong system size dependence. We perform a finite-size analysis based on a dielectric-continuum model with periodic boundary conditions. That analysis results in an estimate of the Born ion size. Remarkably, the finite-size correction applies to systems with only eight water molecules hydrating a sodium ion and results in an estimate of the Born radius of sodium that agrees with the experimental value.

Electron and heavy particle kinetics in a low-pressure nitrogen glow discharge

Abstract: A detailed kinetic model is used to investigate the mechanisms for ionization, dissociation and atomic re-association in a low-pressure positive column. The approach is based on the self-consistent solutions to the electron Boltzmann equation coupled to a system of rate balance equations for the levels, the electronically excited states of and the and ions. The maintenance electric field is self-consistently determined from the continuity equations for electrons and ions. The model provides a satisfactory explanation of measurements conducted in these conditions, in the range p = 0.6 - 2.5 Torr and I = 10 - 100 mA, for the reduced electric field and the concentrations of N atoms and and states. The rate coefficients and are derived here for the two reactions leading to associative ionization by collisions between electronic metastables and , respectively. The dissociation due to the vibration - vibration (V - V) and vibration - translation (V - T) energy exchanges is shown to represent only a minor contribution for the total rate of dissociation, in opposition to previous studies, due to the effects of fast V - T exchanges associated with - N collisions. Finally, it is shown that the reaction does not constitute an effective depopulating mechanism of N atoms as most of the N atoms so created are reconverted to the N by collisions on the wall and quenching.

Correlated ion hopping in single-crystal yttria-stabilized zirconia

Abstract: A study of the effect of correlated ion motion on the electrical conductivity relaxation in single-crystalline yttria-stabilized zirconia is presented. Complex admittance in the radio frequency range show power-law dependencies in the real part of the conductivity at high frequencies of the form ω^(n) and asymmetric electric modulus plots as a result of correlations. An analysis of the frequency dependence of the electric modulus is conducted to obtain time decay functions of the form exp[-(t/τ)^(β)] from an analytical distribution of relaxation times. Correlation times, and parameters n and β characterizing the relaxation in time and frequency domains are compared to show the equivalence of time and frequency representations. The common origin of ac and dc processes is discussed in view of the frequency dependence of the complex conductivity. From a macroscopic activation energy for ion motion E = 1.16 eV and a β value of 0.43, a single-ion microscopic activation energy E_(a) = 0.5 eV is obtained as βE according to Ngai’s coupling model. The microscopic activation energy is related to the association energy of oxygen vacancies.

Chemical modifications of PET induced by swift heavy ions

Abstract: Ion induced chemical modifications of polyethylene terephthalate (PET) were studied by Fourier-transform infrared spectroscopy The irradiations with Kr (86 MeV/u) and with Mo (56 MeV/u) ions were performed under vacuum and in oxygen atmosphere, respectively The overall degradation of the polymer was investigated as a function of the ion fluence in the range from 1 × 1011to 6 × 1012 ions/cm2 A significant loss of crystallinity is related to scission processes of the main chains at the ethylene glycol residue The benzene ring structures show only small changes under irradiation and do not seem to participate in the degradation process significantly While various degradation processes known from photochemical degradation take place, the creation of alkynes near the track core is found to be a unique process induced by heavy ions The presence of oxygen during irradiation enhances the overall degradation of PET and leads to enhanced formation of alkynes and CO2