Showing papers on "Ion published in 1969"

Theory of Sputtering. I. Sputtering Yield of Amorphous and Polycrystalline Targets

Abstract: Sputtering of a target by energetic ions or recoil atoms is assumed to result from cascades of atomic collisions. The sputtering yield is calculated under the assumption of random slowing down in an infinite medium. An integrodifferential equation for the yield is developed from the general Boltzmann transport equation. Input quantities are the cross sections for ion-target and target-target collisions, and atomic binding energies. Solutions of the integral equation are given that are asymptotically exact in the limit of high ion energy as compared to atomic binding energies. Two main stages of the collision cascade have to be distinguished: first, the slowing down of the primary ion and all recoiling atoms that have comparable energies---these particles determine the spatial extent of the cascade; second, the creation and slowing down of low-energy recoils that constitute the major part of all atoms set in motion. The separation between the two stages is essentially complete in the limit of high ion energy, as far as the calculation of the sputtering yield is concerned. High-energy collisions are characterized by Thomas-Fermi-type cross sections, while a Born-Mayer-type cross section is applied in the low-energy region. Electronic stopping is included when necessary. The separation of the cascade into two distinct stages has the consequence that two characteristic depths are important for the qualitative understanding of the sputtering process. First, the scattering events that eventually lead to sputtering take place within a certain layer near the surface, the thickness of which depends on ion mass and energy and on ion-target geometry. In the elastic collision region, this thickness is a sizable fraction of the ion range. Second, the majority of sputtered particles originate from a very thin surface layer (\ensuremath{\sim}5 \AA{}), because small energies dominate. The general sputtering-yield formula is applied to specific situations that are of interest for comparison with experiment. These include backsputtering of thick targets by ion beams at perpendicular and oblique incidence and ion energies above \ensuremath{\sim}100 eV, transmission sputtering of thin foils, sputtering by recoil atoms from $\ensuremath{\alpha}$-active atoms distributed homogeneously or inhomogeneously in a thick target, sputtering of fissionable specimens by fission fragments, and sputtering of specimens that are irradiated in the core of a reactor or bombarded with a neutron beam. There is good agreement with experimental results on polycrystalline targets within the estimated accuracy of the data and the input parameters entering the theory. There is no need for adjustable parameters in the usual sense, but specific experimental setups are discussed that allow independent checks or accurate determination of some input quantities.

Energy of an ion crossing a low dielectric membrane: solutions to four relevant electrostatic problems.

Abstract: The influences on ion energy of membrane thickness, ion-pair formation, “pores” and “carriers” have been estimated. Only “pores” and “carriers” lower the energy barrier significantly.

Analysis of the Electrode Products Emitted by dc Arcs in a Vacuum Ambient

Abstract: We have examined the particles emitted radially by dc arcs drawn in a vacuum ambient on cathodes of several elements, as well as the axial (through‐anode) ion flux from a copper cathode. The axial results for copper agree grossly with the radial copper results. Significant quantities of multiply charged ions were seen for all elements examined. All arc volt‐ampere characteristics were positive in the range of currents observed: 30 to 250 A. We drew the following conclusions (normalizing energy in units of ion energy/ion charge): (1) The energy distributions for the various ions are similar, peaking at potentials well above the arc voltage. (2) The fraction of ions that are singly charged tends to increase with increasing arc current. (3) For a given element, as the degree of ionization increases the location of the ion‐energy distribution peak shifts to lower energies. (4) For a given degree of ionization, the location of the peak tends toward higher energies for elements with greater arc voltages. (5) Th...

Ionic mobility in muscle cells.

Abstract: The diffusivities of ionic potassium, sodium, sulfate, and adenosine triphosphate inside a nmuscle cell are reduced by a factor of 2, relative to diffusivities in aqueous solution. The diffusion coefficients of nonelectrolytes are reduced by the same factor, showing that the diffusion of the ions is retarded by physical, rather than chemical, interactions. In contrast, the diffusivity of the calcium ion, which is taken up by the sarcoplasmic reticulum, is reduced fiftyfold.

Flowing Afterglow Measurements of Ion-Neutral Reactions

Abstract: Publisher Summary This chapter describes the experimental and analytical techniques that have been developed for flowing afterglow applications to the quantitative study of ion–neutral reaction processes. Most other techniques for the measurement of ion–molecule reaction rate constants are inherently unsuited for the examination of an ion reacting with a neutral where the neutral has the lower ionization potential, and sufficient data for this generalization did not exist prior to the flowing afterglow results. Charge transfer reactions of negative ions have sometimes been useful in establishing relative electron affinities of molecules, which are often difficult to measure. Positive ion charge-transfer reactions have, on occasion, been useful in establishing relative ionization potentials of molecules, generally known or better measured in more direct ways. The dc discharge had other advantages over the microwave discharge as well. Its geometrical configuration was more compatible with the detailed flow analysis, and it was more easily incorporated in metal flow tubes, which were soon found to be advantageous.

Ionization equilibrium and radiative cooling of a low-density plasma.

Abstract: Ionization equilibrium and radiative cooling of high temperature low density plasma, noting cosmic gas cooling curve of line emission from oxygen ion transitions

Microwave Detection of Interstellar Formaldehyde

Abstract: Interstellar formaldehyde (${\mathrm{H}}_{2}$CO) has been detected in absorption against numerous galactic and extragalactic radio sources by means of the ${1}_{11}$-${1}_{10}$ ground-state rotational transition at 4830 MHz. The absorbing regions often correspond in velocity with 18-cm OH features. ${\mathrm{H}}_{2}$CO is the first organic polyatomic molecule ever detected in the interstellar medium and its widespread distribution indicates that processes of interstellar chemical evolution may be much more complex than previously assumed.

Ionization Potentials, Appearance Potentials, and Heats of Formation of Gaseous Positive Ions

Abstract: This is a compilation of ionization and appearance potentials of positive ions published from 1955 through June 1966. The compilation lists the ion formed, the parent species from which it was formed, the other products of the process, the threshold energy for the formation of this ion, and the method by which this data was obtained. Where feasible, the heat of formation at 298 K of the positive ion has been computed for each entry using auxiliary thermochemical data. From these computed values “best” values have been chosen.

Coherent Emission from Rare Earth Ions in Electro‐optic Crystals

Abstract: We report the observation of coherent emission from three rare‐earth ions in electro‐optic crystals. Oscillation is observed from Nd3+ ions in Bi4Ge3O12, LiNbO3, and Ba0.75Ca0.25Nb26O at room temperature, and from Tm3+ and Ho3+ ions in LiNbO3 at 77°K. Second‐harmonic radiation is obtained from LiNbO3:Tm3+ and electric‐field modulation of threshold is observed for certain crystals of LiNbO3:Nd3+. The latter effect is believed to arise from crystal strain. The propagation angle for phase‐matched second‐harmonic generation is calculated to lie at 43° to the c axis for both LiNbO3:Tm3+ and LiNbO3:Ho3+ at 77°K. Phasematched second‐harmonic generation from LiNbO3:Nd3+ is not possible, since the maser line is an extraordinary wave. However, the polarization of the maser line in LiNbO3:Nd3+ is appropriate for phasematched parametric frequency conversion in the near infrared. Attempts to observe such radiation were not successful due to the limited power available in the fundamental.

Radiofrequency Spectroscopy of Stored Ions II: Spectroscopy

Abstract: Publisher Summary This chapter focuses on manipulation and investigation of stored ions using radiofrequency spectroscopy (rf). When a charge moves between arbitrarily shaped grounded electrodes, the distribution of field lines originates at the charge and ends on various electrodes changes. This must be accompanied by a change in the surface charges on a given electrode at which these field lines end, and consequently result in currents to and from the electrode. The high-inductance coil connecting the cap electrodes to form the tuned detection circuit acts as a choke to the trapping frequency and because of the finite capacity between the caps and the ring, the electrode usually held at a high rf voltage causes undesirable voltages at Ω to appear at the cap. The trapping rf voltages at Ω and the dc bias are applied to the ring electrode. The capacity of the other cap electrode in this configuration to ground is about 36 pF for the major trap. At the expense of an increase of this capacity by one-third, a low impedance path to ground for the trapping frequency is provided.

Temperature Dependence of Dissociative Attachment in N2O

Abstract: The attachment of low‐energy electrons (<4 eV) by the reaction e+N2O → O−+N2 has been studied as a function of gas temperature from approximately 160 to 1040°K. The ions produced by a monoenergetic electron beam are detected by total ion collection or by mass analysis. The kinetic energy distributions of the O− ions have also been measured and found to be relatively insensitive to the electron energy when the latter exceeds 1.5 eV, in which case the most probable ion energy is 0.38 eV. The shape and magnitude of the cross section below 2 eV is found to be sensitive to gas temperature throughout the range studied. The differences in shape and threshold observed by previous workers occur below 2 eV and to a large extent may be reconciled in terms of the differing gas temperatures employed. The temperature insensitive portion of the cross section is ascribed to electron capture into the highest‐energy N2O− state connected to electronic ground state N2 + O−. The O− kinetic energy distributions arising therefr...

Origin of water cluster ions in the D region

Abstract: Laboratory studies of the ions produced by the addition of water vapor into flowing streams of O2+ (or NO+) ions have led to a vapor phase reaction scheme starting with O2+ that seems likely to be responsible for the formation of the observed water cluster ions H3O+, H5O2+, H7O3+, etc. in the D region of the earth's ionosphere. Experimental difficulties have precluded quantitative rate constant measurements so far, and in addition the D-region water vapor concentration is uncertain so that only a qualitative scheme is possible at this time. The O2(¹Δg) photoionization source recently proposed by Hunten and McElroy is presumed to be the necessary precursor O2+ source. The reaction scheme involves O4+ and O2+·H2O as intermediate ions; these ions are possibly present in observable concentrations in the D region. Nitrous acid (HNO2) production is proposed to be an end result of successive hydrations of atmospheric NO+. The hydration of the atmospheric NO+ and O2+ ions and the formation of O4+ are expected to increase somewhat the effective electron recombination coefficient in the ionosphere.

Localization of Hydrogen Ion and Chloride Ion Fluxes in Nitella

Abstract: Alternating bands of acid and base formation have been detected along the length of the internodal cell of Nitella clavata when it is illuminated, while in the dark this phenomenon is minimal. Chloride influx occurs only or largely in the acid-extruding regions, and this is also a light-dependent ion movement. Chloride efflux is slightly dependent on illumination and is not localized as are H+ efflux and Cl- influx. The results obtained support Kitasato's (1968) proposal that a large passive H+ influx is balanced by an active efflux of this ion. Transport mechanisms suggested by the correlations of Cl- and HCO3- influxes with H+ extrusion are discussed.

Atmospheric electron–ion and ion–ion recombination processes

Abstract: The electron–ion and ion–ion recombination processes of importance in the upper atmosphere are considered, and available laboratory experimental and theoretical information concerning the relevant processes is discussed. For atomic ions the principal electron–ion recombination process is radiative, with theory indicating that the two-body coefficient at ∼200 °K is ∼10−11 cm3/s and decreases with increasing electron temperature. Microwave afterglow/mass spectrometer studies of diatomic ionospheric ions (e.g. NO+, O2+, and N2+) show a loss by dissociative recombination with a coefficient substantially in excess of 10−7 cm3/s at 250 °K and decreasing with increasing electron and ion temperature. There is some evidence from flame studies that H3O+ ions exhibit a very large coefficient (10−6–10−5 cm3/s) at 300 °K. Ion–ion recombination evidently proceeds by mutual neutralization, with laboratory studies of ions such as NO+ and NO2− indicating a two-body coefficient of the order of 10−7 cm3/s at 300 °K. In the ...

Field Ionization in Liquids

Abstract: Field ionization in liquid H2, Ar, He, N2, and O2 has been studied and approximate mobilities obtained for the respective positive ions. In the case of He, field ionization leads to charge multiplication because of sufficient ion acceleration in the high‐field region near the emitter. In the case of H2 and Ar linear lni or lni / V vs 1 / V plots could be obtained at low currents, indicating that current generation was controlled by tunneling. Simple models for tunneling from the valence band and from noninteracting molecules are derived and applied to these systems. For the experimental data applied to Ar the band model yielded a bandgap of 13 eV, in excellent agreement with theoretical and optical measurements. For H2 the noninteracting molecule and the band model give equally reasonable results.

Optical Properties of Cerium-Activated Garnet Crystals*

Abstract: The optical properties of cerium-activated garnet crystals have been investigated. In the garnet crystals showing luminescence, a bright-yellow fluorescence has been observed. The fluorescence exhibits structure at low temperatures, and the wavelength maxima of the fluorescence-spectrum profiles are found to be host-dependent. Energy transfer has been observed from the cerium ion to the neodymium ion.

Studies of Heisenberg Spin Exchange in ESR Spectra. I. Linewidth and Saturation Effects

Abstract: A compact general theory for the effect of Heisenberg spin exchange on ESR linewidths and saturation parameters is detailed. The effects of Heisenberg exchange on the linewidths of the tetracyanoethylene anion (TCNE−) radical and the di‐tert‐butyl nitroxide (DTBN) radical in both dimethoxyethane (DME) and tetrahydrofuran (THF) are investigated. From comparative studies of linewidth as a function of temperature and of radical concentration, TCNE− in DME is shown to undergo strong exchange with a second‐order rate constant of 4.1 ± 0.6 × 109M−1·sec−1 at 15°C. The TCNE− radical in THF exhibits an anomalous concentration‐dependent linewidth effect when compared to the theory and to the experiments employing DME as the solvent. The uncharged DTBN radical shows similar spin‐exchange properties in both solvents. Possible mechanisms for the anomalous linewidth effect are discussed. The effect of spin exchange on the saturation parameters of the TCNE− radical in DME is investigated in detail, and the experimental ...