Charged Projectile Ions Research Articles

Ionization of CH[formula omitted] under fast He-like Si-ion impact

We have studied the absolute double differential cross section (DDCS) of electrons ejected from the methane molecule (CH4) under the impact of highly charged fast projectile ions. The energy and angular distributions of electrons have been measured in interactions with 70 MeV Si12+ ions. These DDCS data together with the derived single differential cross sections and the total cross section (TCS) have been compared with predictions of the continuum distorted wave-eikonal initial state model using different approaches for the molecular orbitals. It has been found that the theories overestimate the measured DDCS values at low ejection energies, which may indicate shortcomings of the model in the case of strong perturbation. At the same time, applying the model to a previous measurement using fast C ions having a lower perturbation strength, excellent agreements have been obtained with the experimental data. Considering the scaling properties, the available TCS data have been plotted with a scaled parameter, namely the perturbation strength, q/v (where q= charge state, v=velocity of the projectile). The KLL Auger e-emission as well as the KLL hyper-satellite peaks are analyzed for different emission angles. The double K-vacancy production shows a considerable enhancement i.e. 37% of the single production cross section which is consistent with some of the recent experiments on K-ionization using x-ray techniques.

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.

Multicharged-ion–water-molecule collisions in a classical-trajectory time-dependent mean-field theory

A recently proposed classical-trajectory dynamical screening model for the description of multiple ionization and capture during ion--water-molecule collisions is extended to incorporate dynamical screening on both the multicenter target potential and the projectile ion. Comparison with available experimental data for $\mathrm{He}{}^{2+}$ $+$ $\mathrm{H}{}_{2}\mathrm{O}$ collisions at intermediate energies (10--150 keV/u) and $\mathrm{Li}{}^{3+}$ $+$ $\mathrm{H}{}_{2}\mathrm{O}$ at higher energies (100--850 keV/u) demonstrates the importance of both screening mechanisms. The question of how to deal with the repartitioning of the capture flux into allowed capture channels is addressed. The model also provides insights for data on highly charged projectile ions $(\mathrm{C}{}^{6+},$ $\mathrm{O}{}^{8+},$ $\mathrm{Si}{}^{13+})$ in the MeV/u range where the question of saturation effects in net ionization was raised in the literature.

An analytical formula for the Barkas correction to the theory of ion stopping

Nonperturbative methods of nonrelativistic quantum mechanics have been used to obtain a simple analytical form of the Barkas correction to the stopping factor for arbitrary values of the charge of projectile ion and collision velocities greater than or comparable with atomic values. The proposed theory involves the notion of the effective interaction radius that is borrowed from the well-known Bohr theory of stopping of fast charged particles.

Stability of the glycine cation in the gas phase after interaction with multiply charged ions

This study presents a coupled experimental and theoretical work on the stability of singly charged glycine molecules (NH2CH2COOH+, the simplest amino acid) produced in the collision of neutral glycine with low-energy multiply charged ions in the gas phase. The main fragments observed experimentally result from Cα-Ccarboxyl bond cleavage. Additionally, some fragments are produced after an intramolecular rearrangement of the glycine cation, in particular the loss of a neutral water molecule following a H-migration. Moreover, this work discusses qualitatively the energy transferred to the molecule in the collision with a comparative study of the fragmentation patterns for different projectile ion charge states.

Coulomb excitation of highly charged projectile ions in relativistic collisions with diatomic molecules

We investigate the Coulomb excitation of highly charged ions colliding with diatomic molecules. In this process, the coherent interaction between the projectile electron and two molecular centers may cause clear interference patterns in the (collision) energy dependencies of the total cross sections and alignment parameters. We discuss such a Young-type interference for the particular case of the $K\ensuremath{\rightarrow}L$ excitation of hydrogen- and helium-like projectile ions. Calculations, performed for the scattering of these ions on nitrogen molecules, indicate that the interference effects are extremely sensitive to the collisional geometry and are pronounced only if the molecular axis is aligned almost parallel to the incident beam trajectory.

Influence of the projectile charge state on the ionization probability of sputtered particles

We present a computational study of the effect of the projectile charge state on secondary ion formation in sputtering. A molecular dynamics simulation of an atomic collision cascade is combined with a kinetic excitation model including electronic friction and electron promotion in close atomic collisions. The model is extended to account for potential excitation following the bombardment with a highly charged ion (HCI). The spatial spreading of the excitation generated in the cascade is treated in an diffusive approach. The excitation energy density profile obtained this way is parametrized via an effective electron temperature, which is then used to calculate the ionization probability of each sputtered atom in terms of a simple charge exchange model. The results obtained for the impact of a 5 keV Ag atom onto a solid silver surface show that the average ionization probability increases from 4.7 × 10 - 4 for a neutral projectile to 5.4 × 10 - 4 for a highly charged projectile ion with a total ionization energy of 576 eV.

Charge exchange, surface-induced dissociation and reactions of doubly charged molecular ions SF 42+ upon impact on a stainless steel surface: A comparison with surface-induced dissociation of singly charged SF 4+ molecular ions

Collisions of SF 4 2+ and SF 4 + ions with hydrocarbon-covered stainless steel surface at room temperature were investigated. The projectile ions were mass selected by a two-sector-field mass spectrometer and decelerated to incident energies of 60 to a few eV. Product ions were measured with the use of a time-of-flight spectrometer and their relative abundances determined as a function of the incident energy of the projectile ions (collision-energy-resolved mass spectra, CERMS curves). The mass spectra of product ions were dominated by fragment ions SF 3 +, SF 2 +, and SF + at incident energies below 40 eV, while sputtering of contaminant adsorbates prevailed at higher energies. The results indicate that the likely major reaction sequence responsible for the observed CERMS curves of product ions from SF 4 2+ collisions is charge exchange to form singly charged projectile ions followed by subsequent unimolecular fragmentation. In addition, chemical reactions between projectile ions and hydrocarbon adsorbates were observed leading to SF 2CH 3 +, SFCH 2 +, and SCH + ions.

Numerical simulations of the projectile ion charge difference in solid and gaseous stopping matter

AbstractThe dependence of calcium ion subshell populations on the target density during the ion stopping process was analyzed using a five charge-state collisional-radiative model. The model, which consists of the ground and three excited states for every ion charge, was successfully compared with the experiment. The gas-solid difference of calcium ion charge state distribution has been numerically demonstrated. For Ca projectiles with energies of 4–11 MeV/u, the increase of the mean ion charge in solid target is explained by the increase of the total ionization rate and by suppression of the bound electron capture process at high densities of the stopping medium.

Separation of potential and kinetic electron emission from Si and W induced by multiply charged neon and argon ions

The total secondary electron emission yields, γT, induced by impact of the fast ions Neq+ (q=2–8) and Arq+ (q=3–12) on Si and Neq+ (q=2–8) on W targets have been measured. It was observed that for a given impact energy, γT increases with the charge of projectile ion. By plotting γT as a function of the total potential energy of the respective ion, true kinetic and potential electron yields have been obtained. Potential electron yield was proportional to the total potential energy of the projectile ion. However, decrease in potential electron yield with increasing kinetic energy of Neq+ impact on Si and W was observed. This decrease in potential electron yield with kinetic energy of the ion was more pronounced for the projectile ions having higher charge states. Moreover, kinetic electron yield to energy-loss ratio for various ion-target combinations was calculated and results were in good agreement with semi-empirical model for kinetic electron emission.

A setup for probing ion–molecule collision dynamics

We describe a setup for studies of dissociation dynamics of multiply charged molecules formed due to highly charged ion impact. The setup consists of a multihit, position-sensitive, time of flight measurement system along with an electrostatic analyzer with offset detector. Brief description of different components of the setup and electronics for multihit data acquisition is presented in this paper. Coulomb explosion of multiply ionized N 2 and charge exchange studies on Ar with highly charged projectile ion is carried out to test the performance of the spectrometer and its multifold coincidence capabilities.

Electron angular and energy distributions following the ionization of highly charged projectile ions

The projectile ionization of highly charged, hydrogen-like ions is studied within the framework of first-order perturbation theory and Dirac's relativistic equation. Emphasis is placed, in particular, on the angular and energy distributions of the emitted electrons as observed in both the projectile and the laboratory frame. Detailed computations are carried out to investigate the effects from the excited states of the projectiles upon the energy-differential ionization cross sections. For relativistic U91+ ions, for instance, a small partial population of the excited 2s1/2 state results in a much narrower energy spectrum of the emitted electrons than obtained for the case of a pure K-shell ionization. This behaviour of the differential cross sections might help the understanding of a recent observation by Vane et al (2000 Proc. 21st Int. Conf. of the Physics of Electronic and Atomic Collisions (Sendai, Japan) (New York: AIP) pp 709–14) on the ionization of heavy, hydrogen-like projectiles in ultra-relativistic ion–atom collisions.

Ionization of helium by relativistic highly charged ions within the symmetric eikonal approximation

We consider ionization of helium targets by impact of highly charged projectile ions in collisions at relativistic energies. The overwhelming majority of electrons emitted in such collisions have velocities in the target frame, which are much less than the speed of light. Therefore, considering the collisions in this frame we treat the electron dynamics nonrelativistically. The projectile–target coupling in the collision is described within the symmetric eikonal approximation taking explicitly into account the interaction between the projectile and all the target constituents. The relation between the symmetric eikonal and the first Born approximation is discussed in detail. Helium single ionization is considered by developing an effective three-body (projectile, ‘active’ electron, target core) model of this process in which (undistorted) initial and final states of the ‘active’ electron are described in a Hartree–Fock approximation. Using a four-body model we also briefly discuss the contribution to single ionization from simultaneous ionization–excitation. A good agreement with available experimental data has been found.

Nanoscopic surface modification by slow ion bombardment

We present systematic scanning tunneling microscopy (STM)/atomic-force microscopic (AFM) investigations on nanoscopic defect production at atomically clean surfaces of SiO 2, Al 2O 3 and highly oriented pyrolytic graphite (HOPG) after bombardment by slow (impact energy≤1.2 keV) singly and multiply charged ions under strict ultra-high vacuum (UHV) conditions. Combined STM and AFM studies show that on HOPG only “electronic” but no visible topographic defects are created by such ion bombardment. On the monocrystalline insulator surfaces, well-defined topographic features of typically nm extensions are produced (“potential sputtering”). For Al 2O 3 and HOPG, a clear dependence of the defect size on the projectile ion charge is demonstrated. These results are discussed in view to possible new nanoscopic surface structuring and modification methods for which the kinetic projectile energy plays a minor role only.

Asymptotic energy dependence of projectile excitation in relativistic ion-atom collisions

The Coulomb excitation of inner-shell electrons around heavy and highly charged projectile ions impinging on light targets is investigated in the highly relativistic energy regime for projectiles as heavy as gold $(Z=79).$ First-order time-dependent perturbation theory is employed for treating the interaction of the active electron with the light target, and exact relativistic Coulomb-Dirac wave functions are used for the description of electronic states in the strong field of the projectile nucleus. The energy dependence of transition probabilities and partial cross sections is studied up to extreme relativistic energies of hundreds of GeV/u and beyond. An analytic representation of the asymptotic energy dependence of the projectile excitation cross section complements the numerically calculated impact-parameter-dependent probabilities and cross sections, providing additional insights.

Observation and analysis (synthesis) of X-ray spectrum originated from electron capture of low-energy, highly charged Xeq+ (q = 26–43) ions in single collisions with Ar atom

X-rays have been observed in collisions of low-energy (keV/u), highly charged Xeq+ (q = 26–43) ions with neutral Ar atoms. These X-rays are understood to be produced through electron capture by highly charged projectile ions from target atoms, the electrons then cascade down to the ground state. It is clearly noted that the most intense X-ray peaks correspond to M-shell – N-shell transitions with different numbers of M-shell vacancies and that X-ray intensities decrease significantly toward high energies near the ionization limit. This observation indicates that the direct transition of an electron captured in a highly excited state to M-shell vacancies is negligibly small. To obtain a better understanding of X-ray production mechanisms, we tried to synthesize the expected X-ray spectrum and compare that with the observed spectrum. The synthesized spectra were found to reproduce the observed spectra reasonably well. PACS Nos: 32.30Rj, 32.70Cs, 32.80Rm, 34.70+e

In-situ Observation of Surface Modification Induced by Highly Charged Ion Bombardment

Nanoscale dots created on the surface of a highly oriented pyrolitic graphite sample by individual highly charged xenon projectile ions were observed using an in-situ scanning tunneling microscope. This is the first time that such features have been created and imaged without exposure to air. The dots vary in size with the charge state of the ion, up to a diameter of 6.6 nm for Xe44+. The results are discussed in terms of the energy density deposited in the near surface region.

Total electron emission from metals due to the impact of highly-charged Xe ions with energies up to MeV

Total electron emission from metals due to the impact of multiply charged ions, γ, may significantly influence quantitative measurements of ion current in corpuscular diagnostics. The value of γ (γ/q) was determined for Xe ions impacting clean polycrystalline copper as a function of ion charge state \(\left( {q = 6 - 28} \right)\) and of ion kinetic energy, \(E_i /q = \left( {5 - 150} \right)\) keV/q, i.e. in the energy region up to \(E_i /A \approx 30\) keV/amu, where there is a lack of such data. For highly charged projectile ions, \({\gamma}\) was found to have a clear minimum as a function of Ei. With decreasing charge state of the projectile ion this minimum shifts to a lower energy and becomes shallower. This observation is in agreement with compiled results of other authors. Limits for values of \({\gamma /}q\) are estimated and discussed.

An experimental and computational study of the double ionization of CH 3Cl, CH 2Cl 2, and CHCl 3 molecules to singlet and triplet electronic states of their dications

Experimental and computational methods were applied to study the double ionization of the molecules CH 3Cl, CH 2Cl 2, and CHCl 3. Double-charge-transfer spectroscopy was applied to measure the double-ionization energies of the molecules. The translational energies of the negative ions generated when fast-moving singly charged positive projectile ions acquire two electrons in collisions with the molecules provides information on the states of the dications populated. The use of H +, OH +, and F + projectile ions allowed transitions to singlet and triplet electronic states of the dications to be studied. Double-ionization energies to those states were calculated using the second order algebraic diagrammatic construction Green’s function method. The calculated and measured values for CH 3Cl were found to be in good agreement. The computed data predicted that the density of states for CH 2Cl 2 2+ and CHCl 3 2+ were much higher than for CH 3Cl 2+. On grouping the calculated double-ionization energies to close-lying states together, however, good agreement with the measured values was evident. The combined experimental and computational study reported here thus provides a detailed understanding of the double ionization of the three molecules to singlet and triplet electronic states of their dications.

Production and relaxation pathways of multiply excited states in slow highly charged ion-atom collisions

When a slow multiply charged projectile ion moving at a velocity below that of typical outer-shell atomic electrons collides with a many-electron neutral target, a large number of electrons can become active during the collision. Such collisions are dominated by the transfer of a number of target electrons to the projectile, resulting in the formation of projectile multiply excited states. Substantial progress has been made toward understanding one- and two-electron processes in slow ion-atom collisions during the past three decades ~see, e.g., Ref. @1#, and references therein!. Although slow collisions involving more than two active electrons have also been investigated for over two decades as well, our understanding of multielectron processes is by no means comparable to that of one- and two-electron processes. The vast majority of experimental studies of multielectron processes involved measurements of cross sections for projectile charge-change and recoil-ion production, both in a singles’ and in coincidence modes, and a limited number involved energy gain and visible photon spectroscopy ~see, e.g., @2‐5#, and references therein!. Moreover, since autoionization is a main decay mode of multiply excited states, Augerelectron spectroscopy has been employed in a singles’ mode @6,7# to study such collisions. While such measurements have played a significant role in understanding two-electron processes ~see, e.g., Ref. @8#, and references therein!, the situation is drastically different when many electrons are involved. For example, Benoit-Cattin et al. @6# obtained a singles’ electron spectrum for the 70-keV N 71 1Ar collision system. The analysis of the spectrum was rather difficult since it contained contributions from doubly, triply, quadruply, and quintuply excited states that rendered the interpretation nontrivial. During the last six years, however, Morgenstern and co-workers have made significant contributions @9‐12# toward understanding Auger-electron spectra obtained in multiple-electron capture processes by means of the coincident detection of Auger electrons and target ions. They obtained partial Auger spectra corresponding to the different target ion charge states that are much more informative than singles’ spectra. These spectra can provide further information if the final projectile charge state is also determined. From a theoretical point of view, understanding multielectron processes is a twofold problem. First, the different mechanisms involved in the collision process that lead to the production of the multiply excited states must be recognized and described. Second, the radiative and nonradiative properties of the resulting multiply excited states must be known. Concerning the first problem, quantum-mechanical or semiclassical treatment of collisions involving more than two electrons is prohibitively difficult due to the large number of channels involved. Therefore, extended classical overbarrier ~ECB! models have been developed @13,14# to account for multiple-electron capture processes. These models are limited to giving the capture state distribution on the projectile and possible simultaneous target excitation. There has been, until recently @15‐17#, a severe lack of theoretical work on the radiative and nonradiative properties of multiply excited states, due in part to the extremely large number of states that need to be taken into account, and in part to the lack of experimental data to which the calculations can be directly compared. Therefore, relaxation schemes @6,12# based on simple arguments, such as autoionization to the nearest continuum limits and minimum electron rearrangement ~twoelectron transitions!, have been invoked. This Rapid Communication reports triple-coincidence measurements of Auger electrons, scattered projectile, and target recoil ions in slow multiply charged ion-atom collisions. The measurements provide insights into the relaxation pathways of multiply excited states populated in such collisions.