Showing papers on "Ion published in 1987"

Principles of Plasma Diagnostics

Abstract: Preface to first edition Preface to second edition 1. Plasma diagnostics 2. Magnetic diagnostics 3. Plasma particle flux 4. Refractive-index measurements 5. Electromagnetic emission by free electrons 6. Electromagnetic radiation from bound electrons 7. Scattering of electromagnetic radiation 8. Neutral atom diagnostics 9. Fast ions and fusion products Appendices Glossary.

Pressure-sensitive ion channel in Escherichia coli.

Abstract: We have used the patch-clamp electrical recording technique on giant spheroplasts of Escherichia coli and have discovered pressure-activated ion channels. The channels have the following properties: activation by slight positive or negative pressure; voltage dependence; large conductance; selectivity for anions over cations; dependence of activity on the species of permeant ions. We believe that these channels may be involved in bacterial osmoregulation and osmotaxis.

Ion beam modification of insulators

Abstract: 1. Ion Ranges and Energy Deposition in Insulators (J.P. Biersack). 2. The Sputtering of Insulators (R. Kelly). 3. Characterization Techniques for Ion Bombarded Insulators (J.A. Borders). 4. Defect Creation in Ion Bombarded Inorganic Insulators (A. Perez, P. Thevenard). 5. Ion Beam Modification of Glasses (G.W. Arnold, P. Mazzoldi). 6. The Mechanical and Tribological Properties of Ion Implanted Ceramics (C.J. McHargue). 7. Synthesis of Dielectric Layers in Silicon by Ion Implantation (I.H. Wilson). 8. Ion Beam Effects in Organic Molecular Solids and Polymers (T. Venkatesan, L. Calcagno, B.S. Elman, G. Foti). 9. Condensed Gases (W.L. Brown). 10. Optoelectronic Materials (G. Gotz). 11. Ferromagnetic Garnets (P. Gerard). 12. Nuclear Waste Materials (Hj. Matzke). 13. Role of Defects on the Aqueous Dissolution of Ion-Bombarded Insulators (G. Della Mea, J.-C. Dran, J.-C. Petit). 14. Chemical Effects of Ion Bombardment (G.K. Wolf, K. Roessler). 15. Ion Beam Effects on Thin Film Adhesion (J.E.E. Baglin). 16. Astrophysical Implications of Ions Incident on Insulators (L.J. Lanzerotti, R.E. Johnson). Subject Index. Appendix: Tables of Ion Ranges in Insulators.

An intrinsic stress scaling law for polycrystalline thin films prepared by ion beam sputtering

Abstract: The intrinsic stresses of Al, Ti, Fe, Ta, Mo, W, Ge, Si, AlN, TiN, and Si3N4 films prepared by ion beam sputtering were investigated at low Td/Tm values. The intrinsic stress is compressive and its origin is explained in terms of the ion peening model. Knock‐on linear cascade theory of forward sputtering is applied to derive a simple scaling law with the film’s physical properties. The results show that the stress is directly proportional to the elastic energy/mole, given by the quantity Q=EM/(1−ν)D, where E is Young’s modulus, M the atomic mass, D the density, and ν Poisson’s ratio. Stress data taken from the literature for a variety of materials deposited by low‐pressure magnetron sputtering, and rf and ion beam sputtering also fit the correlation with Q. Furthermore, the model predicts a square‐root dependence on the incident ion energy, suggesting that the stress is momentum rather than energy driven.

Rational modification of enzyme catalysis by engineering surface charge.

Abstract: Changing the surface charge of subtilisin by site-directed mutagenesis produces enzymes with significantly shifted pH-activity profiles, higher catalytic activities and altered specificities. Insight into water as a dielectric, the role of ions in electrostatic shielding and the field effects on catalysis is obtained and suggests rules for tailoring pH-activity profiles.

Observation of a phase transition of stored laser-cooled ions.

Abstract: Clouds of two to about fifty simultaneously stored, laser-cooled ${\mathrm{Mg}}^{+}$ ions in a Paul trap were observed in two phases, which are clearly distinguishable by their excitation spectra. Transitions between these phases can be induced either by a variation of the power of the laser radiation used to cool the ions or by a change of the size of the radio-frequency voltage applied to the trap. Transitions between a "crystalline" phase and a "gaseous" phase can be repeatedly observed by variation of the appropriate parameters. The two phases and the transitions between them have also been recorded by a photon-counting image system.

Temporary Anion States of Polyatomic Hydrocarbons

Abstract: The experimental methods used to characterize molecular anions differ substantially depending on the stability of the species. The application of laser spectroscopic methods, discussed elsewhere in this issue, is generally limited to anions possessing hound electronic ground states or to those with lifetimes against autodetachment of the electron in excess of a few microseconds. For a great many molecules, including such important prototypes as ethylene, butadiene, benzene, naphthalene, formaldehyde, and acetylene, the ground states of the anions are known to be unstable in the gas phase with lifetimes less than lo-'* s, typically in the neighborhood of s. Even for those molecules with hound ground-state anions, nearly all of the excited anion states lie in the continuum and hence decay by electron detachment. Thus from consideration of numbers alone, the manifold of temporary anion states

Photoelectron spectroscopy of mass-selected metal cluster anions. I. Cu−n, n=1–10

Abstract: Negative ion photoelectron spectra of Cu−n (n=1–10) are reported for the 0–2.4 eV region at an instrumental resolution of 10 meV. The cluster anions were prepared in a flowing afterglow ion source incorporating a cold cathode dc discharge. This very simple source provides a convenient, general method to prepare continuous beams of near‐thermal metal cluster ions at intensities (up to 10−11 A) sufficient for spectroscopic or chemical studies. Photoelectron spectra of the copper cluster anions yield measurements for vertical electron binding energies and adiabatic electron affinities as a function of cluster size. The overall trend observed is well described by the classical spherical drop electrostatic model. In addition, quantum effects are apparent in the higher electron affinities generally observed for clusters containing odd numbers of atoms. Excited electronic states in the photoelectron spectra show that the transition energy in the neutral molecule decreases rapidly with cluster size. Vibrational structure resolved in the Cu−2 spectrum yields measurements for the vibrational frequency (210±15 cm−1), bond length (2.345±0.010 A), dissociation energy (1.57±0.06 eV), and vibrational temperature (450±50 K) of the anion.

Zeolite SSZ-25

Abstract: A crystalline zeolite SSZ-25 is prepared using an adamantane quaternary ammonium ion as a template.

Ion-beam-induced epitaxial vapor-phase growth: A molecular-dynamics study

Abstract: Low-energy ions which bombard a vapor-deposited film of low adatom mobility during growth mobilize surface atoms in the vicinity of the ion impact, causing a modification in the evolving microstructure. In a two-dimensional molecular-dynamics simulation where inert-gas ions strike a growing film of Lennard-Jones particles, it is demonstrated that ion bombardment during growth causes the filling of voids quenched in during vapor condensation and induces homoepitaxial growth. The dependence of film density and degree of homoepitaxial growth on the ion-to-vapor arrival rate ratio and ion energy is studied in detail.

Photoionization mass spectrometric studies of AsHn (n=1–3)

Abstract: A photoionization mass spectrometric study of SiH4 at T=150 K reveals the presence of SiH+4 with an adiabatic threshold at 11.00±0.02 eV. The implications for the structure of this Jahn–Teller split state are discussed. The appearance potentials of SiH+2 and SiH+3 are 11.54±0.01 eV and ≤12.086 eV, respectively. The reaction of F atoms with SiH4 generates SiH3 (X 2A1), SiH2 (X 1A1 and a 3B1), and SiH (X 2Π) in sufficient abundance for photoionization studies. The measured adiabatic ionization potentials (eV) are: SiH3, 8.01±0.02; SiH2 (X 1A1), 9.15±0.02 or 9.02±0.02; SiH2 (a 3B1), 8.244±0.025; SiH, 7.91±0.01. The singlet–triplet splitting in SiH2 is either 0.78±0.03 or 0.91±0.03 eV. The dissociation energy of SiH is 2.98±0.03 eV. A Rydberg series is observed, converging to SiH+ (a 3Π) at 10.21±0.01 eV. Heats of formation of the various neutral and ionic species are presented, as are the stepwise bond energies of SiH4.

Absolute Cross Sections for Molecular Photoabsorption, Partial Photoionization, and Ionic Photofragmentation Processes

Abstract: A compilation is provided of absolute total photoabsorption and partial‐channel photoionization cross sections for the valence shells of selected molecules, including diatomics (H2, N2, O2, CO, NO) and triatomics (CO2, N2O), simple hydrides (H2O, NH3, CH4), hydrogen halides (HF, HCl, HBr, HI), sulfur compounds (H2S, CS2, OCS, SO2, SF6), and chlorine compounds (Cl2, CCl4). The partial‐channel cross sections presented refer to production of the individual electronic states of molecular ions and also to production of parent and specific fragment ions, as functions of incident photon energy, typically from ∼20 to 100 eV. Total photoabsorption cross sections above the first ionization threshold are reported from conventional optical measurements obtained using line and continuum sources and from ‘‘equivalent‐photon’’ dipole (e,e) electron scattering experiments. Partial photoionization cross sections for production of electronic states of molecular ions are obtained from photoelectron spectroscopy and from dip...

The heavy ion storage and cooler ring project ESR at GSI

Abstract: The Experimental Storage Ring ESR [1] is designed for accumulation, storage and cooling of heavy ion beams up to uranium with energies up to 500 MeV/u either directly out of the heavy ion synchrotron SIS [2] or via a stripper target, where electron-free or electron-poor ions are produced. The intense, cold beams may be used for a large variety of experiments, preferably in-ring experiments making use of the special features of the storage ring. Due to circulation frequencies of up to 2 MHz high luminosities will be attained using thin internal gas jet or electron target. Energies can be varied over a wide range from 500 MeV/u down to 3 MeV/u. The SIS/ESR-facility will also be capable of delivering beams of unstable nuclei produced by fragmentation or fission of heavy projectiles. After preselection in a large acceptance Z / A -separator the secondary beams can be prepared for internal or external experiments by stochastic precooling, intensity accumulation and final electron cooling.

Optical effects of ion implantation

Abstract: Ion implantation is a powerful technique for surface analysis and material modification and this review discusses the effects that specifically relate to optical properties of insulators. By selection of ion energy and ion dose one can inject trace impurities that control luminescence, generate optical absorption bands or complex defect aggregates, stimulate production of new crystalline phases or destroy crystallinity. Implantation invariably stimulates luminescence which provides a sensitive means of analysis to measure purity and near-surface defect concentrations. Post-implantation measurements reveal changes in many physical and chemical properties of the materials. Some, such as chemical reactivity, can increase a thousandfold, others such as birefringence, electro-optic and acoustic wave parameters are reduced. One major property which can be controlled is the refractive index; thus optical waveguides, and ultimately, complex integrated optical devices can be precisely defined by ion implantation.

The thermodynamics of solvation of ions. Part 2.—The enthalpy of hydration at 298.15 K

Abstract: So-called ‘absolute’ standard molar enthalpies of hydration of individual ions at 298.15 K have been obtained from the conventional data by means of the widely employed TATB extrathermodynamic assumption, i.e. that ΔhydH°[(C6H5)4As+]=ΔhydH°[(C6H5)4P+]=ΔhydH°[B(C6H5)–4]. This is possible by the evaluation of the standard molar enthalpies of hydration of salts involving the tetraphenyl ions with suitable counter-ions from their lattice enthalpies and heats of solution and appropriate combinations of these data. The resulting value of the standard molar enthalpy of hydration of the hydrogen ion, ΔhydH°[H+]=–1103 ± 7 kJ mol–1, agrees with the values obtained by other reliable methods. The values of ΔhydH° on this basis of numerous ions are tabulated.

Effects of radiation damage in ion‐implanted thin films of metal‐oxide superconductors

Abstract: The effects of ion implantation into thin films of the superconductor YBa2Cu3Ox have been studied. Using oxygen and arsenic ions, the superconducting transition temperature Tc, the change in room‐temperature electrical properties from conducting to insulating, and the crystalline to amorphous structural transition in the films were studied as a function of ion dose. The deposited energy required to change Tc was found to be 0.2 eV/atom, while 1–2 eV/atom was required to affect the room‐temperature conductivity, and 4 eV/atom to render the film amorphous. This hierarchy of effects is discussed in terms of the damage mechanisms involved.

Shock wave mechanism for cluster emission and organic molecule desorption under heavy ion bombardment

Abstract: Mechanism for large cluster emission during sputtering of solids and bioorganic molecule desorption under ion bombardment via formation and emergence of shock waves originating from some high density energy spikes due to collision cascades or Coulomb explosions on fast heavy ion paths is proposed. The peculiarities of large cluster emission and the dependence of the bioorganic molecule desorption yield on energy loss, angle of incidence, and charge state of the primary ions are compared with the available experimental data. The energy and the angular distributions of the desorbed molecules are discussed.

Ab Initio MO Calculations of Effective Exchange Integrals between Transition-Metal Ions via Oxygen Dianions: Nature of the Copper-Oxygen Bonds and Superconductivity

Abstract: The superexchange interaction between transition-metal ions via oxygen dianion was investigated by the ab initio molecular orbital (MO) method. It is found that the magnitude of the effective exchange integral (Jab) for the CuOCu unit is far larger than those of the NiONi, CrOCr and FeOFe units. Implications of this result are discussed in relation to the high Tc superconductivity for Ba-La-Cu-O and R-Ba-Cu-O (R=Y, etc.).

High resolution zero kinetic energy photoelectron spectroscopy of benzene and determination of the ionization potential

Abstract: We report the direct high‐resolution determination of the ionization potential of benzene and the observation of rotational structure in the vibronic ground state of the benzene ion. We employ a new technique of two‐color high‐resolution zero kinetic energy photoelectron spectroscopy (ZEKE‐PES) via selection of an intermediate resonant (neutral) state. The ZEKE‐PES method allows for the investigation of rotationally resolved photoionization dynamics and the direct high‐precision determination of ionization potentials, the precision being derived directly from the laser wavelength calibration. For benzene we determine an IP of 74 555.0±0.4 cm−1. We compare this method to ionization potential measurements from ion yield measurements under various conditions.

Multiple ionization of He, Ne, and Ar by fast protons and antiprotons.

Abstract: Single and multiple ionization of He, Ne, and Ar has been studied experimentally by impact of fast protons and antiprotons. The single-ionization cross sections obtained with protons and antiprotons are found to be the same. The double-ionization cross sections obtained with antiprotons, however, are much larger than those obtained with protons at equal velocity. This difference is found for all three gases but the effect is largest for He and Ne, where the difference is about a factor of 2 at 1 MeV/amu. The difference is discussed in terms of interference between two collision mechanisms which both result in double-electron escape. Experimental information on the magnitude of the interference term is obtained by inclusion of double-ionization data, partly obtained in this work, for fast electron and \ensuremath{\alpha}-particle impact. For triple ionization of Ne, we also find that antiprotons yield much larger cross sections than protons do. Identical cross sections, however, are found for triple ionization of Ar with protons and antiprotons. This is believed to be due to the fact that triple ionization of Ar is mainly a consequence of a single vacancy produced in an inner shell followed by electronic rearrangement. This observation supports the interpretation that the observed charge effect is due to an interference effect in the outermost shell.

Structure/Reactivity and Thermochemistry of Ions

Abstract: Capture Collision Theory.- Collision Theory: Summary of the Panel Discussion.- Optical Studies of Product State Distributions in Thermal Energy Ion-Molecule Reactions.- Crossed-Molecular Beam Studies of Charge Transfer Reactions at Low and Intermediate Energy.- Production, Quenching and Reaction of Vibrationally Excited Ions in Collisions with Neutrals in Drift Tubes.- Kinetic Energy Dependence of Ion-Molecule Reactions: From Triatomics to Transition Metals.- Reactions of Transition Metal Ions with Cycloalkanes and Metal Carbonyls.- Gas Phase Metal Ion Chemistry: Summary of the Panel Discussion.- Dynamics of Dissociation and Reactions of Cluster Ions.- Growing Molecules with Ion/Molecule Reactions.- AB Initio Studies of Interstellar Molecular Ions.- Structures and Spectroscopic Properties of Small Negative Molecular Ions - Theory and Experiment.- AB Initio Calculations on Organic Ion Structures.- Formation of Anions in the Gas Phase.- Proton Transfer Reactions of Anions.- Assignment of Absolute Gas Phase Basicities of Small Molecules.- Kinetics and Equilibria of Electron Transfer Reactions: A? + B = A + B?. Determinations of Electron Affinities of A and B and Stabilities of.Adducts A2? and (A * B)?.- Ion Thermochemistry: Summary of the Panel Discussion.- Entropy-Driven Reactions: Summary of the Panel Discussion.- Organic Ion/Molecule Reactions: Summary of the Panel Discussion.- Experimental and Theoretical Studies of Small Organic Dications, Molecules with Highly Remarkable Properties.- Structure and Reactivity of Gaseous Ions Studied by a Combination of Mass Spectrometric, Nuclear Decay and Radiolytic Techniques.

Covalent Group IV Atomic Clusters

Abstract: Atomic clusters containing from two to several hundred atoms offer the possibility of studying the transition from molecules to crystalline solids. The covalent group IV elements carbon, silicon, and germanium are now being examined with this long-range objective. These elements are particularly interesting because of the very different character of their crystalline solids and because they are intermediate between metals and insulators in the nature of their bonding. Small mass-selected atom cluster ions are formed by pulsed laser techniques and identified by time-of-flight methods. Laser photoexcitation is used to study the relative stability of these clusters and their modes of fragmentation. These modes for C n + clusters, which tend to fragment with a characteristic loss of a neutral C 3 , are found to be different from the modes for Si n + and Ge n + clusters, which tend to fragment to "magic" clusters such as Si 4 + , Si 6 + and Si 10 + . These experimental results can be accounted for by recent theoretical calculations of the ground-state structure and stability of small silicon and carbon clusters. Several theoretical approaches give consistent results, showing that small silicon clusters are compact and different from small fragments of the bulk crystal. Calculations show that carbon clusters change from linear structures toward cyclic structures as the cluster size increases, but with significant odd-even differences.