Charge State Dependence Research Articles

Study of the average charge states of 188Pb and 252,254No ions at the gas-filled separator TASCA

The average charge states of 188Pb and 252,254No ions in dilute helium gas were measured at the gas-filled recoil separator TASCA. Hydrogen gas was also used as a filling gas for measurements of the average charge state of 254No. Helium and hydrogen gases at pressures from 0.2mbar to 2.0mbar were used. A strong dependence of the average charge state on the pressure of the filling gases was observed for both, helium and hydrogen. The influence of this dependence, classically attributed to the so-called “density effect”, on the performance of TASCA was investigated. The average charge states of 254No ions were also measured in mixtures of helium and hydrogen gases at low gas pressures around 1.0mbar. From the experimental results simple expressions for the prediction of average charge states of heavy ions moving in rarefied helium gas, hydrogen gas, and in their mixture were derived.

Electron capture by hydrogen like projectile ions from ground state atomic hydrogen

Electron capture from ground state atomic hydrogen by the impact of hydrogen like projectile ions is investigated by means of the four-body boundary corrected continuum intermediate state approximation in the energy range 20–1000keV/amu. In this formalism, the electron–electron interaction explicitly appears in perturbation potential of the prior transition amplitude. Distortion in the initial channel related to the Coulomb continuum states of the target ion and the active electron in the field of residual projectile ion are included. In all cases, total capture cross sections have been calculated by summing over all contributions up to n=3 shells and sub shells, respectively. The results obtained both in prior and post forms are compared with other recent theoretical and experimental findings. Numerical results for total electron capture cross sections show good agreement with the available experimental findings. Post-prior discrepancy is well within 25% in the entire energy range. As suggested by Kim et al. in connection with the oscillations in the charge state dependence of total electron capture cross sections, a slight oscillatory structure for the collision of hydrogen like projectile ion with H atom at high energies has been found in the present investigation.

Simple model for the ionization of helium atoms by fully and partially stripped ions in the strong interaction region

Theoretical calculations employing the modified over-barrier model to investigate single and double ionization cross sections of helium in collisions with highly charged ions are presented in the strong perturbation region. It shows the velocity and charge state dependences of cross sections, which coincides with the available experimental data. In order to determine the reliability of this model for partially stripped projectiles, the cross-section ratios have been measured for C Z p + , O Z p + – He ( Z p = 1 – 4 ) by the coincidence technique in the velocity range of 1–4.5 a.u. The calculated results are approximately in agreement with the experimental data.

Transport studies using laser blow-off injection of low-Z trace impurities injected into the TJ-II stellarator

Trace (<5%) impurity seeding of boron and carbon by laser blow-off is used to study low-Z parallel and radial impurity transport in the TJ-II stellarator. Toroidal transport is observed to be quite rapid τϕ ∼ 0.1 ms, roughly consistent with expectations of collisionless (kinetic) parallel ion transport. Radial impurity de-confinement of injected impurities to the plasma wall is measured to be much slower, τr ∼ 10 ms. The data suggest a slightly higher τr for carbon than for boron, but no clear charge state dependence is observed in contrast with expectations for neoclassical transport in the low collisionality regime.

Revealing of hydrodynamic and electrostatic factors in the center-of-mass velocity of an expanding plasma generated by pulsed laser ablation

AbstractTime-of-flight spectra of C, Fe, and Si ions produced with the use of a KrF excimer laser have been analyzed. Ion currents were collected by Faraday cups and their responses were analyzed using a detector signal function. This function was derived from shifted Maxwell-Boltzmann velocity distribution, in order to uncover the contribution of partial currents of all the ionized species constituting the expanding plasma plume. The deconvolution method allowed to estimate parameters of the plasma, such as the ion temperature and the center-of-mass velocities of expanding ionized species. Furthermore, the linear charge-state dependence of the center-of-mass velocity has revealed the contribution of hydrodynamic and electrostatic forces to the expansion velocity of the plasma. The nearly isotropic distribution of the center-of-mass velocity indicates that the shape of the plasma plume is determined mainly by the angular distribution of the ionization degree of ions.

High-charge-state limit for the double-to-single ionization ratio of helium in the strong-coupling regime

We measured the double-to-single-ionization ratios $R$ of helium impacted by the intermediate velocity ${\mathrm{C}}^{4+}$ ion and found that $R$ decreases as 1/$v$. This type of velocity dependence is consistent with the data for very highly charged ions (N${}^{7+}$ \ensuremath{\sim} ${\mathrm{U}}^{92+}$). The charge state and velocity dependences of $R$ are interpreted well by the classical over-barrier-ionization (COBI) model. It is found that the ratio $R$ can be written in a manner of $R=0.28\sqrt{q}/v$ for very high-$q$ projectiles in the strong coupling (q/v g 1) regime. This theoretical prediction is in excellent agreement with extensive experimental data.

Single and multiple ionization ofC60fullerenes and collective effects in collisions with highly charged C, F, and Si ions with energy 3 MeV/u

We have measured absolute cross sections for single, double, triple, and quadruple ionization of ${\mathrm{C}}_{60}$ in collisions with 3 MeV/u C, F, and Si projectile ions at various projectile charge states. The experiment was performed using the recoil-ion time-of-flight technique. Projectile charge state dependence of the ionization yields was compared mainly with a model based on the giant dipole plasmon resonance (GDPR). In some cases, the continuum-distorted-wave--eikonal-initial-state (CDW-EIS) model which is normally applied for ion-atom collisions was also used as a reference. An excellent qualitative agreement between the experimental data for single and double ionization and the GDPR model predictions was found for all projectile charge states.

EquilibriumK-,L-, andM-shell ionizations and charge-state distribution of sulfur projectiles passing through solid targets

In the present work, an alternative approach for the evaluation of the equilibrium K-, L-, and M-shell ionizations and the mean charge state q for projectiles passing through various targets has been proposed. The approach is based on measured K x-ray energy shifts and line intensity ratios and utilizes the theoretical analysis of projectile spectra using multiconfiguration Dirac-Fock calculations. It was applied for the satellite and hypersatellite K lines in the x-ray spectra emitted by sulfur projectiles passing with energies of 9.6-122 MeV through carbon, aluminium, titanium, and iron targets, recorded by a Si(Li) detector. It was found that only in the high projectile energy region there was significant dependence of mean equilibrium K-shell ionization on the target atomic number. The equilibrium L-shell ionization rises with the increase of sulfur energy until 32 MeV, but for higher energies the changes are very weak. The equilibrium M-shell ionization changes very weakly for low projectile energy while for higher energies this ionization is practically constant. For each target, the estimated value of q rises with the increase of the sulfur energy value. The dependence of the sulfur charge state on the target atomic number was discussed by taking into account the crossmore » sections for ionization, decay, and electron capture processes. The data were compared with the experimental data measured by other authors and with the predictions of Shima's and Schiwietz and Grande's semiempirical formulas. The presented good agreement points out that this alternative approach delivers quantitative results.« less

Core Impurity Transport Studies from Different Diagnostic Approaches in LHD

Impurity transport has been studied in the Large Helical Device (LHD) with different diagnostic approaches based on an active method that combine carbon pellet injection with visible bremsstrahlung measurement and three passive methods for radial profile measurements of Ar and Fe Kα X-ray lines, Zeff, and extreme ultraviolet (EUV, ≤500 Å) impurity line emissions, in addition to usual passive spectroscopy. The existence of an inward convective velocity is confirmed in the edge region (ρ > 0.6) using the active method, whereas no convection is required in the core region (ρ < 0.6). The electron density dependence is weak for the diffusion coefficient (typically D = 0.15 to 0.25 m2/s) for densities of 1 to 5 × 1013 cm-3 but is strong for the inward convective velocity, which varies in the range of V(a) = −0.2 to −1.5 m/s. The inward V in helium plasmas (−0.4 m/s at ρ = 0.8 and the central density, ne ~ 4.0 × 1013 cm−3) is nearly half that in hydrogen plasmas (−0.7 m/s). This difference suggests a charge state dependence of fuel ions predicted by the neoclassical theory. Radial profiles of impurity transport coefficients of argon and iron have been studied using spatially resolved soft X-ray pulse-height analyzers. The impurity transport has also been studied in extremely high density discharges achieved by H2 pellet injection based on the passive spectroscopy and Zeff profile measurement. A flat Zeff profile is obtained at ne = 2.5 × 1014 cm−3 with values of 1.1 ≤ Zeff ≤ 1.2, suggesting no existence of impurity accumulation and radially constant impurity partial pressure. Finally, radial profiles of impurity lines in the EUV range are analyzed with the transport coefficients.

Elucidating the tertiary structure of protein ions in vacuo with site specific photoinitiated radical reactions.

A new method for identifying residue specific through space contacts as a function of protein secondary and tertiary structure in the gas phase is presented. Photodissociation of a non-native carbon-iodine bond incorporated into Tyr59 of ubiquitin yields a radical site specifically at that residue. The subsequent radical migration is shown to be highly dependent on the structure of the protein. Radical-directed dissociation (RDD) of low charge states, which adopt compact structures, generates backbone fragmentation that is prominently distributed throughout the protein sequence, including residues that are distant in sequence from Tyr59. Higher charge states of ubiquitin, which adopt elongated, unfolded structures, yield RDD that is primarily nearby in sequence to Tyr59. Regardless of which structure is probed, information at the residue-level is obtained by examining specific radical-donor and radical-acceptor pairs. The relative importance of a particular interaction pair for a specific conformation can be revealed by tracking the charge state dependence of the dissociation. Structurally important contact pairs exhibit strong and concerted changes in relative intensities as a function of charge state and can also be used to reveal structural features which persist among different protein structures. Moreover, incorporation of distance constraint information into molecular mechanics conformational searches can be used to drive the search toward relevant conformational space. Implementation of this approach has revealed highly stable, previously undiscovered structures for the +4 and +6 charge states of ubiquitin, which bear little resemblance to the crystal structure.

Fast ion collisions with C 60 in vapour phase and collective excitation: Comparison with other gaseous targets

The single and double ionization of a free C 60 molecule in collisions with fast heavy (F and Si) ions is investigated using a recoil ion time-of-flight mass spectrometer. The projectile charge state ( q p) dependence has also been investigated. A linear q p-dependence has been explained in terms of a plasmon excitation model. In addition, continuum electron spectroscopy has been used to detect the electron emission from fullerenes. The measured electron angular distribution for the fullerene target is compared with that for a gaseous target at a fixed electron energy. The ratio of forward-to-backward cross section for C 60 is quite different from that for Ne.

Ion-ion coincidence studies on multiple ionizations of N2 and O2 molecules irradiated by extreme ultraviolet free-electron laser pulses

We have investigated multiple ionization of N(2) and O(2) molecules by 52 nm extreme-ultraviolet light pulses at the free-electron laser facility SCSS in Japan. Coulomb break-up of parent ions with charge states up to 5+ is found by the ion-ion coincidence technique. The charge-state dependence of kinetic energy release distributions suggests that the electrons are emitted sequentially in competition with the elongation of the bond length.

EUV emission spectra in collisions of multiply charged Sn ions with He and Xe

Extreme ultraviolet emission spectra of multiply charged Sn ions were measured in the wavelength range 5–38 nm, following electron capture into the excited states of slow Snq + (q = 5–21) ions passing through He and Xe gas targets. Identification of the transitions was carried out by comparison with calculations using the Hebrew University Livermore Laboratory Atomic physics Code. The target and the charge-state dependences of 4d–nℓ (nℓ = 4f, 5p and 5f) and 4p–4d transitions were observed.

Dynamics simulation on the interaction of intense laser pulses with atomic clusters

Under classical particle dynamics, the interaction process between intense femtosecond laser pulses and icosahedral noble-gas atomic clusters was studied. Our calculated results show that ionization proceeds mainly through tunnel ionization in the combined field from ions, electrons and laser, rather than the electron-impact ionization. With increasing cluster size, the average and maximum kinetic energy of the product ion increases. According to our calculation, the expansion process of the clusters after laser irradiation is dominated by Coulomb explosion and the expansion scale increases with increasing cluster size. The dependence of average kinetic energy and average charge state of the product ions on laser wavelength is also presented and discussed. The dependence of average kinetic energy on the number of atoms inside the cluster was studied and compared with the experimental data. Our results agree with the experimental results reasonably well.

Absolute detection efficiencies of an ion-counting system with a channel-electron multiplier

Absolute detection efficiencies of an ion-counting system have been examined for Li+, Rb+, Cs+, Ar+, and Ar2+ ions as a function of the incident ion-impact energy. The ion-counting system consists of a converter plate (SUS), an electrostatic-lens system, and a channel-type secondary-electron-multiplier. For all ions examined in this study, the detection efficiencies are about unity if the ion energy is greater than 5 keV. The charge-state dependence is not as high for Ar+ and Ar2+.

Potential and kinetic sputtering of alkanethiol self-assembled monolayers by impact of highly charged ions

Highly charged ions have been used to study the sputtering of positive molecular fragments from mercaptoundecanoic acid and dodecanethiol self-assembled monolayers on gold surfaces. The samples were bombarded with ${\mathrm{Ar}}^{q+}$ $(4\ensuremath{\leqslant}q\ensuremath{\leqslant}10)$ ions with kinetic energies from $2\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}18\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$. The main fragments detected were ${\mathrm{H}}^{+}$, ${\mathrm{C}}_{n}{\mathrm{H}}_{2n}^{+}$, and ${\mathrm{C}}_{n+1}{\mathrm{O}}_{2}{\mathrm{H}}_{2n+1}^{+}$ from mercaptoundecanoic and ${\mathrm{H}}^{+}$, ${\mathrm{C}}_{n}{\mathrm{H}}_{2n}^{+}$, and ${\mathrm{C}}_{n+1}{\mathrm{H}}_{2n+3}^{+}$ from dodecanethiol. The proton yields were increased with larger charge state $q$ of the highly charged ion (HCI) in both samples, scaling as ${q}^{\ensuremath{\gamma}}$, with $\ensuremath{\gamma}\ensuremath{\sim}5$. The charge state dependence is discussed in terms of electron transfer to the HCI. The final yield of protons depends on molecular functional group characteristics, orientation on the surface, and reneutralization phenomena.

A quantum chemistry study of diethynylbenzene macrocycles: Structural and electronic properties

The geometric and electronic structures of a series of diethynylbenzene macrocycles (DBMs) in neutral, cationic, and anionic states have been studied theoretically by density functional theory at the B3LYP level. The results show that the model molecules may have planar conformation in both neutral and ionic states. However, bigger macrocycles, for example, 7DBM and 8DBM, the charge states result in more energy minima. The most energetically favored conformation also shows charge state dependence. In addition, electronic structure analysis demonstrates an odd–even difference behavior of LUMO–HOMO gap for the planar DBMs in both neutral and ionic states.

Projectile charge-state dependence of methane fragmentation

The projectile charge-state dependence of methane fragmentation is studied at a constant projectile (${\mathrm{Ar}}^{q+}$, $1\ensuremath{\le}q\ensuremath{\le}7$) velocity of $0.42\phantom{\rule{0.3em}{0ex}}\text{a.u.}$ using the technique of time-of-flight mass spectroscopy. The relative cross sections for fragment production are reported as a function of the incident projectile charge state. The fragments appearing as molecular ions show a decrease in cross sections with increasing projectile charge state as opposed to the carbon ions which show an increase with increasing projectile charge state.

Which Electrospray-Based Ionization Method Best Reflects Protein-Ligand Interactions Found in Solution? A Comparison of ESI, nanoESI, and ESSI for the Determination of Dissociation Constants with Mass Spectrometry

We present a comparison of three different electrospray-based ionization techniques for the investigation of noncovalent complexes with mass spectrometry. The features and characteristics of standard electrospray ionization (ESI), chip-based nanoESI, and electrosonic spray ionization (ESSI) mounted onto a hybrid quadrupole time-of-flight mass spectrometer were compared in their performance to determine the dissociation constant (KD) of the model system hen egg white lysozyme (HEWL) binding to N,N',N''-triacetylchitotriose (NAG3). The best KD value compared with solution data were found for ESSI, 19.4 +/- 3.6 microM. Then, we determined the KDs of the two nucleotide binding sites of adenylate kinase (AK), where we obtained KDs of 2.2 +/- 0.8 microM for the first and 19.5 +/- 8.0 microM for the second binding site using ESSI. We found a weak charge state dependence of the KD for both protein-ligand systems, where for all ionization techniques the KD value decreases with increasing charge state. We demonstrate that ESSI is very gentle and insensitive to instrumental parameters, and the KD obtained is in good agreement with solution phase results from the literature. In addition, we tried to determine the KD for the lymphocyte-specific kinase LCK binding to a kinase inhibitor using nanoESI due to the very low amount of sample available. In this case, we found KD values with a strong charge state dependence, which were in no case close to literature values for solution phase.

Dependence of the Sn0/2+ charge state on the Fermi level in semi-insulating CdTe

We explored the growth and characteristics of CdTe doped with Sn to heighten our understanding of the role of deep levels on electrical compensation and trapping. We demonstrated, for the first time, the strong dependence of the SnCd charge state on the Fermi-level variation (2–3kT) in high-resistivity CdTe. The concentration of deep traps for electrons was determined by the number of doubly positively charged Sn2+ atoms. Thermoelectric-effect spectroscopy and photovoltage measurements revealed the conversion of the SnCd defect from the electron SnCd2+ trap to the hole SnCd0 trap. The results agree well with the existence of a negative U-center in the SnCd0/2+ defect. We also showed that the neutral Sn defect is responsible for the near midgap C-band → bound hole radiative transitions band with a maximum at 0.76 eV.