Projectile Charge State Research Articles

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.

Investigation of the transfer ionization process in collisions of partially stripped ions on He

In this paper a projectile ion-recoil ion coincidence technique is used to investigate the transfer ionization processes in collisions of 0.22–6.30 MeV Cq+ ions and 0.25–6.35 MeV Oq+ ions (q = 1, 2, 3, 4) with the He atom separately. The cross section ratio f of transfer ionization to single electron transfer is measured, and the dependence of f on both charge state q and energy E of the projectiles is investigated. The electron-structure and the mechanisms leading to transfer ionization affect the dependence of f on q and E. Our measurements, along with other data published previously, suggest a similar dependence of f on charge state and energy of projectile for partially stripped ions over a large energy range. The maximum value of f is approximately 0.17q0.60; the energy corresponding to maximum f is about 160q0.60 keV/u.

Differential and total ionization cross sections due to impact of swift heavy ions

In the present work, the Binary stopping power theory was used to calculate total ionization cross section and differential cross section for energy transfer from a swift heavy ion to the target electrons. The dependence on the projectile charge state is studied. The calculated values are compared with the experimental values collected from the literature.

Mass spectrometry (fragmentation ratios) of DNA base molecules following 80 keV proton impact with separation of direct ionization and electron capture processes

The first fragmentation ratios are presented for the ionization of gas-phase DNA bases by 80 keV (1.8 v 0 in Bohr velocity units) proton impact. Event-by-event determination of the projectile charge state post-collision enables branching ratios to be determined for electron capture (EC) by the projectile and for direct ionization (DI) of the target molecule (without projectile neutralization). Results are compared with similar experiments on uracil [11] and water [14]. In all cases, whereas both processes (EC and DI) produce the same fragment ion groups, greater fragmentation ratios are observed for EC than for DI. Moreover the fragmentation ratio is greater for thymine than for adenine, cytosine, and uracil.

Energy loss of argon in a laser-generated carbon plasma

The experimental data presented in this paper address the energy loss determination for argon at 4 MeV/u projectile energy in laser-generated carbon plasma covering a huge parameter range in density and temperature. Furthermore, a consistent theoretical description of the projectile charge state evolution via a Monte Carlo code is combined with an improved version of the CasP code that allows us to calculate the contributions to the stopping power of bound and free electrons for each projectile charge state. This approach gets rid of any effective charge description of the stopping power. Comparison of experimental data and theoretical results allows us to judge the influence of different plasma parameters.

Strong anisotropy in the proton emission following fragmentation of H2O molecules by impact with slow, highly charged Xenon ions

We measured the energy and angular distribution of ionic fragments produced by the interaction of 1–220 keV Xe22+ ions with water molecules. The measured distributions strongly depend on the projectile charge state and energy, as seen from the comparison of the results with previously published data for 5 keV He2+, and 2 and 90 keV Ne(3–9)+ ions. A significant forward–backward asymmetry of the energy and intensity of the H+ fragments is observed. The interpretation of the experimental results is guided by means of classical trajectory simulations based on a Coulomb explosion model. The experimental finding of a strong enhancement of the H+ yield at 90° is attributed to an alignment of the molecular axis during the collision and the momentum transfer from the slow projectile.

Double ionization of helium in collision with 20–500-keV/amuCq+andOq+(q=1–3)ions

Target ionization and projectile charge changing were investigated for 20--500 keV/u ${\text{C}}^{q+}$, ${\text{O}}^{q+}+\text{He}$ $(q=1--3)$ collisions. Double- to single-ionization ratios ${R}_{21}$ of helium associated with no projectile charge change (direct ionization), single-electron capture, and single-electron loss were measured. The cross-section ratio ${R}_{21}$ depends strongly on the collision velocity $v$, the projectile charge state $q$, and the outgoing reaction channel. Meanwhile, a model extended from our previous work [J. X. Shao, X. M. Chen, and B. W. Ding, Phys. Rev. A 75, 012701 (2007)] is presented to interpret the above-mentioned dependences. Good agreement is found between the model and the experimental data.

Secondary ions from condensed gas solids by singly and multiply charged ion impacts

Ion desorption from a condensed gas solid Ne impacted by singly- and multiply-charged Arq+ ions (q = 1−7) has been investigated. Various secondary ions such as cluster ions, Ne+n (n = 2−20) as well as atomic ions are observed. Mass spectral patterns, thickness dependence of the yields, and kinetic energy distributions of the desorbed Ne+n (n = 1,2) depend strongly on the projectile charge state. These results indicate that the dissipation of the projectile potential energy on the surface leads to the desorption of the monomer and small cluster ions by a Coulomb repulsion between adjacent target ions.

Kinetic energy release in CO fragmentation by electron loss and capture collisions of fast B2+ ions

Ionization and fragmentation of CO molecules have been investigated in charge-changing collisions of B2+ ions at v = 1.69 a.u.. Fragment ions from CO were measured by using a momentum 3D imaging technique in coincidence with outgoing projectile charge states. We deduced the kinetic energy release (KER) associated with charge-changing collisions of B2+ to B3+, B1+ and B0+. The KER spectra in the fragmentation to C+ and O+ ions are found to be different to some extent for these loss and capture collisions. It is concluded that the electron loss collision populates the intact CO molecule to highly excited states of (CO)2+1 more effectively than electron capture collisions, resulting from different effective impact parameters associated with loss and capture collisions.

Molecular and dissociative ionization in 1.0-2.0 MeV/u O5+ + O2 collisions

Yields for molecular ionization and dissociation associated with specific outgoing projectile charge states have been investigated for O5+ + O2 collisions at projectile energies of 1.0, 1.5, and 2.0 MeV/u. Time-of-flight spectra for recoiling target ions were recorded in coincidence with projectiles undergoing no charge change (direct), single-electron capture, or single-electron loss. Yields of the observed recoil-ion products are compared for specific outgoing projectile charge states as a function of energy.

X-ray Spectra Excited in Ar~(q+)(q=7—14) Incidence on Au Surface

The Au atomic Mα characteristic Xray spectrum has been measured for the slow highly charged ions Arq+ (q= 7—14) impacting on Au surface. The result shows that as long as the charge state of projectile is higher（q=11）， the characteristic Xrays of heavy atomic can be effectively excited even though the ionic beam is very weak(nA magnitude)， and the single ionic Xray yield is in the order of 10-8 and increases with potential energies of projectiles.

Calculated depth-dose distributions for H + and He + beams in liquid water

We have calculated the dose distribution delivered by proton and helium beams in liquid water as a function of the target-depth, for incident energies in the range 0.5–10 MeV/u. The motion of the projectiles through the stopping medium is simulated by a code that combines Monte Carlo and a finite differences algorithm to consider the electronic stopping power, evaluated in the dielectric framework, and the multiple nuclear scattering with the target nuclei. Changes in projectile charge-state are taken into account dynamically as it moves through the target. We use the MELF–GOS model to describe the energy loss function of liquid water, obtaining a value of 79.4 eV for its mean excitation energy. Our calculated stopping powers and depth-dose distributions are compared with those obtained using other methods to describe the energy loss function of liquid water, such as the extended Drude and the Penn models, as well as with the prediction of the SRIM code and the tables of ICRU.

Electron loss of fast heavy projectiles in collision with neutral targets

The multiple electron loss of heavy projectiles in fast ion-atom collisions has been studied in the framework of the sudden perturbation approximation. Especially, a model is developed to calculate the cross sections for the loss of any number of electrons from the projectile ion, including the ionization of a single electron and up to the complete stripping of the projectile. For a given collision system, that is specified by the (type and charge state of the) projectile and target as well as the collision energy, in fact, the experimental cross sections for just three final states of the projectile are required by this model in order to predict the loss of any number, $N$, of electrons for the same collision system, or for any similar system that differs only in the energy or the initial charge state of the projectile ion. The model is simple and can be utilized for both, the projectile and target ionization, and without that large computer resources are requested. Detailed computation have been carried out for the multiple electron loss of Xe$^{18+}$ and U$^{6+,\, 10+,\, 28+}$ projectiles in collision with neutral Ar and Ne gas targets.

Surface nanostructure formation by the interaction of slow xenon ions on HOPG surfaces

Experiments on nanostructure formation on highly oriented pyrolytic graphite (HOPG) surfaces irradiated by slow highly charged Xeq+ (q = 23-29) ions have been carried out at a new experimental terminal for surface physics on the 320 kV ECR platform at the Institute of Modern Physics (IMP). In the tapping mode atomic force microscope (AFM) image, the nano-sized hillocks protruding from the surfaces are probed. The height and diameter of the nanostructures increase with the projectile charge state. The present results reveal a similarity between the nanostructure formation produced by slow highly charged ions and the track formation induced by swift heavy ions.

The influence of projectile charge state on ionization probabilities of sputtered atoms

The ionization probability of atoms sputtered from a clean polycrystalline metal surface was measured for different charge states of the projectile used to bombard the sample. More specifically, a polycrystalline indium surface was irradiated with Ar+ and Ar0 beams of energies between 5 and 15keV, and In+ secondary ions and neutral In atoms emitted from the surface were detected under identical experimental conditions regarding the sampled emission angle and energy. The resulting energy integrated ionization probability of sputtered In atoms is consistently found to be smaller for neutral projectiles, the difference decreasing with decreasing impact energy. The observed trends agree with those measured for kinetic electron emission, indicating that secondary ion formation is at least partly governed by kinetic substrate excitation.

Direct Evidence for Projectile Charge-State Dependent Crater Formation Due to Fast Ions

We report on craters formed by individual 3 MeV/u Au (q(ini)+) ions of selected incident charge states q_(ini) penetrating thin layers of poly(methyl methacrylate). Holes and raised regions are formed around the region of the impact, with sizes that depend strongly and differently on q_(ini). Variation of q_(ini) of the film thickness and of the angle of incidence allows us to extract information about the depth of origin contributing to different crater features.

Dissociative and non-dissociative ionization of the O2 molecule by He+ impact

Absolute cross sections for the dissociative and non-dissociative ionization of the O2 molecule, accompanied or not by the projectile electron loss, have been measured in coincidence with the final projectile charge states for the impact of 0.75–3.5 MeV He+ ions. The fragmentation patterns are analyzed in comparison with data for photon, electron and heavy-ion impact available in the literature for different collision channels. This analysis shows that the fragmentation pathways depend not only on the projectile energy and charge state, but also on the particular collision channel involved. It is also shown that the fragmentation fractions follow the same scaling rule obtained previously for the N2 molecule by Melo et al (2006 J. Phys. B: At. Mol. Opt. Phys. 39 3519).

Potential electron emission induced by multiply charged ions in thin film tunnel junctions

Thin film metal-insulator-metal tunnel junctions are used to investigate the electronic excitation process induced by the impact of multiply charged ions onto a metallic surface. Hot charge carriers (electrons and holes) generated by the dissipation of the kinetic and potential energies of the projectiles are detected as an ion induced internal emission current from the bombarded ``top'' metal film into the ``bottom'' substrate electrode. Results are presented for ${\text{Ar}}^{q+}$ ions with a kinetic impact energy of 1 keV and charge states $q=1--8$ impinging onto an ${\text{Ag-AlO}}_{x}\text{-Al}$ junction. It is shown that the internal emission yield exhibits an approximately linear dependence on the potential energy of the projectile. At low potential energy, a bias voltage applied between the two metal films is found to strongly influence the internal emission current, whereas this influence becomes much weaker with increasing projectile charge state. The results are shown to be qualitatively well described in the framework of a thermodynamical free-electron model.

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.

X-ray spectra induced by 129Xe q+ impacting the metal surface

Using the slow highly charged ions Xe-129(q+) (q = 25, 26, 27; initial kinetic T-0 <= 4.65 keV/a.u.) to impact Au surface, the Au atomic M alpha characteristic X-ray spectrum is induced. The result shows that as long as the charge state of projectile is high enough, the heavy atomic characteristic X-ray can be effectively excited even though the incident beam is very weak (nA magnitude), and the X-ray yield per ion is in the order of 10(-8) and increases with the kinetic energy and potential energy of projectile. By measuring the Au M alpha-X-ray spectra, Au atomic N-level lifetime is estimated at about 1.33x10(-18) s based on Heisenberg uncertainty relation.