Showing papers on "Ion published in 1998"

Electron Capture Dissociation of Multiply Charged Protein Cations. A Nonergodic Process

Abstract: Neutralization-reionization mass spectrometry (MS)1 is of unique value for preparing and characterizing highly reactive and unstable neutral species, such as the intermediate in the dissociative-recombination reaction H3O + ef H2O + H + 6.4 eV.2 Following an earlier suggestion,3 using neutralization accompanying surface-induced dissociation (SID)4 to form an unstable site did not yield new cleavage reactions5 in multiply charged protein cations from electrospray ionization (ESI) with Fourier transform (FT) MS.6 Serendipitously, we now find that one or more charges on such protein cations can be neutralized with low-energy electrons to cause specific cleavage of the amine bond to form c, z products,7 in contrast to the amide cleavage b, y products formed by collisionally activated dissociation (CAD),8 infrared multiphoton (IRMPD)9 and UV10 photodissociation, 70 eV electron impact excitation,11 and SID.5 The b, y products are formed by the lowest energy backbone cleavage of ESI protein ions.6-9 An attempt to cleave stronger bonds using high-energy (6.4 eV) 193 nm photons gave mainly b, y products for 2 kDa protein ions,10 but for 2.8 and 8.6 kDa protein ions12 gave small yields of c, z amine bond cleavage products not previously observed. In this further investigation, extra electrodes were placed outside the ion cell electrodes that trap the positively charged ions. With the outside electrodes at +9 V,13 extensive 193 nm laser irradiation of SWIFT-selected14 (M + 11H)11+ ubiquitin ions (8.6 kDa) only produces b, y, not c, z, ions. However, with the outside electrodes at -1 V, the c, z products are formed along with 10+ molecular ions; unexpectedly, these are mainly (M + 11H)10+• ions (Figure 1d), 1 Da heavier than the (M + 10H)10+ ions formed by ESI (Figure 1c). The 4+ mellitin ion spectrum measured under the same conditions (Figure 1b) similarly contains (M + 4H)3+•, consistent with capture of secondary electrons formed by the 193 nm photons impinging on metal surfaces and trapped by the -1 V electrodes: (M + 4H)4+ + ef (M + 4H)3+•.15 Electrons were produced instead (no laser) by a conventional heated filament source outside the FTMS magnet opposite the ESI source.11 With a 10-5 Torr Ar pulse for ecooling (energy < 0.2 eV; an SF6 pulse lowered the efficiency), the 11+ ions of ubiquitin gave a spectrum that showed c, z cleavage of 50 out of 75 backbone positions; CAD8/IRMPD9 gave b, y cleavage of eight of these positions plus seven others. Cooled electrons plus the 15+ ions of FeIII equine cytochrome c16 produced (Figure 2) c, z fragment ions from cleavages at all but 40 of the 103 possible backbone sites (e.g., N-terminal side of Pro, none; of Ile, Leu, Val, few); CAD produces b, y cleavages (total 19) at eight additional sites. The 21+ apomyoglobin ions (17 kDa) yielded 33 c, z cleavages, but the 34+ ions of bovine carbonic anhydrase (29 kDa) as yet has given only 33+, 32+, and 31+ molecular ions. Electron capture dissociation (ECD)1-3 rationalizes these results. The capture cross section should be proportional to the ionic charge squared, consistent with the minimal secondary fragmentations to produce internal ions and the predominance of cleavages in the central∼70% of the protein chain. Charge values and masses17 of the complementary product ions are consistent with dissociation after ecapture, such as c39/z37 from the 76-residue ubiquitin 11+ ions and c69/z35 from the 104residue cytochrome c 15+ ions. The most favored protonation sites are the side chains of Lys, Arg, and His;18 neutralization to form hypervalent species1-3 at Lys and Arg would account for ions (Figure 2) representing losses of 17, 44, and 59 Da from (M + nH)(n-x)+ (eq 1; neutralization of protonated His gives a more stable radical site).

Ionization balance for optically thin plasmas: Rate coefficients for all atoms and ions of the elements H to NI

Abstract: We present in this paper new and updated calculations of the ionization equilibrium for all the elements from H to Ni. We collected for these elements all the data available in the literature for the ionization and radiative plus dielectronic recombination rates. In particular, the dielectronic rates have been fitted with a single formula and the related coefficients are tabulated. Our results are compared with previous works.

Defect production in collision cascades in elemental semiconductors and fcc metals

Abstract: A comparative molecular dynamics simulation study of collision cascades in two elemental semiconductors and five fcc metals is performed to elucidate how different material characteristics affect primary defect production during ion irradiation. By using simulations of full 400 eV-10 keV collision cascades and contrasting the results on different materials with each other, we probe the effect of the mass, melting temperature, material strength, and crystal structure on the modification of the material due to the cascade. The results show that the crystal structure has a strong effect on many aspects of damage production, while other material characteristics are of lesser overall importance. In all materials studied, isolated point defects produced by the cascade are predominantly interstitials. In semiconductors, amorphous clusters are produced in the cascade core, whereas in metals most of the crystal regenerates, leaving only small vacancy-rich clusters. Large interstitial clusters found in a few events in the heavy metals were observed to form by the isolation of a high-density liquid zone during the recrystallization phase of a cascade.

Calculated natural band offsets of all II–VI and III–V semiconductors: Chemical trends and the role of cation d orbitals

Abstract: Using first-principles all-electron band structure method, we have systematically calculated the natural band offsets ΔEv between all II–VI and separately between III–V semiconductor compounds Fundamental regularities are uncovered: for common-cation systems ΔEv decreases when the cation atomic number increases, while for common-anion systems ΔEv decreases when the anion atomic number increases We find that coupling between anion p and cation d states plays a decisive role in determining the absolute position of the valence band maximum and thus the observed chemical trends

Minimization of ion micromotion in a Paul trap

Abstract: Micromotion of ions in Paul traps has several adverse effects, including alterations of atomic transition line shapes, significant second-order Doppler shifts in high-accuracy studies, and limited confinement time in the absence of cooling. The ac electric field that causes the micromotion may also induce significant Stark shifts in atomic transitions. We describe three methods of detecting micromotion. The first relies on the change of the average ion position as the trap potentials are changed. The second monitors the amplitude of the sidebands of a narrow atomic transition, caused by the first-order Doppler shift due to the micromotion. The last technique detects the Doppler shift induced modulation of the fluorescence rate of a broad atomic transition. We discuss the detection sensitivity of each method to Doppler and Stark shifts, and show experimental results using the last technique.

Quantum dynamics of cold trapped ions with application to quantum computation

Abstract: The theory of interactions between lasers and cold trapped ions as it pertains to the design of Cirac-Zoller quantum computers is discussed The mean positions of the trapped ions, the eigenvalues and eigenmodes of the ions' oscillations, the magnitude of the Rabi frequencies for both allowed and forbidden internal transitions of the ions, and the validity criterion for the required Hamiltonian are calculated Energy level data for a variety of ion species are also present- ed

Complex Nature of the UV and Visible Fluorescence of Colloidal ZnO Nanoparticles

Abstract: Time-resolved spectroscopy of colloidal ZnO nanoparticles has been carried out with a laser excitation at 248 nm. UV and visible fluorescence has been analyzed. Except for the known band gap 370-nm and impurity 510-nm emissions, we have found additional continua at ∼300, 430, and 545 nm. These continua were developed in solutions of different composition, with excess Zn2+ and OH- ions, and in function of time. All but 510-nm fluorescence bands exhibit short nanosecond or subnanosecond decays. The green fluorescence at 510 nm originating from 4-nm particles in Zn2+-rich solutions is a much longer-lived, 1.0 μs. The band natures are discussed. Cluster size distribution and growth kinetics have been recovered from spectral measurements.

Localized vibrational modes in metallic solids

Abstract: Filled skutterudite antimonides1,2 are cubic compounds with the formula RM4Sb12, where R is a rare-earth element (such as La or Ce), and M is a transition metal (for example, Fe or Co). The rare-earth ion is weakly bound in an oversized atomic cage formed by the other atoms. Its presence has been shown to cause a dramatic reduction in the lattice component of the thermal conductivity, while having little effect on the electronic properties3,4,5 of the compound. This combination of properties makes filled skutterudites of interest as thermoelectric materials. It has been suggested4 that localized, incoherent vibrations of the rare-earth ion are responsible for the reduction in thermal conductivity, but no direct evidence for these local vibrational modes exists. Here we report the observation of local modes in La-filled skutterudites, using heat capacity, elastic constant and inelastic neutron scattering measurements. The La atoms show unusual thermodynamic behaviour, characterized by the presence of two low-energy localized modes. Our results suggest that consideration of local modes will play an important role in the design of the next generation of thermoelectric materials.

Self-Assembled Lanthanide-Cored Dendrimer Complexes: Enhancement of the Luminescence Properties of Lanthanide Ions through Site-Isolation and Antenna Effects

Abstract: The site isolation of lanthanide cations (Er3+, Tb3+, and Eu3+) has been achieved through the self-assembly of three convergent polyether dendrons, each with a carboxylate anion focal point, around the central trivalent cation. Evidence for the self-assembly of the dendritic complexes can be obtained by a variety of spectroscopic and other analytical means both in solution and in the solid state. The luminescence properties of these new dendrimers measured both in solution and in the bulk show a dependence of luminescence activity on the size of the dendritic shell. The observed enhancement in luminescence properties can be attributed both to a large antenna effect, involving the nonconjugated phenyl benzyl ether dendrimer framework, and to a shell effect that results from the effective site isolation of each lanthanide cation within a dendritic sphere, preventing their mutual interaction and decreasing their rate of self-quenching. The site isolation afforded by self-assembly and the antenna effect provi...

Synthesis and physical-chemical properties of ionic liquids based on 1-n-butyl-3-methylimidazolium cation

Abstract: The reaction of 1-n-butyl-3-methylimidazolium chloride (BMI.Cl) with sodium tetrafluoroborate or sodium hexafluorophosphate affords the molten salts BMI.X ( 1 , X= BF 4 and 2 , X= PF 6 ). Compounds 1 and 2 are viscous liquids within a wide range of temperature (down to 192 K). IR, NMR, density, viscosity and conductivity measurements suggest that compound 2 behaves quasi-molecular. Compound 1 is quasi-molecular below 279 K, but at higher temperatures is probably composed of imidazolium and tetrafluoroborate ions in an extended hydrogen-bonded network.

Quantum-Mechanical Study of Thermodynamic and Bonding Properties of MgF2

Abstract: The structural and thermodynamic properties of MgF2 have been investigated in a wide range of pressures (0−80 GPa) and temperatures (0−850 K) by coupling quantum-mechanical ab initio perturbed ion ...

Structure of the dissipation region during collisionless magnetic reconnection

Abstract: Collisionless magnetic reconnection is studied using a 2 1/2-dimensional hybrid code including Hall dynamics and electron inertia. The simulations reveal that the dissipation region develops a two-scale structure: an inner electron region and an outer ion region. Close to the X line is a region with a scale of c/ωpe, the electron collisionless skin depth, where the electron flows completely dominate those of the ions and the frozen-in magnetic flux constraint is broken. Outside of this region and encompassing the rest of the dissipation region, which scales like c/ωpi, the ion inertial length, is the Hall region where the electrons are frozen-in to the magnetic field but the ions are not, allowing the two species to flow at different velocities. The decoupling of electron and ion motion in the dissipation region has important implications for the rate of magnetic reconnection in collisionless plasma: the ions are not constrained to flow through the very narrow region where the frozen-in constraint is broken so that ion flux into the dissipation region is large. For the simulations which have been completed to date, the resulting rate of reconnection is a substantial fraction of the Alfven velocity and is controlled by the ions, not the electrons. The dynamics of the ions is found to be inherently nonfluid-like, with multiple ion beams present both at the X line and at the downstream boundary between the inflow and outflow plasma. The reconnection rate is only slightly affected by the temperature of the inflowing ions and in particular the structure of the dissipation region is controlled by the ion inertial length c/ωpi and not the ion Larmor radius based on the incoming ion temperature.

Electron Attachment Energies of the DNA Bases

Abstract: Injection of electrons into the empty π* molecular orbitals of uracil and the DNA bases creates short-lived anion states whose energies have been determined by electron scattering. A common range of attachment energies into the lowest orbitals is observed in all the bases. Evidence for nuclear motion during the lifetimes of the anions is found in all the compounds except adenine. These properties of the bases as bridge sites along the π-stack of DNA, namely, the effective degeneracy of the anion energies and the strong excitation of vibration, are key parameters for theories of electron-transfer rate, some of which lead to inverse rather than exponentially decreasing bridge-length dependences.

Three-Dimensional Ion Mobility/TOFMS Analysis of Electrosprayed Biomolecules

Abstract: An ion mobility/mass spectrometry technique has been developed to record mass-resolved ion mobility distributions for multiple ions simultaneously. The approach involves a new instrument that couples an electrospray ion source to an injected-ion drift tube/time-of-flight mass spectrometer. Individual components in a mixture of ions are separated by mobility differences in a drift tube and subsequently dispersed by mass-to-charge ratios in a time-of-flight instrument. Flight times in the mass spectrometer are much shorter than residence times in the drift tube, making it possible to record mass-resolved ion mobilities for all ions simultaneously. The result is a three-dimensional spectrum that contains collision cross section, mass-to-charge, and ion abundance information. The instrument and data acquisition system are described. Examples of combined ion mobility/time-of-flight data are presented for distributions of electrosprayed bradykinin and ubiquitin ions.

Precision and accuracy in isotope ratio measurements by plasma source mass spectrometry

Abstract: Precise and accurate isotope ratio measurements are an important task in many applications such as isotope dilution mass spectrometry, bioavailability studies or the determination of isotope variations in geological and cosmic samples. There is much more interest in ICP-MS for isotope ratio determinations at present compared with GDMS, which is preferred for the direct measurement of the isotopic composition of metallic solid samples. Spectroscopic interferences and a limited abundance sensitivity can influence the accuracy of isotope ratio determinations by GDMS and ICP-MS. In addition, in ICP-MS the space charge effect always influences the accuracy and a nozzle separation effect may also contribute to the total mass bias. Using a quadrupole ICP-MS the mass discrimination per mass unit can be >10% for elements with mass numbers <10, about 1–5% for mass numbers in the range of 20–120 and only <1% for heavier elements. Mass discrimination is strongly dependent on the potential of the different lenses of the ion optics and on the nebulizer gas flow. Even in magnetic sector field ICP-MS instruments a distinct mass discrimination is observed. Different procedures such as calibration by substances of consistent natural isotopic composition of the same or a neighbouring element and by isotopic standard reference materials, respectively, in combination with various mathematical functions, linear and exponential ones, are used for mass bias correction. In addition to the ion counting statistics, stability of the ion current is one of the most important topics which influences the precision of isotope ratio determinations. Magnetic sector field instruments, producing flat topped peak shapes, coupled with a multi-collector system for simultaneous measurement of different isotopes achieve the best relative standard deviation in the range of typically 0.005–0.02%, which is only comparable with precisions obtained by TIMS. However, ICP-TOFMS also has the potential for similar results. Typical relative standard deviations for other types of plasma source mass spectrometers for isotope ratio determinations are as follows: GDMS 0.1–1, quadrupole ICP-MS 0.1–0.5, and high resolution ICP-MS 0.05–0.2%.

Ultraviolet Photoelectron Spectroscopy of the o-, m-, and p- Benzyne Negative Ions. Electron Affinities and Singlet−Triplet Splittings for o -, m -, and p -Benzyne

Abstract: The 351 nm photoelectron spectra of the negative ions of o-, m-, and p-benzyne (1,2-, 1,3-, and 1,4-dehydrobenzene, respectively) and their perdeuterated isotopomers have been obtained. The o-benzyne ions were generated by the reaction of benzene and benzene-d6 with O-, while the m- and p-benzyne ions were prepared by the gas-phase reaction between the corresponding 3- and 4-(trimethylsilyl)phenyl anions and molecular fluorine, F2. The photoelectron spectra of the benzyne anions each contain two features, corresponding to formation of the singlet and triplet states of the biradicals. The electron affinities of o- and p-benzyne are found to be 0.564 ± 0.007 and 1.265 ± 0.008 eV, respectively, while the electron affinities of deuterated o- and p-benzyne are found to be 8 and 5 meV lower, respectively. The electron affinity of m-benzyne could not be determined from the photoelectron spectrum because the origin peak could not be assigned unequivocally. For o- and p-benzyne, the singlet−triplet energy splittin...

Impedance Study on the Electrochemical Lithium Intercalation into Natural Graphite Powder

Abstract: Electrochemical lithium intercalation into natural graphite powder of different sizes was studied by alternating current impedance spectroscopy. Impedance spectra at various potentials were fitted with a modified Randles equivalent circuit including a pseudocapacitance to express the observed finite diffusional behavior. The variations of electrochemical parameters with electrode potential, such as the charge-transfer resistance, the pseudocapacitance, the Warburg prefactor, and, finally, the chemical diffusion coefficient of lithium ion within graphite, were evaluated and discussed. It was shown that the charge-transfer reaction takes place on the whole surface of graphite particles, whereas lithium ion is intercalated from the edge plane and diffuses to the interior. The kinetics of the charge-transfer reaction was independent of the structure of the host. In contrast, the diffusivity of lithium ion within graphite was strongly dependent on the host structure, and the dependence was explained in terms of differences in in-plane and stacking order of lithium-graphite intercalation compounds formed by the intercalation.

The Fast Auroral SnapshoT (FAST) Mission

Abstract: The FAST satellite mission investigates plasma processes occurring in the low altitude auroral acceleration region, where magnetic field-aligned currents couple global magnetospheric current systems to the high latitude ionosphere. In the transition region between the hot tenuous magnetospheric plasma and the cold, dense ionosphere, these currents give rise to parallel electric fields, particle beams, plasma heating, and a host of wave-particle interactions. FAST instruments provide observations of plasma particles and fields in this region, with excellent temporal and spatial resolution combined with high quantitative accuracy. The spacecraft data system performs on-board evaluation of the measurements to select data “snapshots” that are stored for later transmission to the ground. New measurements from FAST show that upward and downward current regions in the auroral zone have complementary field and particle features defined by upward and downward directed parallel electric field structures and corresponding electron and ion beams. Direct measurements of wave particle interactions have led to several discoveries, including Debye-scale electric solitary waves associated with the acceleration of upgoing electron beams and ion heating, and the identification of electrons modulated by ion cyclotron waves as the source of flickering aurora. Detailed quantitative measurements of plasma density, plasma waves, and electron distributions associated with auroral kilometric radiation source regions yield a consistent explanation for AKR wave generation.

Calculation of the aqueous solvation free energy of the proton

Abstract: The value of the proton hydration free energy, ΔGhyd(H+), has been quoted in the literature to be from −252.6 to −262.5 kcal/mol. In this article, we present a theoretical model for calculating the hydration free energy of ions in aqueous solvent and use this model to calculate the proton hydration free energy, ΔGhyd(H+), in an effort to resolve the uncertainty concerning its exact value. In the model we define ΔGhyd(H+) as the free energy change associated with the following process: ΔG[H+(gas)+H2nOn(aq)→H+(H2nOn)(aq)], where the solvent is represented by a neutral n-water cluster embedded in a dielectric continuum and the solvated proton is represented by a protonated n-water cluster also in the continuum. All solvated species are treated as quantum mechanical solutes coupled to a dielectric continuum using a self consistent reaction field cycle. We investigated the behavior of ΔGhyd(H+) as the number of explicit waters of hydration is increased from n=1 to n=6. As n increases from 1 to 3, the hydration free energy decreases dramatically. However, for n=4–6 the hydration free energy maintains a relatively constant value of −262.23 kcal/mol. These results indicate that the first hydration shell of the proton is composed of at least four water molecules. The constant value of the hydration free energy for n⩾4 strongly suggests that the proton hydration free energy is at the far lower end of the range of values that have been proposed in the literature.

Explosion Dynamics of Rare Gas Clusters in Strong Laser Fields

Abstract: We have studied the ionic outcome from the interaction of intense laser light with large argon and xenon clusters. Ions with initial energies of several 100 keV are charge and energy selected using a magnetic deflection time-of-flight mass spectrometer. For argon clusters, Coulomb repulsion is the key process in the explosion mechanism, whereas for xenon we observe a mixture of Coulomb repulsion and hydrodynamic expansion. Coulomb explosion is the preferred decay channel for smaller clusters and it is also responsible for the production of the most energetic ions. Our results can be understood on the basis of a charged sphere model of cluster-sized plasmas.

Programmable sub-surface aggregating metallization structure and method of making same

Abstract: A programmable sub-surface aggregating metallization structure (100) includes an ion conductor (110) such as a chalcogenide glass which includes metal ions and at least two electrodes (120, 130) disposed at opposing surfaces of the ion conductor (110). Preferably, the ion conductor (110) includes a chalcogenide material with Group IB or Group IIB metals. One of the two electrodes (120, 130) is preferably configured as a cathode and the other as an anode. When a voltage is applied to between the anode and cathode, a metal dendrite (140) grows from the cathode through the ion conductor (11) toward the anode. The grow rate of the dendrite may be stopped by removing the voltage or the dendrite may be retracted back toward the cathode by reversing the voltage polarity at the anode and the cathode. When a voltage is applied for a sufficient length of time, a continuous metal dendrite grows through the ion conductor (110) and connects the electrodes (120, 130), thereby shorting the device. The continuous metal dendrite then can be broken by applying another voltage.

Solvation Free Energy of Biomacromolecules: Parameters for a Modified Generalized Born Model Consistent with the AMBER Force Field

Abstract: The generalized Born (GB) model provides rapid estimates of the electrostatic free energies of solvation for diverse molecules and molecular ions. This method is expected to be of considerable utility for studies of solvation in macromolecular and biological systems. Calculations on biological molecules are typically based on empirical energy functions, each of which have their own prescriptions for determining net atomic charges. For maximum compatibility, GB parameters tailored to specific force fields are required. The development of parameters compatible with the AMBER force field is described. The method is used to estimate free energies of A and B form structures of DNA obtained from molecular dynamics simulations. The results provide an account of the conformational preferences of right-handed DNA in solution.