Effect Of Charge State Research Articles

The effect of charge state and the localization of charge on the collision-induced dissociation of peptide ions

The effect that charge state has on the collision-induced dissociation (CID) of peptide ions is examined in detail for several representative peptides under high-energy collision conditions. The CID spectra of singly and doubly charged precursor ions (generated by fast-atom bombardment and electrospray ionization, respectively) are compared for several peptides with similar primary structure. It is shown that for peptides that contain highly basic amino acids, the dissociation of doubly charged ions is strongly influenced by the position of these residues within the peptide and the general observations reported concerning the dissociation of singly charged ions can be extended to precursors with higher charge states. Based on the dissociation behavior of the doubly charged ions of these peptides, it is demonstrated that two charges can reside in close proximity in the precursor ions, overcoming possible repulsion effects, when favored by a high concentration of basic sites. In addition, this work illustrates that in the case of doubly charged ions, the charge state of some fragment ions can be determined directly from the mass-to-charge ratio assignments of the CID spectrum.

Binary collision simulations of ion transmission through silicon single crystal films

Abstract Computer simulation studies of the energy distribution of transmitted ions such as alpha-particles, He-, and B-ions through crystalline silicon, using the enhanced binary-collision cascade simulator MARLOWE, will be reviewed. The enhancement includes an additional electronic-energy loss (EEL) model which takes into account explicitly both the target electron density variation via the structure factors and the electron density of the projectile. Investigations of the stopping power for He ions and protons in silicon, at intermediate- and high-energies, based on the adapted EEL model and a velocity-dependent effective charge will be presented. The overall agreement between the calculated and experimentally determined stopping power data and the simulated and measured transmission spectra will be demonstrated. Effects of energy-loss straggling, core-electron contribution to the energy loss at high-energies and charge-state effects at low energies on the transmission spectra will also be discussed.

Stability of oblique modulation of ion-acoustic waves in a multicomponent plasma

The stability of oblique modulation of ion-acoustic waves in a collisionless plasma consisting of two cold-ion species with different masses, concentrations, and charge states, and hot isothermal electrons is studied. Using the Krylov–Bogoliubov–Mitropolosky (KBM) perturbation technique, a nonlinear Schrödinger equation governing the slow modulation of the wave amplitude, is derived for the system. It is found that the presence of second-ion species significantly changes the instability domain in the k-φ plane. The effect of charge state, concentration, and mass of second-ion species on the modulational instability is discussed in detail. The predictions of the theory are found to be in good agreement with the experimental observations.

Angular distribution of ions axially channeled in a very thin crystal: Experimental and theoretical results.

We have studied the angular distributions for 6--30-MeV C[sup [ital q]+] ([ital q]=4--6) and 2--9-MeV H[sup +] ions axially channeled in the [001] direction of a thin silicon crystal (1792 and 1900 A). We report highly structured two-dimensional angular distributions that depend sensitively on the projectile's velocity and incident charge state and the target's thickness and azimuthal orientation. Some structure in the angular contour maps is the result of a rainbow effect in axial channeling (i.e., extrema in the classical deflection function). State-to-state charge-state distributions, which are required to interpret the data accurately, have also been measured. All measured angular distributions have been explained via Monte Carlo trajectory calculations using Moliere's approximation to the Thomas-Fermi screening function and a screening length given by target electrons alone. The calculations indicate that all projectile velocity and charge-state effects and the target-thickness effects observed are the result of the projectile's transverse oscillatory motion in the channel. Using this information, we show that swift heavy-ion and proton angular distributions are simply related using a scaling law that depends only on the projectile's velocity and charge-to-mass ratio and on the crystal thickness.

Mixed silicon carbide clusters studied by laser ablation Fourier transform ICR mass spectrometry

AbstractIons produced by the 1064 nm Nd:YAG laser ablation of various Si–C samples include mixed silicon carbon (SixCy) cluster ions of low molecular mass. Unique SixCy cluster ion distributions are observed. A charge state effect for SixCy clusters is evident from differences between the respectively charged distributions. This may be accounted for by either differing energetics and/or differing formation mechanisms for positive and negative SixCy cluster ions. Several specific SixCy clusters of probable high stability consistently dominate the mass spectra. The chemistry of the more abundant SixCy ions was probed by collision‐induced dissociation and ion–molecule reactivity experiments. Similar SixCy+˙ dissociation results in the common loss of an Si ion. Parent SixCy ion and small neutral reagent reactants give rise to oxidation, addition and exchange reactions.

The effect of high charge states on the stopping and ranges of ions in solids

Recently, new types of ion sources have been developed which can deliver ions with extremely high charge states, for example Xe q+ with q≈30 in ECR sources, and q≈44 in an EBIT source. Therefore, it becomes important to assess the influence of high charge states on the stopping and ranges, as well as on sputtering and damage production. In a first series of experiments, the ranges of 308 keV Xe q+ , q = 1–44, in SiO 2 were measured, and shall find their interpretation in the present theoretical treatment of nuclear and electronic stopping powers.

Binary-collision-cascade simulation of hyperchanneling of high-energy helium ions in silicon.

The binary-collision-cascade code, marlowe, was modified to include an accurate electron-density distribution of silicon and an electronic-energy-loss (EEL) model that is suitable for high-energy studies. The EEL model is based on a theory that relates the energy loss and local valence-electron density via a phenomenological band-structure model. The enhanced marlowe is applied to simulate transmission spectra due to the hyperchanneling of high-energy \ensuremath{\alpha} particles in silicon along the [110] and [111] directions. The transmission spectra due to a ``random'' incident direction was also studied. The channeling peak positions in the energy-transmission spectra agree fairly well with the measurements. The effects of transverse-energy distribution of the incident beam, the effects of electron-multiple scattering and energy-loss straggling, and the effects of nonuniformity of the electron density on the transmission spectra are discussed. The electron-density effects reproduce the stopping-power measurements based on the peak and leading edge of the energy spectrum. The importance of energy-loss straggling, of core-electron contribution to the energy loss at high energies, and of charge-state effects at intermediate energies is also discussed.

Time dependence of p-MOSFET hot-carrier degradation measured and interpreted consistently over ten orders of magnitude

Hot-carrier degradation is measured and analyzed over ten orders of magnitude in time for three buried-channel p-MOSFET types with different oxide thicknesses. The effects of oxide charge and interface states are separated by using the charge-pumping technique. Two dominating effects are sufficient to account for the degradation. For worst case degradation, negative oxide charge and interface states are generated by electrons near the drain. This charge is distributed homogeneously over the oxide thickness and it attracts an inversion layer that extends the drain and reduces the effective transistor length logarithmically in time. Simultaneously, this inversion layer prevents substantial degradation related to the interface states, since it masks their effects. A simple model for the logarithmic time dependence is presented. At more negative gate voltages, holes cause interface states that reduce the transconductance with a power-law time dependence, comparable to the worst case n-MOSFET degradation. >

An impedance spectroscopy study of the anodically formed barrier layer on aluminium substrates

Al 99.996 and Al-0.4% Mn specimens were anodically oxidized in a boric acid-borate solution (pH 7.4, 15°C). The electronic properties of the thin oxide films were investigated using electrochemical impedance spectroscopy in the potential range 0.3 V ⩽ E ⩽ 1.8 V. The impedance of the system is characterized by two capacitive contributions in the frequency range 0.1 Hz ⩽ f ⩽ 50 kHz. The dynamic characteristics of the electrochemical system are discussed in terms of a model which accounts for the presence of space charge and surface state effects.

Charge-state-dependent diffusion and carrier-emission-limited drift of iron in silicon.

The behavior of iron in silicon during low-temperature annealing was studied. The influence of electric fields on the underlying defect reactions is detailed. The depth profiles reveal ${\mathrm{Fe}}_{\mathit{i}}$ outdiffusion and no precipitation in the bulk up to 470 K. In the presence of electric fields, the kinetics differ considerably and can be understood in terms of carrier-emission-limited iron drift. Consequences of this mechanism are discussed. The outdiffusion and drift data indicate a charge-state-dependent diffusion mechanism, in contrast to the generally accepted lack of any charge-state effect.

Internal friction and symmetry of intrinsic point defects in GaAs.

To study the structure and symmetry of intrinsic point defects in GaAs, internal-friction measurements on n-type single crystals of different doping levels were performed in the range 80--440 K for a frequency of about 105 kHz. Actually, we detected damping peaks due to relaxation of point defects which are shown to be of 〈110〉 orthorhombic symmetry. The present analysis of the peak spectra supports an explanation in terms of midgap defect reorientations where charge-state effects seem to play a central role. The determination of the kinetic parameters of the relaxations yields activation energies of 0.6--0.8 eV for the ${\ensuremath{\nu}}_{13}$ and 0.85--1 eV for the ${\ensuremath{\nu}}_{12}$ reorientation. Remarkably large ${\mathrm{\ensuremath{\tau}}}_{0}^{\mathrm{\ensuremath{-}}1}$ values of ${10}^{18}$ ${\mathrm{sec}}^{\mathrm{\ensuremath{-}}1}$ are found for the latter. They are attributed to activation entropies of the order 10k.

Response to: ‘‘Comment on ‘The effect of charge state on the local vibrational mode absorption of the carbon acceptor in semi-insulating GaAs’ ’’

In a previous report we showed the effect of photoquenching the EL2 donor in GaAs on the carbon acceptor electronic absorption and local vibrational mode (LVM) absorption spectra. We agree that for the sample used to illustrate these effects, only a small portion (9% or less) of the total carbon was neutralized by the photoquench. Other samples have been studied, however, in which a much larger fraction (30% or more) of the carbon has been neutralized with no effect on the carbon LVM. We therefore maintain that our previous conclusion, i.e., a change in the charge state of carbon has no significant effect on the LVM structure, is still valid.

Comment on: ‘‘The effect of charge state on the local vibrational mode absorption of the carbon acceptor in semi-insulating GaAs’’

A recent study of the effect of photoquenching of EL2 on the compensation of carbon acceptors in GaAs and the resulting effect on the carbon local vibrational mode, LVM, concluded that there was no broadening of the LVM associated with bound holes on neutral carbon acceptors. The failure to observe broadening of the LVM in this study was a result of the failure to neutralize a significant fraction of the carbon atoms by photoquenching EL2. We show that, in the experiment reported, at most only about 3%–4% of the carbon acceptors were neutralized. Such very small modifications of the ionized carbon concentration are unobservable in the LVM spectra unless the spectral stability and signal-to-noise ratio are exceptional.

Sulconazole reactions with peracetic acid and hydrogen peroxide

Sulconazole (1-[2-{[4-chlorophenyl)methyl]thio}-2-(2,4-dichlorophenyl)ethyl]-1H-imidazole) is one member of a family of antimycotic drugs. Because oxidative degradation frequently limits the shelf lives of sulfur-containing drugs, we investigated the oxidation kinetics of sulconazole and sulconazole sulfoxide in aqueous peracetic acid and hydrogen peroxide. The oxidations were studied as a function of pH (7–13) and of the concentration of peracetic acid (0.025–25 mM) and of hydrogen peroxide (100–1000 mM). Our objective was to probe the effects of peroxide structure and charge state on sulconazole loss rates and product distributions. In the presence of peroxyacetic acid or hydrogen peroxide, sulconazole converts quantitatively to the corresponding sulfoxide and sulfone. Solution pH strongly influences the overall product distribution at intermediate extents of conversion. Thus, alkaline pH favors sulfone accumulation whereas sulfoxide predominates in acidic solution. The observed relative reactivities toward sulconazole are: peroxyacetic acid (105), peroxyacetate anion (103), hydrogen peroxide (1), and hydroperoxy anion (1). Transition state intramolecular proton transfer remains the most straightforward explanation for the dependence of sulconazole oxidation rates on peroxide structure and charge state.

On shell effects of ionic charge states in solids

Current models describing the ionic charge states in solids have been reconsidered by noticing the shell effect of ionic charge distributions observed behind a solid target. For this purpose, the source of the shell effect has been investigated by performing Monte Carlo calculations. As a result, the shell effect, which originated inside the foil, has been found to be preserved outside the foil, which leads to the argument that the post-foil Auger electron emission plays only a minor role in the explanation of the difference of heavy ion mean charges between solid and gaseous media. It is concluded that the Betz-Grodzins (BG) model representing the density effect of ionic charge states is not adequate for most of the collisions.

Electrical conductivity and thermodynamic functions of weakly nonideal plasma

The effect of partial screening due to electric microfields within the shielding cloud is examined and used as the basis for determination of electrical conductivity and certain thermodynamic functions of weakly nonideal plasma. A modified screening radius (non-Debye radius) that is greater than the Debye radius is used to take into account short-range Debye–Huckel-type interactions. The evaluation of nonideal plasma electrical conductivity includes corrections to the Spitzer model and accounts for localization of low-energy electrons in the continuum due to incomplete screening. Excellent argeement with experimental results is obtained in the weakly nonideal plasma regime. Corrections to certain thermodynamic functions, such as, chemical potential and internal energy, are evaluated using numerical methods. The effect of charge state on transport coefficients and thermodynamic functions is also analyzed.

The effect of charge state on the local vibrational mode absorption of the carbon acceptor in semi-insulating GaAs

We use infrared absorption spectroscopy to study both the electronic excitation and the local vibrational mode (LVM) of the carbon acceptor in semi-insulating GaAs as a function of EL2 photoquenching. The behavior of the far-infrared electronic lines shows that the normally compensated carbon becomes neutralized (i.e., changes its charge state from singly negative to neutral) when the EL2 absorption is photoquenched. At the same time, the LVM absorption band shows no change in spectral form or vibrational frequency of its fine structure after photoquenching which indicates that, contrary to expectations, a charge-state change has no measurable effect on this band.

Effects of peptide hydrophobicity and charge state on molecular ion yields in plasma desorption mass spectrometry

Plasma desorption mass spectra were obtained for a series of peptides, grouped in four mass ranges having approximately 9, 20, 30, and 40 amino acid residues. Within each group, the individual peptides differed in hydrophobicity, charge state, and retention time, as measured on a reversed-phase HPLC column. Comparison of the molecular ion intensities in the positive ion mass spectra of peptides from each group showed a strong dependence upon hydrophobicity and no correlation with charge state. Plasma desorption mass spectra of mixtures of all the peptides within each mass range generally resulted in the desorption of a single residue and suppression of the ion signal from other components. In most cases, this could be correlated with hydrophobicity, as calculated from the Bull and Breese index; however, a better correlation existed when the results were compared with reversed-phase retention times. In general the spectra of mixtures were not influenced by charge state (except in the absence of hydrophobic peptides), as the same component in each peptide mixture produced the most abundant ions in both positive and negative ion spectra.

Stopping power ratios less than unity?

We present calculations of the stopping power ratio R for H2+ molecular ions incident on graphite. These calculations include contributions from both free valence electrons and bound core electrons. It is known from experiment [W. Brandt et al., Phys. Rev. Lett. 33 (1974) 1325; 35 (1975) 130] that R is greater than unity for ν > 1 (a.u.), although it can fall below unity at lower velocities [J.C. Eckhardt et al., J. Phys. C11 (1978) L851]. We predict, however, that in appropriately chosen conditions R can be less than unity for ν ⋍ 2, even though we have assumed the constituent protons to remain bare throughout the thin solid target so that this is not a charge state effect. We have used both pure and screened Coulomb potentials to describe the repulsion process of the two protons. We also compare our computations with the data of Steuer et al. [ IEEE Trans. Nucl. Sci. NS-30 (1983) 1069] for H2+ on graphite, ν = 4. Unfortunately, their interesting results for N2+ and O2+ are obtained at velocities which are too low for our th be appropriate.

Comparison of Theoretical and Experimental Determinations of cis/trans Proline Isomer Ratios: Effect of Molecular Charge State

The resonances of the 1H and 13C n.m.r. spectra of the peptide Pro-Pro-Pro in aqueous solutions have been assigned to specific nuclei by employing one- and two-dimensional techniques and pH titrations. Minimum energy conformations of the tripeptide in different ionization states were estimated by conformational energy calculations with two effective dielectric constants. The cis/trans isomer ratios of the lowest energy conformers obtained for each set of conditions were compared to the isomer ratios measured from the 13C n.m.r. spectra of the peptide in the cationic, zwitterionic and anionic forms. The theoretical predictions did not agree with the experimental data. The conformations and geometry of triprolyl sequences in the uncharged pentapeptide Ac- Gly-Pro-Pro-Pro-Gly-NHMe were investigated for different isomer configurations of the proline residues. The structure of the molecule was characterized by its conformational energy and by two other parameters estimated from the atomic coordinates of each conformation: the distance between atoms located at the end groups, and the direction cosine between vectors defined at the N- and C-terminal ends of the molecule. About 90% of the statistical weight of the lowest energy conformers corresponded to extended geometries of the triprolyl sequence.