Showing papers on "Ion published in 1975"

Three-Dimensional Structure of Immunoglobulins

Abstract: Publisher Summary This chapter discusses a study analyzing the three-dimensional structure of immunoglobulins. Heavy atom derivatives were obtained. The compounds used were (1) p -chloromercuribenzene sulfonate (PCMBS), (2) mercuric cyanide Hg(CN) 2 , and (3) chloroplatinate ion, (PtCl 6 −− ). X-ray diffraction data were measured for the native crystals and each of the heavy atom derivatives to a Bragg spacing of 6 A. Electron micrographs were prepared of sections of the Dob crystals, cut perpendicular to the short c axis. The crystals were fixed with glutaraldehyde, washed, postfixed in osmium tetroxide, and embedded in Maraglas prior to sectioning. The sections were further stained with uranyl acetate and lead citrate solutions. The method of optical integration was applied in which the periodicity of the crystal was used to reduce the background noise and reveal the molecular outline. An electron density map was calculated with phases obtained from the heavy atom derivatives. The examination of the map showed that one pair of asymmetric units (containing one complete molecule) had density corresponding to three globular regions. One region lay on the twofold axis relating the two halves of the Dob molecule.

Negative surface charge near sodium channels of nerve: divalent ions, monovalent ions, and pH.

Abstract: Evidence is given for a high density of negative surface charge near the sodium channel of myelinated nerve fibres The voltage dependence of peak sodium permeability is measured in a voltage clamp The object is to measure voltage shifts in sodium activation as the following external variables are varied: divalent cation concentration and type, monovalent concentration, and pH With equimolar substitution of divalent ions the order of effectiveness for giving a positive shift is: Ba equals Sr less than Mg less than Ca less than Co approximately equal to Mn less than Ni less than Zn A tenfold increase of concentration of any of these ions gives a shift of +20 to +25 mV At low pH, the shift with a tenfold increase in Ca-2+ is much less than at normal pH, and conversely for high pH Soulutions with no added divalent ions give a shift of minus 18 mV relative to 2 mM Ca-2+ Removal of 7/8 of the cations from the calcium-free solution gives a further shift of minue 35 mV All shifts are explained quantitatively by assuming that changes in an external surface potential set up by fixed charges near the sodium channel produce the shifts The model involves a diffuse double layer of counterions at the nerve surface and some binding of H+ions and divalent ions to the fixed charges Three types of surface groups are postulated: (1) an acid pKa equals 288 charge density minus 09 nm- minus 2; (i) an acid pKa equals 458, charge density minus 058 nm- minus 2; (3) a base pKa equals 628, charge density +033 nm- minus 2 The two acid groups also bind Ca-2+ ions with a dissociation constant K equals 28 M Reasonable agreement can also be obtained with a lower net surface charge density and stronger binding of divalent ions and H+ ions

Energy gap law in the solvent isotope effect on radiationless transitions of rare earth ions

Abstract: Data on the fluorescent yields of tervalent ions of Pr, Sm, Eu, Gd, Tb, Dy, and Tm in H2O and D2O, containing perchlorate ions, and isotope effects on the fluorescence are reported. The results are interpreted in terms of the mechanisms responsible for radiationless processes in solutions of rare earth ions [Y. Haas and G. Stein, J. Phys. Chem. 75, 3668 (1971); 76, 1093 (1972)]. It is shown that along the series of the closely related rare earth ions, all of which exhibit well shielded f−f electronic transitions, the results can be correlated in terms of a single variable: the energy gap between the lowest fluorescent and highest nonfluorescent level. For several rare earth ions, the matching of this gap by a single high energy (OH or OD) vibration of one solvent molecule is the decisive factor accounting for observed yields and isotope ratios.

Interpretation of S-state ion E.P.R. spectra

Abstract: Although a considerable body of data exists on the parametrization of the ground-state splittings of S-state ions in crystals, relatively little progress has been made in obtaining a quantitative understanding of the mechanisms which determine these parameters. In the course of summarizing our present understanding, we emphasize the need for making planned experiments explicitly aimed at testing theoretical models, such as those proposed in this article. The variable frequency E.P.R. technique is described in some detail, as it has proved to be particularly useful in this respect.

A computer simulation of charged particles in solution. I. Technique and equilibrium properties

Abstract: A computer technique is presented for simulating the translational motion of ions in a liquid solution. In the model the diffusive motion of each ion is perturbed by the electrostatic force of the surrounding ions. Several polyelectrolyte systems of spherical polyions (10–50 A in radius) and small ions (∼1 A in radius) have been studied. For each system the polyion electrostatic shielding length and the average potential energy of each ion species was calculated. When the shielding length was sufficiently short, the computer results and the predictions of the zero polyion concentration Debye–Huckel theory were in good agreement.

Ionic selectivity, saturation, and block in sodium channels. A four-barrier model.

Abstract: Ionic fluxes in Na channels of myelinated axons show ionic competition, block, and deviations from simple flux independence. These phenomena are particularly evident when external Na+ ions are replaced by other permeant or impermeant ions. The observed currents require new flux equations not based on the concepts of free diffusion. A specific permeability model for the Na channel is developed from Eyring rate theory applied to a chain of saturable binding sites. There are four energy barriers in the pore and only one ion is allowed inside at a time. Deviations from independence arise from saturation. The model shows that ionic permeability ratios measured from zero-current potentials can differ from those measured from relative current amplitudes or conductances. The model can be fitted to experiments with various external sodium substitutes by varying only two parameters: For each ion the height of the major energy barrier (the selectivity filter) determines the biionic zero-current potential and the depth of the energy well (binding site) just external to that barrier then determines the current amplitudes. Voltage clamp measurements with myelinated nerve fibers are given showing numerous examples of deviations from independence in ionic fluxes. Strong blocks of ionic currents by guanidinium compounds and Tl+ ions are fitted by binding within the channel with apparent dissociation constants in the range 50-122 mM. A small block with high Na+ concentrations can be fitted by Na+ ion binding with a dissociation constant of 368 mM. The barrier model is given a molecular interpretation that includes stepwise dehydration of the permeating ion as it interacts with an ionized carboxylic acid.

Effect of electric fields on the daytime high-latitude E and F regions

Abstract: We have obtained solutions of the coupled continuity, momentum, and energy equations for NO(+), O(+), and O2(+) ions for conditions appropriate to the daytime high-latitude E and F regions. Owing to the rapid increase of the reaction O(+) + N2 yielding NO(+) + N with ion energy, high-latitude electric fields and consequent perpendicular-E x B drifts deplete O(+) in favor of NO(+). For electric field strengths less than about 10 mV/m the depletion of O(+) is small, and the altitude profiles of ion density are similar to those found at mid-latitudes. However, for moderate electric field strengths (50 mV/m), NO(+) is substantially increased in relation to O(+) and becomes an important ion throughout the F region. For large electric fields (200 mV/m), NO(+) completely dominates the ion composition to at least 600 km, decreasing at high altitudes with a diffusive equilibrium scale height. Since the overall F region electron density decreases markedly with increasing electric field strength, it appears that high-latitude, daytime electron density troughs are directly related to the presence of ionospheric electric fields.

Electrochemical and Spectroscopic Properties of Cation Radicals III . Reaction Pathways of Carbazolium Radical Ions

Abstract: Previous work had elucidated the anodic oxidation pathways of carbazole and several N‐substituted derivatives. These studies have now been extended to seventy‐six ring‐substituted carbazoles using electrochemical and spectroscopic techniques to study the reactivity of the various substituted carbazole cation radicals. Generally, it was found that 3, 6, and 9 (N) positions are extremely reactive; if these sites are not blocked by inert substituents the cation radicals generated by electrolytic oxidation react rapidly via coupling‐deprotonation. In some cases, substituents are eliminated from the 3 and 6 positions in the cation radicals followed by coupling to form substituted bicarbazyls. In other cases, relatively stable cation radicals were obtained and their EPR and visible absorption spectra were recorded. It was found that the reactivities of substituted carbazole cation radicals are considerably greater than those of analogous di‐ and triphenylaminium ions due to the planarity of the carbazole aromatic rings.

Kinetic Process of Plasma Heating Due to Alfvén Wave Excitation

Abstract: We elucidate the kinetic process of plasma heating by the resonant excitation of the shear Alfv\'en wave. The heating occurs as a result of the damping of the modified (by the finite ion Larmor radius and electron inertia) Alfv\'en wave which is excited by mode conversion at the resonant layer. The heating rates of ions and of electrons are comparable in the collisional regime; otherwise electrons are predominantly heated.

Charge densities and interionic potentials in simple metals: Nonlinear effects. II

Abstract: We present nonlinear self-consistent calculations of the charge density induced by isolated ${\mathrm{Li}}^{+}$, ${\mathrm{K}}^{+}$, ${\mathrm{Mg}}^{++}$, ${\mathrm{Al}}^{+++}$, and ${\mathrm{Ca}}^{++}$ ions when placed in an electron gas of the appropriate metallic density. By comparison with linear-response theory we show that in the first four metals nonlinear effects in the response of the conduction electrons to the ionic perturbations play an important role in determining the charge density and the interionic potential. However as in the case of Na studied in the previous paper these nonlinear effects can be simulated by using a suitably adjusted model potential. The calculated phonon dispersion curves for Li, K, and Al agree very well with experiment. Nonlinear effects are also very likely to be important in Ca but further work is necessary before conclusions can be drawn.

Collisional ionization of Na, K, and Cs by CO2, COS, and CS2: Molecular electron affinities

Abstract: The negative ion products resulting from collisions between orthogonal beams of alkali metal atoms (Na, K, Cs) and the linear triatomic molecules CO2, COS, and CS2 have been studied from threshold to ∼400 eV (lab). Ions with masses corresponding to the parent molecules CO2, COS, and CS2 are detected for all collision permutations except for Na colliding with CO2. The following electron affinities are deduced from measurements of the threshold for the ion pair production reactions: CO2(−0.60±0.2 eV), COS(+0.46±0.2 eV), and CS2(1.0±0.2 eV). The CO−2 ion was found to be metastable with respect to autodetachment. This result is compatible with the negative electron affinity for CO2 and in agreement with our earlier observations of CO−2* and with recent theoretical calculations. The lifetime of CO−2* (9±2×10−5 sec) was measured to be independent of collision energy over the region of energy studied (threshold to ∼20 eV c.m.). The fragment ions O−/CO2, O−/COS, S−/COS, and S−/CS2 were detected at a threshold ene...

Theory and simulation of resonant absorption in a hot plasma

Abstract: A detailed theoretical and simulation study of resonant absorption in a hot plasma is presented which isolates the behavior of the plasma for times short compared to an ion response time. The extent to which an electron fluid model can describe the absorption process in the kinetic regime is discussed. At high intensities the absorbed energy is observed to be deposited in a suprathermal tail of electrons whose energy varies approximately as the square root of the incident power. The density profile modification due to the ion response to the pondermotive force is also discussed.

Principles of the stored ion calorimeter

Abstract: The properties of a harmonically bound radiatively thermalized ion gas were investigated by studying the behavior of an electron cloud stored in a Penning trap. A simple model characterizing ions contained in an electromagnetic trap is proposed and tested by investigating the electromagnetic−dynamic behavior of these electrons subject to various external perturbations. The ion calorimeter realized in such a system is also discussed; particular attention is devoted to sensitivity to heat inputs into the various degrees of freedom.

The structure of borosilicate glasses studied by Raman scattering

Abstract: Raman spectra of alkali and alkaline earth borosilicate glasses are reported. These spectra are used to discuss the molecular structure of the glasses. The influence of Al 2 O 3 additions on the structure of borosilicate glass is also discussed. It is shown that the same type of groups are present in borosilicate glasses as in borate and silicate glasses. The presence of large borate groups such as tetraborate and metaborate groups is strongly suggested by the Raman spectra. It appears that boron ions are hardly taken up in the silicon-oxygen network. Our results suggest that the region of phase separation is larger than the region presently acknowledged.

Electromagnetic ion cyclotron instability driven by ion energy anisotropy in high‐beta plasmas

Abstract: Low frequency (ω≈ωci=eiB0/mic) transverse electromagnetic perturbations propagating parallel to a confining magnetic field B0ez are shown to exhibit instability in the presence of ion energy anisotropy with Ti⊥≳Ti∥. The characteristic maximum growth rate for Ti⊥≫Ti∥ is γM≈(βi⊥/2)1/2ωci, where βi⊥=8πniTi⊥/B20, and the wavelengths corresponding to instability are of order c/ωpi, where ωpi is the ion plasma frequency. Within the context of a quasi‐linear model, it is shown that the characteristic time scale for energy isotropization through nonlinear response of the ions to the instability is several γ−1M. Since γ−1M≪τii (the ion‐ion binary collision time) in typical high‐density pinch experiments, this instablity appears to provide a viable collective mechanism for ion energy isotropization during the implosion or post‐implosion phases of these experiments. It is also shown that the instability persists in the limit of weakly magnetized ions (‖ω+iγ‖≫‖ωci‖, k2zr2Li≫1) and strongly magnetized electrons (‖ω+iγ...

R-matrix theory of photoionization. Application to neon and argon

Abstract: The R-matrix method, which has been used recently to calculate electron-atom and ion collision cross sections and atomic polarizabilities is extended to enable atomic photoionization processes to be studied. Both the initial atomic bound state and the final atomic continuum state are expanded in terms of R-matrix bases. The method is programmed for a general atomic system and then used to calculate the photoionization cross sections of ground state neon and argon atoms leaving the residual ions in their ground or first excited states. Good agreement is obtained with recent experiments using synchrotron radiation both in resonant and non-resonant regions, showing that the method has a wide range of applicability.

On the formation of auroral arcs and acceleration of auroral electrons

Abstract: It is suggested that the highly structured auroral arc is caused by a current-driven laminar electrostatic shock oblique to the geomagnetic field. Electrons are accelerated by the potential jump associated with the shock. The shock is assumed to be confined to a plane. Self-consistent solutions to the Poisson-Vlasov systems are calculated for the electrostatic potential. Adiabatic theory is used to calculate the ion number density in terms of the electrostatic potential and its derivatives. The electrons are assumed to be highly magnetized so they can only move parallel to the magnetic field. Solutions are exhibited for two plasma models: (1) streaming electrons and a two-temperature distribution of ions and (2) streaming electrons and ions and thermal electrons and ions. In the latter model, solutions can be obtained for an arbitrary potential jump across the shock. The shock is identified with the linear electrostatic ion cyclotron wave, and stability of these waves is examined to determine conditions for the formation of oblique shocks. Finally, the theory is discussed in the context of the magnetosphere, and possible model shocks are exhibited and discussed in terms of auroral arc formation.

Configuration interaction studies of O3 and O+3. Ground and excited states

Abstract: We report the results of extensive configuration interaction calculations (double ζ basis) on the ground and 15 excited states of ozone and the eight lowest states of the ozone positive ion. The studies on ozone are carried out as a function of both bond length and bond angle. We find that three excited states of ozone may be bound with respect to the ground states of O2 and O. Additional information is provided on the ring state (60 ° bond angle) previously reported. Two of the excited states of ozone are known spectroscopically and the ionization potentials of the first three states of O+3 are now known experimentally. These experimental results are in good agreement with the theoretical calculations.

The behaviour of surfaces under ion bombardment

Abstract: The processes involved when ions bombard surfaces are divided into two categories: those due to elastic collisions, i.e., sputtering and ion scattering; and those due to inelastic collisions, leading to excitation and ionization, resulting in the emission of X-rays, optical photons and electrons. The mechanisms involved in ion scattering and sputtering are reviewed in detail. The experimental energy and angular distributions of the secondary particles are considered and compared with current theories. The effect of sputtering on the erosion of surfaces and the effects of radiation blistering and radiation damage on surface topography are discussed. Considering inelastic effects due to incident ions, results for total yields, statistics of secondary electron emission, and their energy distribution are outlined. X-ray emission, including molecular X-rays and other solid state effects, are discussed. Finally optical photon emission due to de-excitation of excited atoms is treated.

Gas phase ion chemistry of HNO3

Abstract: The ion chemistry of HNO3 is studied at 298 °K. The attachment of thermal electrons to HNO3 to produce NO−2 is found to have a rate constant of (5±3) ×10−8 cm3/sec. HNO3 reacts rapidly (k∼10−9 cm3/sec) with a large variety of negative ions including Cl−, NO2−, O2−, and CO−3. HNO3 is also found to bond strongly to NO3−. The proton affinity of HNO3 is determined to be 176±7 kcal/mole. The present results indicate a strong similarity between protonated HNO3 and hydrated NO2+, suggesting that these ion species are identical. The implication of the present results for atomospheric ion chemistry is discussed.

Oscillatory Cross Sections in Low-Energy Ion Scattering from Surfaces

Abstract: Oscillations in the low-energy noble-gas ion-scattering cross sections versus primary ion energy have been observed for a number of solid surfaces. This behavior is ascribed to resonant electron exchange between electronic states of the surface atom and the primary ion. The oscillations are sensitive to the particular electronic energy levels and to the chemical environment of the surface atom.

On the ionic conduction in β‐alumina: Potential energy curves and conduction mechanism

Abstract: Potential energy curves for various carrier ions in β‐alumina have been calculated. All ions except the carrier ions (Li+, Na+, Ag+, K+, Rb+, Cs+) were assumed fixed in their equilibrium positions. These ionic positions were taken from Roth’s work. In our calculation the motion of carrier ions is restricted on lines of the two dimensional hexagonal network. We show that the site energy difference between the anti‐Beevers–Ross site and the Beevers–Ross site is approximately 2 eV in stoichiometric crystals. In nonstoichiometric crystals the extra carrier ions may be paired to form interstitialcy pairs. The potential energy barriers for the in‐phase motion of the interstitialcy pairs are found to be comparable to the experimental activation energies. Attempt frequencies have been calculated, and a simple random walk model is used for the electrical conductivities.

High oxide ion conduction in sintered oxides of the system Bi2O3-WO3

Abstract: The ionic conduction in sintered oxides of the system Bi2O3-WO3 was investigated by measuring conductivities and ion transference numbers under various conditions. The ion transference number was measured by an oxygen concentration cell employing the specimen tablet as the electrolyte.

Cross section for single and double ionization of N2 and O2 molecules by electron impact from threshold up to 170 eV

Abstract: Absolute ionization cross sections have been measured for the processes N2+e→N2++2e, N2+e→N2+++3e, O2+e→O2++2e, and O2+e→O2+++3e as a function of electron energy up to 170 eV in a crossed molecule–electron beam apparatus. The ionization cross section curve for producing the singly charged parent ions N2+ and O2+ is found to have at least one inflection of the type first observed by Lawrence for mercury. Using nth root extrapolation the following minimum ionization potentials were derived from the low energy cross section behavior: N2+(X 2Σg+) =15.6±0.1 eV, N2++(3Πu) =42.9±0.3 eV, O2+(X 2Πg) =12.1±0.1 eV, and O2++(X 1Σg+) =35.6±0.3 eV. From distinct breaks in the single ionization cross section curve the following excited states were observed; N2+(B 2Σ+u) =18.8±0.2 eV, O2+(a 4Πu) =16.0±0.2 eV, and O+2(2Σ−g) =21.0±0.2 eV. In the double ionization cases, a square law was found from threshold up to some 20 eV above threshold. All results are compared with previous measurements if available.

Heavy‐ion sputtering yields of gold: Further evidence of nonlinear effects

Abstract: The sputtering yield of vacuum‐deposited gold targets has been measured with 18 different 45‐keV ions throughout the periodic system. The results are compared with previously reported results obtained with copper, silicon, and silver targets and with the predictions of the Sigmund theory. The variation in the yield with projectile atomic number is found to be consistent with theory although rather large deviations are found for both heavy and light projectiles. The large experimental yields for heavy projectiles as well as the maximum in the energy dependence of the yield, which is more pronounced than predicted by theory, are explained by nonlinear effects in very dense collision cascades. This interpretation is supported by data obtained from irradiation with atomic and molecular ions of the same elements. The deviations for light projectiles are considered mainly to be due to the lack of a surface correction term in the theory.