Initial Charge State Research Articles

Interaction of slow highly charged ions with the inner surface of nanocapillaries

The transmission of 1–10 keV Ne 7+ ions through nanocapillaries in a highly insulating PET polymer was studied. Capillaries with a diameter of 100 nm and a length of 10 μm in PET were produced using chemical etching techniques. For slow projectiles the angular distributions of the transmitted ions show that they are predominantly transported in their initial charge state along the capillary axis even when the capillaries are tilted with respect to the incident beam direction. This result indicates ion-guiding, which is produced by charge-up effects influencing the ion trajectories in a self-supporting manner. Emphasis is given to results studying the projectile-energy dependence of the capillary guiding. A relatively strong energy dependence is observed for the fraction of guided ions. Guiding effects for the studied Ne 7+ ions are found to diminish at projectile energies above about 10 keV.

Electron transfer and multi-ionization in slow collisions of C60 with highly charged ions

In the collisions of 20qkeV Arq+ with C60, the relative cross-section for the production of higher charge of C60r+ increases with the initial charge state of the projectiles increasing. By comparing the TOF spectra produced by 56keV and 160keV Ar8+ ion impact, respectively, we found that the intensity of the fragments C(60-2n)r+ via evaporation at higher energy was much less.

Photoionization and electron-impact ionization ofXe3+

Photoionization and electron-impact ionization of ${\mathrm{Xe}}^{3+}$ ions in the region of $4d$ inner-shell excitation-autoionization have been studied using interacting-beam spectroscopic techniques. Absolute cross sections for photoionization of ${\mathrm{Xe}}^{3+}$ have been obtained and the electron-impact spectrum has been normalized to previously published absolute data. The ${\mathrm{Xe}}^{3+}$ spectra show large contributions from $4d\ensuremath{\rightarrow}nf$ excitation-autoionization processes compared to the lower charge states of Xe due to the significant collapse of the $nf$ wave functions into the ion core as the initial charge state of the ion is increased. In addition, a tentative value for the ionization potential of ${\mathrm{Xe}}^{3+}$ of $42.2\ifmmode\pm\else\textpm\fi{}0.2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ was obtained.

Charge state distribution and its equilibration of 2 MeV/u sulfur ions passing through carbon foils

Charge state distributions of 2.0MeV/u sulfur ions of various initial charge states (6, 10, 11, 13+) after passing through 0.9, 1.5, 2.0, 3.0, 4.7, 6.9 and 10μg/cm2 carbon foils have been studied. It is observed that the processes involving the L-shell electrons are equilibrated within the target thickness of ∼5μg/cm2 and the charge equilibration over this thickness is ruled by the K-shell processes. Measured charge state distributions do not flat off to establish equilibrium within the measured thicknesses, but the mean charge states almost saturate to 12.4 for all initial charge states examined. Calculation with ETACHA code, developed by Rozet et al. [Nucl. Instr. and Meth. B 107 (1996) 67], is employed, although the present impact energy is lower than the assumed region for the code.

Guiding of slow Ne 7+ ions through nanocapillaries in a PET polymer: dependence on the capillary diameter

The transmission of 3 keV ions through capillaries of 100 and 200 nm diameters and 10 μm length produced by etching ion tracks in a PET polymer foil were measured. The foils were tilted relative to the incident beam direction by angles up to 25° for which the incident ions are forced to approach the capillary surface. Surprisingly, the majority of Ne 7+ ions were found to be transmitted in their initial charge state. Also, the angular distributions of the transmitted particles exhibit a predominant emission of the Ne 7+ ions along the capillary axis. This capillary guiding of the Ne 7+ ion shows that the ions produce a charge-up within the capillaries in a self-organizing process. The experimental results indicate a significant dependence of the ion transmission on the diameter of the capillaries.

Time evolution of ion guiding through nanocapillaries in a PET polymer

The time evolution of transmitting 1.6 keV H + and 3 keV Ne 7+ ions through nanocapillaries (100 nm diameter and 10 μm length) in PET insulators was studied. By measuring the angular distribution of the transmitted projectiles it is shown that the majority of ions are transported in their initial charge state along the capillary axis even when the capillaries are tilted with respect to the incident beam direction. The results indicate ion guiding effects, which are produced by charge-up effects influencing the ion trajectories in a self-organizing manner. The data analysis reveals that a certain fraction of capillaries is inclined with respect to the foil normal. Emphasis is given to unravel the influence of the capillary inclination on the guided transmission of the different ions species.

Coherent dynamics of a Josephson charge qubit

We have fabricated a Josephson charge qubit by capacitively coupling a single-Cooper-pair box (SCB) to an electrometer based upon a single-electron transistor configured for radio-frequency readout (RF-SET). Charge quantization of 2e is observed and microwave spectroscopy is used to extract the Josephson and charging energies of the box. We perform coherent manipulation of the SCB by using very fast DC pulses and observe quantum oscillations in time of the charge that persist to ~=10ns. The observed contrast of the oscillations is high and agrees with that expected from the finite E_J/E_C ratio and finite rise-time of the DC pulses. In addition, we are able to demonstrate nearly 100% initial charge state polarization. We also present a method to determine the relaxation time T_1 when it is shorter than the measurement time T_{meas}.

Guided transmission of Ne 7+ ions through nanocapillaries in PET: dependence on the tilt angle

We measured the transmission of 3 keV Ne 7+ ions through nanocapillaries of 100 nm diameter and 10 μm length in a PET polymer foil. The capillaries were produced before the transmission experiments by etching tracks of fast xenon ions. The foils were tilted up to 15° for which the incident ions are forced to approach the capillary surface. After a characteristic time the majority of Ne 7+ ions were found to be transmitted in their initial charge state. The angular distributions of the transmitted Ne 7+ projectiles reveals a propagation of the ions parallel to the capillary axis. This indicates that the ions are guided through the capillaries. The capillary guiding is the result of deposit of charges in a self-organizing process. A non-linear model is presented to interpret the experimental observation. The analysis exhibits a significant dependence of the ion transmission on the tilt angle.

Electron loss from 0.74- and 1.4-MeV/u low-charge-state argon and xenon ions colliding with neon, nitrogen, and argon

Absolute total-, single-, and multiple-electron-loss cross sections are measured for $({\mathrm{Ar}}^{+}\ensuremath{-},$ ${\mathrm{Ar}}^{2+}\ensuremath{-},$ ${\mathrm{Xe}}^{3+})\ensuremath{-}(\mathrm{Ne},$ ${\mathrm{N}}_{2},$ Ar) collisions at 0.74 and 1.4 MeV/u. In addition, a many-body classical trajectory Monte Carlo model was used to calculate total- and multiple-electron-loss cross sections for ${\mathrm{Ar}}^{+}$ impact. For ${\mathrm{N}}_{2}$ and Ar targets, excellent agreement between the measured and calculated cross sections is found; for the Ne target the experimental data are approximately 40% smaller than the theoretical predictions. The experimental data are also used to examine cross-section scaling characteristics for electron loss from fast, low-charge-state, heavy ions. It is shown that multiple electron loss increased the mean charge states of the outgoing argon and xenon ions by 2 and 3 respectively. The cross sections decreased with increasing number of electrons lost and scaled roughly as the inverse of the sum of the ionization potentials required to sequentially remove the most weakly bound, next most weakly bound, etc., electrons. This scaling was found to be independent of projectile, incoming charge state, and target. In addition, the experimental total loss cross sections are found to be nearly constant as a function of initial projectile charge state. As a function of impact energy, the theoretical predictions yield an ${E}^{\ensuremath{-}1/3}$ behavior between 0.5 and 30 MeV/u for the total loss cross sections. Within error bars, the data are consistent with this energy dependence but are also consistent with an ${E}^{\ensuremath{-}1/2}$ energy dependence.

Charge driven fragmentation of nucleobases.

We studied multiple ionization of single nucleobases by means of slow highly charged ions (Xe(q+), q=5-25). The products of the subsequent fragmentation were studied using high resolution coincidence time-of-flight spectrometry. We observed a strong dependence of the fragment kinetic energies on the initial charge state of the intermediate parent ions as well as on the initial chemical environment of the respective fragment ions within the parent molecule. The data allow us to shed light on the charge distribution within the molecule as well as on the fragmentation dynamics of these intermediate size systems.

Effects of electron–hole excitations in ion–surface collisions

We investigate effects related to electron–hole pair production and atomic level shift in atom scattering at surfaces by using a recently proposed exactly soluble model. We show that electron–hole pair production weakens Stückelberg oscillations and enhances loss of memory of the initial atomic charge state for narrow bands because of the diffusion of an electron or hole captured by the band. Wide band materials tend to display memory loss at lower velocities than do narrow band materials. Allowing the atomic energy level to shift above the Fermi energy tends to reduce memory loss.

Scattering of sulfur ions by carbon: Classical-trajectory Monte Carlo results

We analyze classically the scattering of sulfur ions by carbon using the classical-trajectory Monte Carlo method. It is assumed that the scatterer and scattered nuclei are coupled to each other as well as to all electrons, but there is no coupling between electrons themselves. To initialize the state of both atoms, quasiexact energies are used that are obtained from the Dirac-Fock method. Effective charges are used to partially take into account the intra-atomic interactions between electrons. We concentrate on the cross sections for production of vacancies in the K and L shells and capture of electrons to K, L, and M shells of the sulfur ions. The dependence of these cross sections on the energy of the projectile sulfur ions and on the initial charge states of these ions is analyzed. Our results will be helpful in the interpretation of x-ray spectra from highly ionized fast sulfur projectiles passing through a carbon foil.

Guided transmission of 3 keV Ne 7+ ions through nanocapillaries etched in a PET polymer

We measured the transmission of 3 keV Ne 7+ ions through capillaries of 100 nm diameter and 10 μm length produced by etching ion tracks in a polyethylene terephthalate polymer foil. The foils were tilted up to ±25° for which the incident ions are forced to interact with the capillary surface. The majority of Ne 7+ ions were found to survive the transmission in their initial charge state. For tilted foils the angular distributions of the transmitted particles indicate propagation of the Ne 7+ ions parallel to the capillary axis. This capillary guiding of the Ne 7+ ion provides evidence that part of the ions deposit charges within the capillaries in a self-organizing process so that a considerable fraction of the ions is transmitted through the capillaries. A non-linear model is introduced to describe the essential features of the capillary guiding.

Emission of singly and doubly charged light fragments fromC60r+(r=4–9)inXe25+−C60collisions

Asymmetrical fission of ${\mathrm{C}}_{60}^{r+}$ $(r=4--9)$ ions are studied in ${\mathrm{Xe}}^{25+}\ensuremath{-}{\mathrm{C}}_{60}$ collisions at 100 keV impact energy $(v\ensuremath{\approx}0.18\mathrm{a}.\mathrm{u}.).$ The branching ratios for the emission of a singly or a doubly charged light fragment, ${\mathrm{C}}_{m}^{+}$ or ${\mathrm{C}}_{m}^{2+},$ are measured for each initial charge state r. The measured m-dependent branching ratios for the singly charged ${\mathrm{C}}_{m}^{+}$ fragment emission channels are reproduced using a simple statistical model. The ionization energies $I({\mathrm{C}}_{60\ensuremath{-}2n}^{q+})$ for even numbered fullerenes ${\mathrm{C}}_{60\ensuremath{-}2n}^{q+}$ $(q=1--5,8;n=1--4)$ and $I({\mathrm{C}}_{2n}^{+})$ for singly charged light fragments ${\mathrm{C}}_{2n}^{+}$ $(n=1--4)$ are estimated by the density-functional theory. The total branching ratio for the ${\mathrm{C}}_{m}^{2+}$ emission channels is found to increase from less than 2 to 25% when the charge r of the parent ion ${\mathrm{C}}_{60}^{r+}$ increases from 5 to 9.

Electron-impact multiple ionization of Baq+ ions (1⩽q⩽13) via resonant 3d excitation

One ionization mechanism of barium ions is the resonant excitation of a 3d inner-shell electron with the subsequent decay of the resulting highly excited intermediate state by multiple autoionization processes. We have experimentally studied this process by observing associated resonance structures – in the energy range 700–800 eV – in the multiple (up to sevenfold) ionization cross-sections of Baq+ ions with 1⩽q⩽13. The 3d-resonance strength is determined quantitatively as a function of initial and final ion charge state. The results suggest a statistical description of multiple ionization via the excitation of inner-shell electrons.

A framework for understanding molecularionization in strong laser fields

The aim of this paper is to examine the role of excited statesand multi-electron interactions in molecular ionization bystrong laser fields. We present new data on the ionization anddissociation of iodine molecules that reveal important aspectsof the strong field-molecule interaction in the short pulseregime. Our data, along with previous studies, is inconsistentwith the simplest and commonly accepted model of molecular ionization in a strong laser field and this has led us toexamine closely the individual ionization steps. We have foundthat a molecule can ionize into several distinct configurationspredominantly through multi-electron interactions and theabundance of such configurations is dependent on internuclearseparation. The ionization appears to be dominated by pairs of states with gerade and ungerade symmetry, as they have a largedipole coupling and the transition is near resonant with thestrong laser field. In a one-electron molecule, this pairconsists of the ground and first excited state whereas in atwo-electron molecule this corresponds to the lowest lying pairof ionic states. In this paper, we propose a framework for organizing the numerous ionization pathways based on theelectronic configuration of the initial charge state.

Transmission of 3 keV Ne7+ ions through nanocapillaries etched in polymer foils: evidence for capillary guiding.

We report unprecedented transmission experiments of 3 keV Ne7+ ions through capillaries of 100 nm diameter and 10 microm length produced by etching ion tracks in a polymer foil. We studied foils tilted up to +/-20 degrees for which the incident ions are forced to interact with the capillary surface. Surprisingly, the majority of Ne7+ ions were found to survive the surface scattering events in their initial charge state. The angular distributions of the transmitted particles indicate propagation of the Ne7+ ions along the capillary axis. This capillary guiding of the Ne7+ ion provides evidence that the inner walls of the capillaries become charged and electron capture from the surface is suppressed in a self-organizing process.

Stopping power of swift neon ions in dependence on the charge state in the non-equilibrium regime

We report on measurements of charge changing cross-sections and energy losses in dependence on the initial and final charge state of Ne ions at an incident energy of 2 MeV/u penetrating thin carbon foils. Different initial charge states could be separated in energy by applying a high voltage in front of the carbon foils; the final charges and their energy loss were measured with a high-resolution magnetic spectrometer. We derived a consistent evolution of the charge state distribution solving the corresponding rate equations for the measured cross-sections. Including the charge state dependent energy losses, ΔE( q i, q f), stopping power values for frozen charge states, S( q), could be extracted. The experimental data are compared with different theoretical predictions.

Heavy Ion Abundances in Impulsive Solar Flares: Influence of Pre-Acceleration in a Current Sheet

Competition between stochastic energy gains and collisional energy losses is known to lead to preferential acceleration of heavy ions in flare loops. Ion acceleration in a reconnecting current sheet is shown to mitigate the influence of collisional energy losses on stochastic particle acceleration in impulsive solar flares. This effect decreases the sensitivity of the resulting abundance ratios on initial ion charge states. The resulting abundances are determined by the fact that the energy loss rate falls off rapidly with increasing energy. As an example, the expected Fe/O enhancement ratios are computed and shown to be comparable with those observed with ACE SEPICA in several impulsive flares in 1998. One consequence of the model is that the preferential acceleration of heavy ions can occur only when the plasma gas pressure is large enough, β≈me/mp, which may explain the observed correlation between the heavy ion enrichment and selective 3He acceleration in impulsive flares.

Multiple electron stripping of 3.4 MeV/amu Kr7+ and Xe11+ in nitrogen

Use of heavy ions beams with ∼10 MeV/amu mass ∼200, and average charge state of 1+ has been proposed as a driver for heavy ion fusion. Stripping of the ion beam by background gas can lead to an increase in the space charge density of the beam, which may make focusing the intense ion beam onto small targets more complex. Knowledge of the electron loss cross sections is essential to understand and address the problem. Currently, there are no 10 MeV/amu mass=200, charge state=1 beams available, and the theories that calculate electron loss cross sections can be experimentally tested only by using available beams of somewhat lower energy and higher initial charge state. The charge state distribution of ions produced in single collisions of 3.4 MeV/amu Kr7+ and 3.4 MeV/amu Xe11+ in N2 have been measured at the Texas A&M Cyclotron Institute using a windowless gas cell. The charge states of the outgoing ions are determined by magnetic analysis using a position-sensitive microchannel-plate detector. The cross sections for single and multiple electron loss are determined, and the results indicate that substantial multiple-electron loss occurs. The relative cross section for loss of i+1 electrons is 0.3–0.7 times that for i electron loss. The average number of electrons removed per one collision (sum of the electron-weighted cross sections normalized to the total cross section) is 1.86 for Kr and 1.97 for Xe.