Cross Sections For Multiple Ionization Research Articles

Electron-impact cross sections for multiple ionization of Kr and Xe.

Partial cross sections were measured for the ionization M+e\ensuremath{\rightarrow}${\mathit{M}}^{\mathit{n}+}$+(n+1)e for M=Kr and Xe (n=1--6) as a function of electron energy (0--470 eV). The experiment is based on a crossed-beam time-of-flight mass spectrometer employing pulsed electron and atomic beams, and pulsed ion extraction. Cross sections for ${\mathrm{Kr}}^{5+}$, ${\mathrm{Kr}}^{6+}$, and ${\mathrm{Xe}}^{6+}$ have not been previously measured over this energy range. The cross sections reported for the remaining ions considerably narrow the uncertainty of previous measurements. The source of discrepancy among recent work appears to be due to improper account of detector-gain effects. The ionization onsets are fitted to threshold power-law expressions to derive ionization potentials.

Independent particle model description of multiple ionization dynamics in fast ion-atom collisions

The time-dependent dynamics of the many-electron density and its interaction with projectile and target nuclei is described within the semiclassical (=0) Vlasov model. Differential cross sections for multiple ionization as a function of projectile scattering angle and transverse target recoil momentum are calculated. Average scattering angles are obtained as a function of impact parameter within the model taking into account a given number of ejected electrons for each recoil ion charge state. Other results include projectile energy loss and the relationship between projectile and target nucleon motion. Comparison with experiment and classical N-particle simulations (N-CTMC) is made for two systems, namely, C6+-Ne and U3+-Ne at collision energies of 0.8 and 1.4 MeV amu-1 respectively.

Angular variation of the partial doubly differential cross sections for multiple ionization of argon atoms by electron impact.

Partial doubly differential cross sections ${\mathit{d}}^{2}$${\mathrm{\ensuremath{\sigma}}}^{(\mathit{n})}$/dEd\ensuremath{\Omega} for the production by electron impact of argon ions having charge states from n=1 to n=4 have been measured as a function of the angle of ejection \ensuremath{\theta} in the range 40\ifmmode^\circ\else\textdegree\fi{}113\ifmmode^\circ\else\textdegree\fi{}, with the energies of the incident and ejected electrons being 1.0 keV and 200 eV, respectively. A comparison has been made with similar data from the literature and explanations have been offered for the behavior of the measured angular variations.

Differential cross sections for multiple ionization of Ne and Ar in collisions with fast protons.

Differential cross sections for single and multiple ionization of Ne and Ar by protons have been measured at impact energies of 3 and 6 MeV, and at proton scattering angles between 0.1 and 0.9 mrad. The recoil ions formed in the collision processes were extracted from the collision region and detected in coincidence with the scattered protons. Their charge states were determined by their flight times. The experimental differential cross sections for single and multiple ionization are qualitatively explained by a simple model that takes into account the contributions of scattering from the electrons and from the target nucleus. The model indicates that ionization by hard proton-electron collisions remains the largest contributor to single and multiple ionization of both Ne and Ar for projectile scattering angles larger than 0.1 mrad. The ratio of double-to-single ionization cross sections for Ne and Ar increases with increasing proton scattering angle, suggesting an increasing importance of inner-shell ionization at small impact parameters.

Charge distribution of Ar recoil ions produced in one- and two-electron capture collisions by 16-MeV Oq+ projectiles.

The yields of Ar recoil ions produced in collisions with 16-MeV ${\mathrm{O}}^{\mathrm{q}+}$ (q=3--8) were measured in coincidence with one- and two-electron capture by the projectile. Time-of-flight analysis identified the charges of the Ar recoil ions and enabled the determination of charge-state distributions. The independent-electron approximation was employed along with a simple exponential parametrization of the ionization and capture probabilities to calculate the cross sections for multiple ionization accompanied by one- and two-electron capture. By accounting for direct L-shell ionization and Auger multiplication of L-shell vacancies, good agreement between calculated and measured charge-state distributions was attained.

Double differential stopping powers of 1.4 MeV/u U33+in Ne and Ar derived from electron production and multiple ionization cross sections

Double differential stopping powers of 1.4 MeV/u U³³⁺in Ne and Ar derived from electron production and multiple ionization cross sections

Electronic transitions in highly charged ion-atom collisions

Three different aspects of electronic transitions in fast, highly charged ion-atom collisions are discussed. First, experimental data and n-CTMC calculations for differential multiple ionization cross sections of 1.4 MeV u U 32+on rare gas atoms are presented. It is shown that the electronic motion has a dramatic influence on the kinematics of the emitted particles (in particular the nuclei). The possibility is discussed to measure in fast ionizing processes by a recoil ion-projectile coincidence technique the internal sum momentum of “electron clusters” in atoms. This new “technique” opens a new field of atomic structure research at high-energy heavy-ion accelerators. Second, the use of the H-like heavy ions as projectiles is discussed to measure, through observable interference structures, static and dynamic properties of transiently formed superheavy quasimolecular systems. Third, the “ancient” gas target-solid target difference in the impact-parameter dependence of K-shell ionization in nearly symmetric ion-atom collisions is presented. This severe discrepancy between gas and solid still remains an unsolved fundamental problem in the field of inner-shell ionization in the MO regime.

Partial doubly differential cross sections for multiple ionization of argon, krypton, and xenon atoms by electron impact.

Partial doubly differential cross sections for multiple ionization ${d}^{2}$${\ensuremath{\sigma}}^{(n)}$/dE d\ensuremath{\Omega} of argon, krypton, and xenon by electron impact have been measured as a function of incident electron energy and ejected electron energy, for argon up to ${\mathrm{Ar}}^{4+}$, for krypton up to ${\mathrm{Kr}}^{5+}$, and for xenon up to ${\mathrm{Xe}}^{8+}$. Incident electron energies between 0.5 and 10 keV were used, while the electrons ejected at an angle of 90\ifmmode^\circ\else\textdegree\fi{} to the incident electron direction were detected with energies between 20 and 270 eV. The doubly differential cross section (sum of partial doubly differential cross sections) for ionization for each gas has been compared with experimental data in the literature.

Inclusive and exclusive cross sections for multiple ionization by fast, highly charged ions in the independent-electron approximation.

Cross sections for the ionization of n of N electrons with equal single-electron ionization probability P are considered. When both N and the projectile charge q are large, the cross sections for single and double ionization are both found to be approximately linear in q at 1 MeV/amu. The ratio of double-to-single-ionization cross sections is independent of q. Moreover, first-order perturbation theory for the single-electron ionization probability P, which varies as ${q}^{2}$, is found to be applicable due to the damping of contributions with large P caused by factors of (1-P${)}^{N\mathrm{\ensuremath{-}}n}$. For large P there are differences between the inclusive probability P and the probability NP commonly used for a target with N electrons. Both of these probabilities differ significantly from the exclusive probability NP(1-P${)}^{N\mathrm{\ensuremath{-}}1}$ for the ionization of only one electron. For large N and large q, the exclusive ionization probabilities for removing exactly n of the N electrons tend to be concentrated in somewhat separate ranges of impact parameters b, defining impact-parameter ``windows.'' The windows which we obtain using the quantum-mechanical semiclassical-Coulomb-approximation (SCA) probabilities are similar to those using classical Monte Carlo calculations. Model calculations, based on analytic fits to the SCA probabilities, are used to obtain approximate analytic expressions for single- and double-ionization cross sections and for the impact-parameter windows. Results of this simple model are in reasonable agreement with measured cross sections for single and multiple ionization of neon atoms by projectiles of charge q varying from 1 to 13 at a velocity corresponding to 1 MeV/amu.

Multiple ionization and capture in relativistic heavy-ion atom collisions

We show that in relativistic heavy-ion collisions the independent electron model can be used to predict cross sections for multiple inner-shell ionization and capture in a single collision. Charge distributions of 82- to 200-MeV/amu Xe and 105- to 955-MeV/amu U ion beams emerging from thin solid targets were used to obtain single- and multiple-electron stripping and capture cross sections. The probabilities of stripping electrons from the K, L, or M shells were calculated using the semiclassical approximation and Dirac hydrogenic wave functions. For capture, a simplified model for electron capture was used. The data generally agree with theory.

Time-of-flight spectrometer for the determination of microradian projectile scattering angles in atomic collisions

A time-of-flight spectrometer (TOFS) has been developed and tested which allows the determination of projectile scattering angles ϑ in the range of 10 −6 rad. The basic idea behind the TOFS and first differential measurements of multiple ionization cross sections as a function of recoil ion energy for the collision system 5.9 MeV/u U 65+ on Ne are presented. The properties of the spectrometer and future applications are discussed.

Exciton fusion in molecular clusters.

Large benzene clusters ionize with anomalously high efficiency when excited by ultraviolet radiation. The enhancement of the ionization and multiple-ionization cross sections over molecular values is quantitatively explained by ultrafast fusion of pairs of excitations within a cluster.

Ionization and charge transfer in He2+-rare-gas collisions. II.

New measurements of absolute cross sections for multiple ionization of helium, neon, argon, and krypton by 50--500-keV/amu ${\mathrm{He}}^{2+}$ impact are presented. By measuring projectile-ion--target-ion coincidences, these cross sections are obtained for the direct ionization as well as the single- and double-electron-capture channels. The present data are combined with previous measurements by the author that covered the energy range between 5 and 67 keV/amu in order to provide a broader description of ${\mathrm{He}}^{2+}$ impact ionization of atomic targets. The earlier measurements were found to require reevaluation in order to correct ion detection efficiencies used in that study. An analysis of the direct-multiple-ionization cross sections demonstrates that a simple impact-parameter, independent-electron model can be used to describe these cross sections over a broad range of low to intermediate impact energies. In particular, this analysis, when applied to fully stripped ion (1\ensuremath{\le}Z\ensuremath{\le}8) impact on helium, yields a single universal curve in the velocity range investigated here.

Ion state selection and preparation by high resolution translational energy spectroscopy

Different applications and modifiications of the translational energy spectroscopy are described which are used for the study of ion-atom- as well as electron-atom-collisions. In the first case charge changing, collisions between multiply charged ions and atoms are analysed with respect to initial and final state populations and results of collision experiments with state prepared multiply charged ions are discussed. In the case of electron-atom-collisions the translational energy spectrometer has been used as metastabledetector yielding, state-selective cross sections for multiple ionization by electron impact.

Multiple-ionization channels in proton-atom collisions.

A detailed investigation of multiple ionization of He (ionization charge states q = 1,2), Ne (q = 1--3), and Ar and Kr (q = 1--4) is presented for proton impact energies ranging from 10 keV to a few MeV. Absolute cross sections for various ionization pathways have been obtained by combining some new measurements with previously published experimental results and, in certain cases, with existing theoretical information. It is shown how each of these pathways contribute to the various stages of target ionization that are observed after the collision and how these experimentally measured quantities are related to the cross sections for initial inner- and outer-shell vacancy production. Areas where additional data are required or where the existing data are not internally consistent are pointed out. In general, it is shown that the existing data are sufficient to describe the ionization of helium as well as the lower levels of ionization of neon, argon, and krypton. However, for the higher degrees of ionization, particularly for Kr, our understanding is hampered by substantial gaps in the available inner-shell ionization data: both in cross-section and branching-ratio information. Nevertheless, the data are sufficient to indicate the relative importance of the various pathways. Formore » all targets, direct multiple outer-shell cross sections were extracted. Analyzing the energy dependences of these cross sections provided some hints as to how to calculate multiple-ionization cross sections, e.g., information as to where the multiple ionization is dominated by the first-order or by a higher-order term in the perturbation expansion of the proton-target interaction is obtained.« less

Multiple ionization in relativistic heavy-ion-atom collisions.

We show that in relativistic heavy-ion collisions the independent-electron model can be used to predict cross sections for multiple inner-shell ionization in a single collision. Charge distributions of 430- and 955-MeV/amu ${\mathrm{U}}^{90+}$, ${\mathrm{U}}^{89+}$, ${\mathrm{U}}^{83+}$, and ${\mathrm{U}}^{68+}$ beams emerging from thin solid targets were used to obtain single- and multiple-electron stripping cross sections. The probabilities of stripping electrons from the K, L, or M shells were calculated using the semiclassical approximation and Dirac hydrogenic wave functions. The data generally agree with theory. An influence of the Auger effect is seen in ${\mathrm{U}}^{68+}$ collisions.

The production of highly charged Ar and Xe recoil ions by fast uranium impact

The multiple ionization of Ar and Xe atoms by fast U 75+ impact has been measured with a projectile-recoil ion time-of-flight technique at 3.6, 5.9, 9.4 and 15.5 MeV/u. Using this technique, the recoil-ion production cross sections can be determined as a function of the final recoil and projectile charge states, i.e. one can distinguish between direct multiple ionization and those processes accompanied by charge transfer. Due to the high projectile charge and high velocity, the direct multiple ionization cross sections to produce highly ionized systems are very large, e.g. the multiple ionization cross section to produce in one collision hydrogen-like Ar exceeds the geometrical Ar-K-shell area. As a consequence of this long range interaction, the kinetic energy of Ar 17+ recoil ions is only of the order of some 10 eV.

A new technique for the measurement of ionization cross sections with crossed electron and ion beams

A new experimental technique has been developed and used to measure cross sections for electron impact ionization of ions. Each single cross section is determined independently absolute by moving the electron gun mechanically across the ion beam with simultaneous registration of the ionization signal and the actual beam current. Different from the method of Brouillard et al. it is not necessary then to provide constant speed of the motion. Typical relative uncertainties of the measured cross sections are of the order of ±2% and absolute total errors are usually within ±6.5%. Very good agreement of ionization cross sections σ 1,2 measured with our new technique for Ar + ions is found with data from Harrison's group. Cross sections for single and multiple ionization of singly and multiply charged ions of Ar, Kr, Sb and Bi have been measured from threshold to 1000 eV.

Multiple ionization of rare gases byH+andHe+impact

Absolute cross sections for multiple ionization (${\ensuremath{\sigma}}_{q}$) of He, Ne, Ar, and Kr are presented for proton-impact energies between 10 and 4000 keV. Charge states up to 6+ are observed for Kr. The results are obtained by measuring relative yields of multiply charged ions with a time-of-flight spectrometer and normalizing to total ion-production cross sections ${\ensuremath{\sigma}}_{T}=\ensuremath{\Sigma}q{\ensuremath{\sigma}}_{q}$. The results are compared with existing proton- and electron-impact data. Relative multiple- to single-ionization cross-section ratios are also presented for ${\mathrm{He}}^{+}$ impact.

Absolute experimental cross sections for the electron-impact multiple ionization of singly charged rubidium ions

The absolute cross sections for the double, triple, and quadruple ionization of ${\mathrm{Rb}}^{+}$ ions by electron impact have been measured from below their respective thresholds to approximately 3000 eV. This determination has been accomplished using a crossed-beam facility in which monoenergetic beams of ions and electrons are caused to intersect at right angles in a well-defined collision volume. The various charged-particle currents, energies, and beam-current density distributions represent the experimental data from which the desired absolute cross sections have been determined. The results obtained with this technique are compared with available theoretical predictions of the appropriate cross sections.