Charged Negative Ions Research Articles

Critical nuclear charges forN-electron atoms

One-particle model with a spherically-symmetric screened Coulomb potential is proposed to describe the motion of a loosely bound electron in a multielectron atom when the nuclear charge, which is treated as a continuous parameter, approaches its critical value. The critical nuclear charge, Z , is the minimum charge necessary to bind c N electrons. Parameters of the model are chosen to meet known binding energies of the neutral atom and the isoelectronic negative ion. This model correctly describes the asymptotic behavior of the binding energy in the vicinity of the critical charge, gives accurate estimation of the critical charges in comparison with ab initio calculations for small atoms, and is in full agreement with the prediction of the statistical theory of large atoms. Our results rule out the stability of doubly charged atomic negative ions in the gas phase. Moreover, the critical charge obeys the proposed inequality, N y 2 F Z F c N y 1. We show that in the presence of a strong magnetic field many atomic dianions become stable. Q 1999 John Wiley & Sons, Inc. Int J Quant Chem 75: 533)542, 1999

Unfolding of proteins monitored by electrospray ionization mass spectrometry: a comparison of positive and negative ion modes

Electrospray ionization (ESI) mass spectrometry (MS) in both the positive and negative ion mode has been used to study protein unfolding transitions of lysozyme, cytochrome c (cyt c), and ubiquitin in solution. As expected, ESI of unfolded lysozyme leads to the formation of substantially higher charge states than the tightly folded protein in both modes of operation. Surprisingly, the acid-induced unfolding of cyt c as well as the acid and the base-induced unfolding of ubiquitin show different behavior: In these three cases protein unfolding only leads to marginal changes in the negative ion charge state distributions, whereas in the positive ion mode pronounced shifts to higher charge states are observed. This shows that ESI MS in the negative ion mode as a method for probing conformational changes of proteins in solution should be treated with caution. The data presented in this work provide further evidence that the conformation of a protein in solution not its charge state is the predominant factor for determining the ESI charge state distribution in the positive ion mode. Furthermore, these data support the hypothesis of a recent study (Konermann and Douglas, Biochemistry 1997, 36, 12296–12302) which suggested that ESI in the positive ion mode is not sensitive to changes in the secondary structure of proteins but only to changes in the tertiary structure.

‘Ascospore O’, oxidant, host assistance: the pathway to carcinogenesis

Sufficient evidence exists to conclude that a cycle of sequential physiological events is the pathway contributing to the transformation of the mammalian cell to its malignant phase. The cycle of events involves a reticulum or network of cells. These consist either of the reticuloendothelial cells, the squamous, or the epithelial group. A second factor requires the presence, intracellularly, of conidia derived from one of the various groups of the anaerobic micro-organisms known as Ascomycetes. These conidia as a separate entity, totally free of any part of their parental origin, constitute the invasive agent which, however, can or may lie quiescent at the onset. They retain genetically an electronegative viable chemical complex (Oxidant) and also an asexual procreative unit. This Oxidant can alternately accept the negative charged ions (anions) while releasing oxygen. The subsequent ensuing circulation and compatible flow of blood by the host provides the source for the electrolytes as well as the metabolites, enzymes, etc. It is this continued flow of blood that promotes the next stage of the conidia's lifecycle and reactivates their anaerobic respiration, metabolism, and reproduction of new asexual conidia. This is the cycle of physiological events that ultimately contributes to the transformation of an animal (human) cell to its malignant phase. It also accounts for the pathological consequences as clinically present in malignantly ill patients.

Search for doubly charged negative ions of small carbon clusters

A search has been conducted for dianions of small carbon clusters C n 2− (2 ≤ n ≤ 8) formed by laser ablation of graphite. A multicoincidence time-of-flight technique has been used to monitor fragment ion-pairs produced in dianion decay. No evidence is found for C 2 2− dianions. However, detection of anion-pairs C n −−C n − ( n > 1) provides evidence for higher- n ( n > 2) cluster dianions which exhibit a preference for symmetrical decay. Ab initio computations of potential energy curves have been carried out for C 2 2− dianion states; results indicate too short a lifetime against autodetachment for detection in our experiments.

Appearance and structure of multiply charged negative ions of hydrogen in intense circularly polarized laser fields

We present a theoretical description of the appearance of bound states of atomic multiply charged negative ions of hydrogen in circularly polarized laser fields, using high-frequency Floquet theory (HFFT). Monochromatic circularly polarized fields with frequencies large compared to the binding energy of the dressed ion can (founded by HFFT) induce doubly and triply charged negative ions of hydrogen. The field conditions required to induce these ions are within experimental reach. A circularly polarized field with a wavelength $\ensuremath{\lambda}=456\mathrm{nm}$ and an intensity $I=3.0\ifmmode\times\else\texttimes\fi{}{10}^{17}{\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$ can induce the appearance of a bound state of ${\mathrm{H}}^{2\mathrm{\ensuremath{-}}},$ whereas a bound state of ${\mathrm{H}}^{3\mathrm{\ensuremath{-}}}$ can be induced by a field with wavelength $\ensuremath{\lambda}=45.6\ensuremath{\mu}\mathrm{m}$ and intensity $I=5.1\ifmmode\times\else\texttimes\fi{}{10}^{13}{\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}.$ We also show that a relativistic description or a low-frequency theory is required in order to determine the appearance of bound states of more highly charged negative ions.

Multiply charged negative ions of hydrogen in linearly polarized laser fields

Motivated by the prediction of the appearance of atomic multiply charged negative ions (AMCNI) of hydrogen, induced by a linearly polarized laser field, we present an analytical quantum mechanical treatment of the appearance and structure of AMCNI in a linearly polarized field, based on high-frequency Floquet theory (HFFT). For the simplest AMCNI of hydrogen, ${\mathrm{H}}^{2\mathrm{\ensuremath{-}}}$ and ${\mathrm{H}}^{3\mathrm{\ensuremath{-}}},$ the values of ${\ensuremath{\alpha}}_{0}$ at which the first bound state appears are ${\ensuremath{\alpha}}_{0}=1.62\ifmmode\times\else\texttimes\fi{}{10}^{2}$ and ${\ensuremath{\alpha}}_{0}=1.02\ifmmode\times\else\texttimes\fi{}{10}^{4},$ where ${\ensuremath{\alpha}}_{0}{=I}^{1/2}/{\ensuremath{\omega}}^{2}$ is the amplitude of the oscillation of a free electron in the field with frequency \ensuremath{\omega} and intensity $I$ (unless stated otherwise, we use atomic units throughout this paper). Whereas in vacuum at least one of the electrons of an AMCNI autodetaches, an intense high-frequency field can change the character of the ion dramatically, such that bound states of AMCNI can appear. Due to the interaction with the field, the electrons of the AMCNI oscillate in phase along the polarization axis. This ``quiver'' motion enables the electrons to be spatially separated over distances of order ${\ensuremath{\alpha}}_{0},$ reducing the repulsive $e\ensuremath{-}e$ interaction as ${\ensuremath{\alpha}}_{0}$ increases. In other words, for ${\ensuremath{\alpha}}_{0}$ large enough, the field enables a configuration in which the electrons, while widely separated, are bound to one proton. For the prediction of bound states of ${\mathrm{H}}^{N\ensuremath{-}}$ with $N>3,$ however, a relativistic description or low-frequency theory is required.

Multiply charged direct recoil spectra from Ar + and Kr + bombarded graphite

Measurements of the energy spectra of multiply charged positive and negative carbon ions (C n ± ) recoiling from graphite surface under 100 and 150 keV argon and krypton ion bombardment are presented. With the energy spectrometer set at recoil angle of 79.5°, direct recoil (DR) peaks have been observed with singly as well as multiply charged carbon ions C n ± , where n = 1 to 6. These C n ± ions have been seen recoiling with the characteristic recoil energy E DR = kE 0cos 2 θ DR, where θ DR is the direct recoil angle, k = 4m 1m 2 (m 1 + m 2) 2 , m 1 and E 0 are projectile mass and energy and m 2 is carbon mass. We have observed sharp DR peaks. A collimated projectile beam with divergence ∼ ±0.2° is supplemented with a similar collimation before the energy analyzer to reduce the background of sputtered ions due to scattered projectiles.

LINAC for charge-symmetrical four-isotopic heavy-ion driver

A linear accelerator (linac) for single charged (positive and negative) ions of the four various Pt isotopes has been proposed. Eight beams of different charges and masses of ions are accelerated in parallel RFQ channels to an energy of 100 MeV. The beams are then brought together by a system of alternating gradient magnets for a 180° bending and matching of the beams. The main channel that accelerates all the beams together consists of three stages. The first stage (until 600 MeV) is a Wideroe structure, followed by two consecutive Alvarex channels (2.5 GeV and 10 GeV) that have different radio frequencies. The characteristics of the output beam for each kind of ions are as follows: average pulse current, 130 mA; horizontal emittance, 0.67π cm mrad; vertical emittance, 0.47π cm mrad; momentum spread, ±07%, bunch length, 3.6 cm; spacing between bunches of each kind, 15.3 m.

Multiply Charged Negative Ions of Hydrogen Induced by Superintense Laser Fields.

Using the high-frequency Floquet theory of laser-atom interactions we predict that in superintense fields a proton can bind more than two electrons. For linear polarization, a self-consistent solution of the structure equation shows that H{sup 2{minus}} becomes bound for {alpha}{sub 0}=155 a.u. The fields and frequencies needed for binding H{sup 2{minus}} are within reach of lasers under construction. By choosing appropriately polarized bichromatic radiation, the production of more highly charged negative ions also lies within experimental reach. The ions are (adiabatically) stabilized against ionization, from their very appearance. {copyright} {ital 1996 The American Physical Society.}

Variation of the ground-state correlation energy of atoms with atomic number

The empirical correlation energy of atoms and singly charged positive and negative ions in their ground state shows a roughly linear dependence with respect to the number of electrons. However, the deviations from this linear behaviour follow a well defined trend. A formulation based on density functional theory allows us to understand these features. This formulation focuses on the number of pairings between antiparallel-spin electrons in the same main shell.

Electron-impact detachment from negative ions.

Electron-impact detachment cross sections have been measured from threshold to about 30 eV for ${\mathrm{D}}^{\mathrm{\ensuremath{-}}}$ and ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$. The purpose was to investigate the cross section near threshold and to reinvestigate earlier claims of resonances due to short-lived states of the doubly charged negative ion. Initial results for ${\mathrm{D}}^{\mathrm{\ensuremath{-}}}$ showed no resonances [L. H. Andersen, D. Mathur, H. T. Schmidt, and L. Vejby-Christensen, Phys. Rev. Lett. 74, 892 (1995)]. Here we show that there is no detectable resonance structure for ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ either. The applied experimental technique is discussed in detail. Classical and semiclassical models for the detachment process are presented and compared with the data. \textcopyright{} 1996 The American Physical Society.

Chemistry of Multiply Charged Negative Molecular Ions and Clusters in the Gas Phase: Terrestrial and in Intense Galactic Magnetic Fields

A review is first given of presently available techniques for generating and observing long-lived dianions of molecules and of clusters in the gas phase. Multiply charged anions that have been unambiguously observed are listed with the technique used to detect them. The remainder of the article is then devoted to the way quantum chemistry can contribute to the continuing search for multiply charged negative ions. This discussion is divided into two parts: (i) ions in zero magnetic fields and (ii) ions in intense magnetic fields. In (i) it is difficult to bind even two electrons to a heavy neutral atom. In (ii), however, a heavy atom with atomic number Z can bind an additional Z electrons, the binding energy of these additional electrons being on the same order as that of the first Z electrons, in the so-called “hyperstrong” field regime. Diatomic homonuclear molecules have binding energies on the order of the separated atom energies. As there should be precursor effects, it is proposed that one should at...

Gas-Phase Multiply Charged Anions

Singly charged negative ions in the gas phase have attracted considerable experimental and theoretical attention over the past decades. However, the existence of free doubly or multiply charged negative ions, in particular those of small systems, has remained a curiosity and a matter of some controversy. Recent experimental and quantum mechanical studies show that multiply charged negative ions of small molecules and clusters can exist as isolated entities.

Mechanistic Interpretation of the Dependence of Charge State Distributions on Analyte Concentrations in Electrospray Ionization Mass Spectrometry

The effect of analyte concentrations upon charge state distributions in electrospray ionization mass spectrometry (ESMS) was investigated using four compounds which showed doubly charged and singly charged positive ions (gramicidin S, 4,4'-bipiperidine), multiply charged negative ions (Trypan Blue), or doubly charged and singly charged negative ions (eosin Y) in ES mass spectra. For each of the above compounds, the charge associated with analyte molecules was calculated to increase in solution with increasing concentration, yet the analyte charge state distributions in ES mass spectra were observed to monotonically shift toward lower values. In order to rationalize these observations, we introduce the ratio N/N o , the ratio of the number of excess charges on all droplets produced by electrospray (N) to the total number of analyte molecules in all droplets (N o ). For each compound, the N/N o value decreases with increasing analyte concentration. The lower N/N o value is proposed to be an underlying factor critical to explaining the desorption of a higher proportion of gas-phase ions of lower charge state at elevated analyte concentrations. We propose that the lower N/N o value is indicative of a decreased efficiency of analyte charging and an increased level of ion pairing of charged analyte molecules with available counterions. Furthermore, in comparing N/N o values and observed analyte charge state distributions as functions of increasing analyte concentration, the decreasing N/N o ratio predicts a much greater extent of shifting of analyte charge states toward lower values than was observed. This implies that the actual degree of ion pairing in droplets was considerably alleviated as compared to that indicated by the calculated N/N o values. This finding conforms to a description of the ES process wherein uneven fission of droplets occurs at the Rayleigh limit, thereby generating offspring droplets of higher charge to mass ratio, hence an augmented N/N o value.

Kohn-Sham theory for orbital dependent exchange-correlation energy functionals: Application to the calculation of ionization potentials and electron affinities

Available approximations to Exc[{nσ}], the exchange-correlation energy functional of spin-density-functional theory, do not include self-interaction corrections (SIC). This leads to Kohn–Sham (KS) potentials, Vxcσ, that fail to satisfy some important analytic properties known to be exhibited by the exact potential. To resolve these difficulties, we consider a KS theory for orbital-dependent exchange-correlation energy functionals that explicitly includes SIC. Recent work by Krieger, Li, and lafrate (KLI), which considers the analytic properties of the spin-polarized optimized effective potentials (OEP), V, i.e., the KS potentials corresponding to Exc = Exc[{ϕiσ,}], is reviewed as well as the properties of VxcσKLI, an easily calculated approximation to the exact result which, unlike Vxcσ0, can be employed for systems of arbitrary symmetry. In addition, we compare the results of the exact and approximate OEP calculation of the properties of the ground state of atoms and singly charged negative ions in the exchange-only case in which Ex = ExHF [{ϕiσ}] where HF = Hartree–Fock. We conclude that V maintains most of the important analytic properties of V, and provides an excellent numerical approximation to the exact result. We also give detailed consideration to the calculation of the ionization potential, I, and the electron affinity, A, in the exchange-only approximation for atoms with Z ≤ 20. We find that the KLI results for both I and A are always within 0.1 milli-au of the exact KS results, whereas both the local spin density (LSD) approximation and the Becke exchange only energy functional lead to deviations which on average are two orders of magnitude larger and significantly exceed the criterion for quantum chemistry accuracy. Finally, using the KLI method for orbital dependent Exc, we compare the KS results for I and A for Z ≤ 20 with the experimental values by employing various approximations for Exc[{ϕiσ] including: (1) HF exchange with LSD correlation with SIC, (2) LSD approximation for exchange and correlation with SIC, (3) the conventional spin density LSD approximation, and (4) the Becke exchange-energy functional with LSD correlation. In addition, we examine how closely the ionization theorem for ∈m, the energy eigenvalue for the highest occupied orbital, is satisfied in these approximations. © 1995 John Wiley & Sons, Inc.

Determination of the electron affinities of fluorinated fullerenes (C 60F 44,46, C 70F 52,54) by Fourier transform mass spectrometry

Negative ion charge exchange reactions in a Fourier transform mass spectrometer were used to “bracket” the electron affinities of the fluorinated fullerenes C 60F x ( x = 44,46) and C 70F y ( y = 52,54). By monitoring the occurrence of charge exchange reactions between negative ions and gas phase neutrals of fluorinated fullerenes and a variety of reagents, electron affinities were determined to be EA(C 60F 44,46) = 4.06±0.25 eV and EA(C 70F 52,54) = 4.06±0.25 eV. These results indicate that fluorination at this level increases the electron affinities of these fullerene derivatives by almost 1.5 eV. It was also determined that the electron affinity of C 70F y ( y = 52, 54) is slightly greater than that for C 60F x ( x = 44, 46), which may be related to the higher number of substituted fluorines and/or the fact that the EA(C 70) is also known to be greater than EA(C 60)

Energy-density relationships for the treatment of ion solvation within density-functional theory.

Useful energy-density relationships, connected with the embedding of singly charged postive or negative atomic ions in polar solvents, are developed. The insertion of the atomic charged system into the polarizable host is modeled through successive isoelectronic processes at the nucleus, involving a varying nuclear charge. In this way, the controversial procedure of selecting appropriate ionic radii, involved in the calculation of solvation energies through the Born formula, is avoided and replaced by integration in [0,\ensuremath{\infty}]. The approximate expressions, derived from a variational procedure proposed by Levy [J. Chem. Phys. 68, 5298 (1978); 70, 1573 (1979)], are reformulated within the nuclear-transition-state model. The classical reaction field expression for the insertion energy is recovered. The quality of the approximations made are discussed within the frame of the Kohn-Sham formulation of density-functional theory.

Stabilized-jellium description of neutral and multiply charged fullerenes C x± 60

A description of neutral and multiply charged fullerenes is proposed based on a stabilized jellium (structureless pseudopotential) approximation for the ionic background and the local density approximation for the σ and π valence electrons. A recently developed shell-correction method is used to calculate total energies and properties of both the neutral and multiply charged anionic and cationic fullerenes. The effect of the icosahedral symmetry is included perturbatively. The calculated single-particle energy level spectrum of C 60 is in good correspondence with experimentally measured ones and previous self-consistent local-density-approximation calculations. For the multiply charged fullerenes, we calculate microscopically the charging energies of C x± 60 for up to x=12 excess charges. A semiclassical interpretation of these results is developed, which views the fullerenes as Coulomb islands possessing a classical capacitance. The calculated values for the first ionization potential and the first electron affinity agree well with the experimental ones. For the second and third ionization potentials, there exist substantial discrepancies in the experimental measurements. Our calculations support the results from charge transfer bracketing experiments and from direct ionization experiments through electron impact. The doubly charged negative ion is found to be very long-lived metastable species, in agreement with observations.

Shell-correction method for calculating the binding energy of metal clusters: Application to multiply charged anions.

Utilizing the nuclear-physics concept of separating, as a function of size, the total energy of a finite system into two parts, a [ital smooth] contribution and a shell correction, we introduce a method of calculating the binding energy of metal clusters. The method consists of a determination of the smooth part of the total energy from an extended Thomas Fermi approach to density-functional theory, and of superimposing on it a shell correction introduced through the kinetic-energy contribution. While circumventing a self-consistent iterative solution of the effective single-particle Kohn-Sham equations, the present method yields results in excellent agreement with such self-consistent calculations, but with considerable savings in computation time, thus allowing for an efficient approach for accurate systematic investigations of cluster properties for a wide range of sizes. As an application of the method, we study energetics and decay modes of multiply charged anionic metal clusters. Singly charged anions are stable for all sizes, but multiply charged negative ions are stable against spontaneous electron decay only above certain critical sizes. Below the border of stability, the cluster anions are metastable against electron tunneling through a Coulombic barrier. Lifetimes for such decay processes are estimated. Fission channels, which may compete with electronmore » autodetachment, are studied for the case of doubly charged anions.« less

A search for doubly charged negative cluster ions in sputtering

The emission of negative cluster ions sputtered under cesium impact from various elemental and multicomponent specimens (B, C, Al, Si, Cu, LiF, MgF 2, CdS, and InP) was investigated by means of secondary-ion mass spectrometry. In all cases abundant yields of singly charged cluster ions were observed in the mass range covered by the instrument ( ≤ 280 amu). On the other hand, doubly charged negative clusters could be detected only for carbon, C 7 2− being the smallest one, in agreement with previous observations by Schauer et al. [Phys. Rev. Lett. 65 (1990) 625]. The intensities of C n 2− ions exhibit odd-even oscillations and even-numbered clusters are more intense than the neighboring odd-numbered ones. Apart from abundance distributions, energy spectra were recorded for selected species. These data revealed the existence of fragmentation processes which dissipate the clusters' internal energy and lead to more stable daughter ions by the evaporation of atomic or diatomic species.