Projectile Charge State Research Articles

Contribution of different electron-capture mechanisms in the fragmentation of CO23+ upon slow ion-molecule collision

Abstract We present recoil ion - projectile ion coincidence study on the three body fragmentation of CO23+ ion into C+ + O+ + O+ fragments upon impact with
Ar3+, Ar6+ and Ar8+ projectile ions having the same velocity of vp = 0.49 a.u. (atomic units) using recoil ion momentum imaging technique. The kinetic energy
release (KER) distributions are presented and effect of different electron-capture associated processes such as pure capture, electron capture followed by target ionization or projectile auto-ionization is discussed based on the analysis of KER corresponding to different final projectile charge state post collision. As the capture stabilization increased the relative yields in the KER distribution are found to decrease in the high KER side for low charged Ar3+ impact, while an increase is observed for Ar6+ and Ar8+ impacts. We discussed the interplay between electron-capture, projectile auto-ionization and target ionization for the dominance of particular process. We also observed a low-KER feature around 15 eV which was observed for electron
and proton impacts, which was not yet observed for highly charged ion impacts

State-selective charge exchange cross sections in collisions of highly-charged sulfur ions with helium and molecular hydrogen

The state-selective cross section data are useful for understanding and modeling the x-ray emission in celestial observations. In the present work, using the cold target recoil ion momentum spectroscopy, for the first time we investigated the state-selective single electron capture processes for S q+–He and H2 (q = 11–15) collision systems at an impact energy of q × 20 keV and obtained the relative state-selective cross sections. The results indicate that only a few principal quantum states of the projectile energy level are populated in a single electron capture process. In particular, the increase of the projectile charge state leads to the population of the states with higher principal quantum numbers. It is also shown that the experimental averaged n-shell populations are reproduced well by the over-barrier model. The database is openly available in Science Data Bank at 10.57760/sciencedb.j00113.00091.

Molecular electronic stopping cross section for H and He beams colliding with water: projectile charge state contribution

Abstract When an ion beam impinges on a target material, electron stripping and capture processes involve different charge fraction states in the beam, such that each projectile charge state produces a different energy-loss. In this work, the energy deposition of swift hydrogen and helium ion beams colliding with a water target in the gas phase is studied. The electronic structure of the molecular target is represented in terms of core, bond, and lone pair orbital decomposition within a Harmonic Oscillator representation. In this way, the stopping cross section becomes only a function of the orbital mean excitation energy, I 0i . The ion beam charge fraction compositions colliding on water is determined from the work of Wedlund et al (2019) Astronomy & Astrophysics, 630, A36) by accounting for the electron transfer cross sections. We find that the larger the projectile charge state, the larger the electronic stopping cross section and that the beam charge fraction determines the position of the maximum of the electronic stopping curve. Also, in agreement with the experiment, evidence is given on the dominant role of the largest projectile charge state in defining the stopping cross section for high energy collisions, while for low collision energies it is the lowest charge state together with all possible charge states contributing at the maximum of the electronic stopping cross section curve. Our results are reported and compared to available experimental data showing an excellent agreement to the available literature.

Publisher’s Note: “Development of a cold target recoil ion momentum spectrometer and a projectile charge state analyzer setup to study electron transfer processes in highly charged ion–atom/molecule collisions” [Rev. Sci. Instrum. 93, 113313 (2022)

Publisher’s Note: “Development of a cold target recoil ion momentum spectrometer and a projectile charge state analyzer setup to study electron transfer processes in highly charged ion–atom/molecule collisions” [Rev. Sci. Instrum. 93, 113313 (2022)

Impact ionization of highly charged ion-atom collisions considering strong magnetic field and plasma screening effect

The impact ionization process of target is a crucial issue in the study of Heavy Ion Beam (HIB)-driven Magnetized Inertial Fusion (MIF). The collision systems between heavy projectile ions (Xeq+, Biq+, Uq+) and light target atoms/ions (H, D, T, 3He, 3He+, 11B) are promising candidate reactions for fusion power. In this study, the impact ionization process in medium-energy (ranges 1–100 MeV/u) is studied numerically based on the extended Classical-trajectory Monte Carlo (CTMC) method, where the effects of projectile charge state, external magnetic field and plasma screening on the single ionization process are considered. The results show that the existed external magnetic field results in a decrease of the yield of ionized high-energy electrons, whereas the yield of backward emitted electrons is increased. The effect of plasma screening on the ionization process is shown to be determined by the competition between the screening charge of projectile ion and the target nucleus. Thus, a significant decrease of ionization cross section is found in the low energy region since the screening charge of projectile ion is dominant. However, the screening effect of target nucleus charge is dominant in high energy region, where a significant increase of ionization cross sections is found.

Exploring the influence of target atomic number (Z2) on mean equilibrium charge state (q¯: A comprehensive study

We have investigated the effect of the target atomic number Z2 on the mean charge state (q¯ using various model predictions such as the Shima–Ishihara –Mikumo, Ziegler–Biersack–Littmark, Schiwietz, Schiwietz–Grande , Fermi-gas-models and theoretical codes with experimental data available in the literature. This investigation makes it possible to determine the best-fit model to calculate q¯. In this work, we discuss the post-collision charge state distribution in different targets used as thin films and projectile beams (Fq+, Siq+, Clq+, and Cuq+) with different charge states (available in literature). A detailed overview of such collision experiments has been explored over a wide energy range of 1.07–3.93 MeV/u. In this contribution, an overview of the mean charge state dependence on the Fermi velocity of target materials is provided. Finally, the influence of the non-radiative electron capture at the target exit surface on the projectile charge state distribution for fast projectiles in different targets is shown, and a comparison is made with experimental data.

Influence of single adsorbed atoms on charge exchange during ion-surface collisions

Partial and final projectile charge states are theoretically analyzed after ${\mathrm{Li}}^{+}$ ions frontally collide with a single-layer graphene surface with potassium atoms adsorbed on it, in the very low deposition limit where the adsorbed atoms negligibly interact with each other. We applied a model based on first principles that accounts for all the details of the surface and is used to describe the resonant charge exchange processes occurring in binary collisions between the projectile and the adsorbate, and between the projectile and the C atoms of the graphene surface located in the surroundings of the adsorption site. Completely different neutral fraction dependences with the outgoing velocity are found for the projectiles scattered by the adsorbate and by carbon atoms located far from the adsorbed atom. In addition, an important influence of the adsorbate is perceived in the neutralization of projectiles colliding with the first and second set of carbon atoms nearest to the adsorption site. For C atoms located at middle-range distances (\ensuremath{\sim}6.7 \AA{}), the final charge state of the projectile is not affected by the adsorbate presence, even when an effective interaction between the projectile and the K adsorbed atom is revealed via appreciable projectile level widths.

The role of partial ionization on the instantaneous charge state of uranium projectiles in fully ionized plasmas from energy loss experiments

In Morales et al. [Phys. Plasmas 24, 042703 (2017)], we analyzed the experimental energy loss of U ions with an initial charge state of 33+ and an energy of 1.4 MeV/u passing through a highly ionized hydrogen plasma from the measurements shown in Hoffmann et al. [Phys. Rev. A 42, 2313 (1990)]. However, in our previous work, we assumed a fully ionized hydrogen plasma and our theoretical predictions overestimated the data at the times of 30 and 35 μs. As the capture of bound electrons was missing in the previous version of the code, we explained those discrepancies due to the possible presence of a higher part of bound target electrons at these times. In such a case, the projectile charge-state should be lower, and therefore, the theoretical energy loss would be also smaller in consequence. The main novelty of this work is to include the partial ionization of the plasma target, which was neglected in our previous analysis, for the theoretical estimation of the ion beam charge-state. Here, we re-analyze the experiment with the new version of our charge-state computer code based on the Cross-Sectional Model (CSM) as described in R. Morales [Phys. Plasmas 29, 093112 (2022)]. We are showing that taking into account the small contribution of bound electrons present in the plasma target affects the projectile mean charge-state and a much better agreement with energy loss data is found with our new CSM code.

Development of a cold target recoil ion momentum spectrometer and a projectile charge state analyzer setup to study electron transfer processes in highly charged ion-atom/molecule collisions.

We report the development and performance of a cold target recoil ion momentum spectrometer (COLTRIMS) setup at TIFR, which is built to study various atomic and molecular processes involving the interaction of slow, highly charged ions from an electron cyclotron resonance based ion accelerator. We give a detailed description of the experimental setup, as well as report some initial results on the electron-capture process in collisions of Ar8+ ions with helium and carbon monoxide targets. Here, we present the longitudinal momentum transfer and the sub-shell resolved Q-value spectrum in the case of 2, 4, and 6 keV/u Ar8+ beams in collision with helium. A longitudinal momentum resolution of 0.27a.u. is achieved in the present system. We also report the state-selective scattering angle distributions for all the collision systems under investigation. We further discuss the fragmentation of the CO2+ molecular ions for different electron capture channels for the 5 keV/u Ar8+ beam. The combination of the COLTRIMS, along with the beam cleaner, the electrostatic deflectors, and the charge state analyzer, is shown to have certain advantages.

Unexplained dissociation pathways of two-body fragmentation of methane dication.

The ion-induced fragmentation of CH4 2+ into H+ and CH3 + is studied using a cold target recoil ion momentum spectroscopy in coincidence with the charge state of the post-collision projectile. Using constant velocity Ar9+ and N3+, results from four different datasets are presented, with a selection on the final charge state of the projectile (Ar8+ or Ar7+ and N2+ or N+). Three distinct dissociation pathways (I, II, and III) are observed for each dataset, with the mean kinetic energy release values of around 4.7, 5.8, and 7.9eV, respectively. The electronic states that are populated correspond to electronic configurations (1t2)-2 and (2a1)-1(1t2)-1 of the methane dication, CH4 2+. The relative branching ratios between the three pathways are discussed as a function of the charge state of the post-collision projectile, and a strong correlation with the specific nature of the ion-molecule interaction is found. The existing ab initio calculations have provided an explanation only for pathway II. In this article, we propose an explanation for pathway III, but pathway I still remains unexplained and requires further theoretical efforts. A discussion of the dependence of dissociation on the mode of excitation is presented.

A recoil ion momentum spectrometer for probing ionization, e-capture, and capture-ionization induced molecular fragmentation dynamics.

The development of a recoil ion momentum spectrometer (RIMS) along with a post-collision projectile charge state analyzer (CSA) and its performance for carrying out studies of molecular fragmentation following direct ionization, electron-capture, and capture-ionization have been demonstrated here. This is a two-stage Wiley-McLaren type spectrometer with slight modification introduced by adding a lens to achieve higher momentum resolution as well as larger angular acceptance. Along with the time and position sensitive detector, it can measure all the three momentum components of singly charged recoil ions of energy up to 10 eV emitted in all directions. The CSA assembly is designed for separating out any neutral or singly or doubly charged post-collision projectiles typically of keV energy. The RIMS with initial trigger ("start") from CSA or an ionized electron can uniquely determine the dynamics of molecular fragmentation following different electron-capture or direct ionization events, respectively. To check the performance of the setup, we carried out an experimental study of the fragmentation of N2 molecules under the impact of 250 keV protons. Apart from the single-electron-capture channel, we could clearly identify three more capture-ionization channels, which lead to fragmentation. The essential features of the momentum distributions and the kinetic energy release distributions of all three fragmentation channels are discussed in detail. These results are compared with the findings from the ionization induced fragmentation experiments and with the available results from theoretical calculations as well as high resolution experiments. The branching ratios of these fragmentation channels are determined.

Charge and excitation state of Na projectiles scattered from Al surfaces

This work addresses the issue of the anomalous neutralization of slow sodium ions at aluminum surfaces by measuring spectra of Auger electrons emitted by collisionally excited sodium projectiles. The anomalous fraction of scattered singly charged ions is due to the Auger decay of singly excited projectiles. Electron emission experiments reveal a sizeable production of doubly charged ions that should be observed in ion fraction measurements. Measurements at varying incidence angles provide a rather detailed description of the successive stages of the inelastic scattering events that determine the charge state of scattered projectiles.

Significance of the high charge state of projectile ions inside the target and its role in electron capture leading to target-ionization phenomena

The $K$ x-ray spectra of different targets (Cu, Zn, and Ge) induced by 3--5 MeV/u Si projectile ions have been measured to determine the $K$-shell ionization cross section. A significant difference is observed between the measurements and theoretical estimates, with the latter being about 50% below the experimental results. This underestimation is attributed to the charge exchange from the target $K$ shell to projectile $K$ and $L$ shells. Such an observation can only be possible if the projectile ions attain up to H- and He-like charge states. Corresponding projectile charge state fractions have been evaluated from the Lorentzian charge state distribution, where the mean charge state is taken from the Fermi gas model [W. Brandt et al., Phys. Rev. Lett. 30, 358 (1973)] and the width from the Novikov and Teplova approach [Phys. Lett. A 378, 1286 (2014)]. The sum of the direct ionization cross section and $K\text{\ensuremath{-}}K+K\text{\ensuremath{-}}L$ capture cross sections gives a good agreement with the measured cross sections. Furthermore, we have validated this methodology with available data for a Si ion on Ti target. Such results may be useful in many solid target-based applications.

Correct calculation of nitrogen charge state passing through highly ionized carbon plasmas.

In the present work, we reanalyze the energy loss experimental data from Cayzac etal. [Nat. Commun. 8, 15693 (2017)10.1038/ncomms15693] using our successful ion charge state theoretical model. We predict lower nitrogen charge values, from 3.5+ to 5.0+, than the ones calculated by Cayzac etal., fitting better to their data. For energy loss estimations, we use the same stopping model, so our predictions agree better with the experimental data only due to our charge state model. Different projectile electron loss and capture processes are taken into account to estimate the projectile charge state. The projectile electron loss, or ionization, with plasma ions and free electrons are considered. On the other hand, the projectile electron capture, or recombination, with plasma free or bound electrons are also considered. The projectile ionization with plasma ions is shown as the main factor that modifies the mean charge of the projectile. Here, the new Kaganovich fitting formula for this projectile ionization is used because it seems to be more accurate than Gryzinsky's fitting in the low energy range. Our charge state model fits better with experimental data than any other model in the bibliography. Thus, it should be considered in any charge state and any energy loss estimation to obtain reliable results in future work.

Velocity and charge-state dependence on the Coulomb explosion of N2, under the impact of highly-charged ions at intermediate velocities

We report the comprehensive measurement of the Coulomb explosion of the double and triple ionization of N2 in collisions with slow highly-charged ions. The experiments are carried out with ion beams of different charge states for a given velocity (1 a.u.) and at three different velocities for a given charge state (O6+). By analyzing the kinetic energy release distributions of and molecular ions, the effects of projectile velocity and charge state on the population of different molecular states are inferred. We explain such a systematic variation in the population of different molecular states considering the energy deposition to N2 by the projectile ions.

Electron capture and loss in the scattering of low-energy protons with a C60 monolayer deposited on Cu(111)

Final projectile charge states are experimentally and theoretically analyzed after ${\mathrm{H}}^{+}$ ions collide with a ${\mathrm{C}}_{60}$ monolayer deposited on Cu(111) with an ample range of incoming energies (2--8 keV) in the low-energy regime. The three possible charge states (negative, positive, and neutral) are experimentally measured by using the low-energy ion scattering technique for two different collisional setups: 45\ifmmode^\circ\else\textdegree\fi{} (90\ifmmode^\circ\else\textdegree\fi{}) and 67.5\ifmmode^\circ\else\textdegree\fi{} (67.5\ifmmode^\circ\else\textdegree\fi{}) incoming (exit) angles, relative to the target surface plane, with a fixed backscattering angle of 135\ifmmode^\circ\else\textdegree\fi{}. Experimental ion fraction magnitudes and energy dependence are practically intermediate between that found in pristine Cu(111) and a thick ${\mathrm{C}}_{60}$ film, revealing the influence of the substrate on the final charge state of the projectile. Unlike these previous systems, the positive and negative ions contribute nearly evenly to the total scattered charged particles. On the theoretical side, we applied a first-principles based model that considers the fine details of the surface under analysis and assumes a projectile trajectory corresponding to a single binary collision with the more exposed carbon atoms of the ${\mathrm{C}}_{60}$ molecule. The theoretical and experimental results are independently compared with the already reported cases: ${\mathrm{H}}^{+}$ on a thick ${\mathrm{C}}_{60}$ film, ${\mathrm{H}}^{+}$ on Cu(111), and ${\mathrm{H}}^{+}$ on graphite. A detailed analysis of the electronic surface band structure allows us to draw a conclusion about the relevance of the substrate in the present system and about the aspects to be improved in our theoretical description. The contrast between experimental and theoretical results allows us to infer that trajectories involving ion penetration and multiple scattering events are particularly relevant for the projectile-target charge exchange process studied.

Dissociative ionization of OCS induced by highly charged ion impact

Dissociation of $\mathrm{OC}{\mathrm{S}}^{q+}$ $(q=3--7)$ produced by $18\text{\ensuremath{-}}\mathrm{keV}/\mathrm{u}\phantom{\rule{0.16em}{0ex}}{\mathrm{Ne}}^{8+}$ ion impact has been investigated using recoil ion momentum spectroscopy. Ten complete three-body Coulomb fragmentation channels were analyzed. Scattered projectile charge state $({\mathrm{Ne}}^{7+} \mathrm{or} {\mathrm{Ne}}^{6+})$ differentiated branching ratios and the kinetic-energy release (KER) distributions were analyzed for different fragmentation channels. Through Dalitz plots, Newton diagrams, and native frames, we have studied the concerted and sequential fragmentation mechanisms for channels leading to ${\mathrm{O}}^{+}+{\mathrm{C}}^{+}+{\mathrm{S}}^{+}$, ${\mathrm{O}}^{+}+{\mathrm{C}}^{+}+{\mathrm{S}}^{2+}$, ${\mathrm{O}}^{2+}+{\mathrm{C}}^{+}+{\mathrm{S}}^{+}$, and ${\mathrm{O}}^{+}+{\mathrm{C}}^{+}+{\mathrm{S}}^{3+}$. For concerted dissociation mechanisms, the molecular bending fragmentation dominates in these channels. For sequential dissociation mechanisms, the KER distribution for the dissociation of intermediate ions $\mathrm{C}{\mathrm{O}}^{2+}/\mathrm{C}{\mathrm{S}}^{2+}$ were determined and the rotation of the intermediate molecular ion $\mathrm{C}{\mathrm{O}}^{2+}$ before the second step dissociation was discussed. Thereby the relative branching ratios of events coming from sequential versus concerted mechanism were also estimated.

The research on optical radiation in the collisions of low energy, highly charged Xeq+ ions with MgOAl2O3 surfaces

In this work, the optical spectra emitted from the magnesia-alumina spinel (MgOAl2O3) surfaces induced by low energy, highly charged Xeq+ (6 ≤ q ≤ 20) ions were measured. In the collisions, the excitation process of sputtered atoms is discussed. In the analysis, a special attention was given to spectral composition of the optical radiation, dependences of lines intensity on projectile charge state (projectile potential energy), relative intensities of different lines, in which mainly present three features: (a) the results do not appear the spectral line of O particle. (b) there were noapparentincreases in the photon yields of atomic Mg and Al lines with the increase of projectile charge state. (c) the ratios of spectral lines Al I 309.27 nm to Al I 396.41 nm of excited Al atoms ejected from the surface of insulator MgOAl2O3 increased with the increasing projectile potential energy, and both of the ratios of spectral lines Mg II 280.40 nm to Mg I 285.42 nm and Mg II 279.73 nm to Mg I 383.37, 383.98 nm increased with projectile potential energy increase.

Time-of-flight mass spectrometry of particle emission during irradiation with slow, highly charged ions.

We describe a setup for the analysis of secondary ions and neutrals emitted from solid surfaces and two-dimensional materials during irradiation with highly charged ions. The ultrahigh vacuum setup consists of an electron beam ion source to produce bunches of ions with various charge states q (e.g., Xe1+-Xe46+) and thus potential energies, a deceleration/acceleration section to tune the kinetic energy of the ions in the range of 5 keV to 20 × q keV, a sample stage for laser-cleaning and positioning of freestanding as well as supported samples, a pulsed excimer laser for post-ionization of sputtered neutrals, and a reflectron type time-of-flight mass spectrometer, enabling us to analyze mass and velocity distributions of the emitted particles. With our setup, contributions from potential and kinetic energy deposition can be studied independently of each other. Charge dependent experiments conducted at a constant kinetic energy show a clear threshold for the emission of secondary ions from SrTiO3. Data taken with the same projectile charge state, but at a different kinetic energy, reveal a difference in the ratio of emitted particles from MoS2. In addition, first results are presented, demonstrating how velocity distributions can be measured with the new setup.

A Model for Projectile Charge State Distribution Inside a Solid-Target

A Model for Projectile Charge State Distribution Inside a Solid-Target