Effect of the target density on the cross section of charge exchange between fast ions and atoms

When fast X^q+ X^{q^ + } ions collide with atomic or molecular targets, the total charge exchange cross section decreases with increasing target density. This is because the excitation levels of resulting X^{(q - 1)+} X^{(q - 1)^ + } ions are suppressed because of ionization by target atoms. The effect of target density on the total charge exchange cross section may amount to one order of magnitude or more depending on the charge and energy of an incident ion, as well as on the density and inner shell configuration of target atoms. Numerical calculations are performed for partial (in the principal quantum number n) cross sections σ(n) and total cross sections σ_tot=Σ_nσ(n) of charge exchange in the case of collisions of fast multiply charged ions having an energy E in the range 100 keV/u-10 MeV/u with gas or solid targets.     

Cross sections for charge exchange of atoms on multiple-charged ions

Processes of charge exchange of Ar^{+ +}(3p ⁵) and Kr^{+ +}(4p ⁵) on He(1s ²) at low collision energies are investigated theoretically. Semiempirical asymptotic calculations of the radial coupling between initial and final states (both neglecting and allowing for the spin-orbit interaction) are presented. The result of semiempirical calculation of the coupling is in reasonable agreement with an earlier ab initio calculation for the system ArHe^{+ +}. The integral cross sections were estimated using the Landau-Zener model. Comparisons are made with available experimental data and numerical quantum calculation.     

Total reaction cross section from the interaction of 4He ions with 28Si at 10–30 MeV

New results have been obtained for the energy dependence of the total reaction cross section σ _R of ⁴He on ²⁸Si in the energy range E _α ≤ 30 meV.     

Electron terms and resonant charge exchange involving oxygen atoms and ions

The electron terms are constructed for oxygen dimer ions at large ion-atom distances taking into account a certain scheme of summation of electron momenta on the basis of a hierarchy of various ion-atom interactions. Because the number of interaction types exceeds that in the Hund scheme, a realistic hierarchy of interactions and corresponding quantum numbers of the diatomic ion are outside the Hund coupling scheme. Electron terms are evaluated for the oxygen dimer ion in the case where the ground and first excited states of an atom and an ion belong to the respective valence electron shells p⁴ and p³ and correspond to the range of separations that determine the cross sections of resonant charge exchange in plasma. These electron terms allow us to calculate the partial and average cross sections for resonant charge exchange involving an oxygen ion and atom in the ground and first excited states in the range of collision energies of interest for oxygen plasmas. The specific features of electron terms of the oxygen ion dimer and the cross section of electron transfer are analyzed.     

Differential cross sections for charge transfer in collisions of He+, Ne+ and Ar+ with Kr

Differential charge transfer cross sections for collisions of He⁺, Ne⁺ and Ar⁺ with Kr were measured at collision energies below 500 eV. A remarkable fraction of these collisions (2–30 %) occurs with large momentum transfer and small impact parameters. These close collisions lead to an excitation of the product particles, the measured reaction channels are strongly endothermic. In the system Ar⁺+Kr one reaction channel may be described in terms of a curve crossing.     

Binary encounter approximation for the electron-impact double ionization cross section of atoms and ions

An binary encounter approximation is formulated for the direct (without any excitation of autoionization levels) double ionization of atoms and ions by electron impacts. Analytical expressions are derived for the cross section, which qualitatively represent the main dependences on the incident electron energy and atomic parameters.     

Cross section for the resonant charge exchange Cs++Cs at thermal energies

A remeasurement of the cross section for the resonant charge exchange Cs⁺ ?Cs at thermal energies is presented which reduces some of the ambiguities left open in previous studies.     

Dissociative charge exchange in collisions of 10keV H 2 + ions on H2, He,and Ar targets

The velocity distributions of H-H fragment pairs arising from dissociative collisions of 10keV H ₂ ⁺ ions incident on H₂, He, and Ar were measured using a flight-time-difference method. The H ₂ ⁺ ions were produced in an electron impact ion source at two different electron energies, at 100eV and 18eV. The influence of the electron energy on the fragment velocity distributions is studied. Conclusions are drawn on the processes which lead to the formation of neutral fragment pairs. Electron capture into the repulsive b³ Σ _u ⁺ state of H₂ plays a dominant role.     

Total charge-transfer cross sections in collisions of Ca+ ions with Mg and Sr atoms

The total charge-transfer cross sections in Ca⁺ + Mg and Ca⁺ + Sr collisions are investigated experimentally by the crossed-beam technique in the energy range 80–1000 eV.     

Energy dependence of the total cross section for the reaction of 4He ions with silicon nuclei

Directly measured data on the total cross section for the reaction of projectile ⁴He ions with silicon nuclei at energies below 25 MeV/nucleon are presented. The energy dependence of the parameters of a semimicroscopic potential is determined from the measured values of this cross section. This investigation was performed at the Flerov Laboratory of Nuclear Reactions at the Joint Institute for Nuclear Research (Dubna, Russia) and at the Department of Physics at the University of Jyväskylä (Finland).     

Asymptotics of the cross section for the electron loss by atoms and ions in light media

The dependence of the cross section of electron loss on the nuclear charge Z of a bombarding ion and on the nuclear charge Z _t of a target atom for fast collisions is studied theoretically using the plane-wave Born approximation and the sum rule. The results of calculations show that the cross section of electron loss for fast collisions increases monotonically as Z and Z _t increase, which can be used to interpolate cross sections for the processes for which there are no experimental data.     

Sputtering of Cu atoms by Ar ions

Sputtering of Cu single-crystal, polycrystal and amorphous targets by 5 keV Ar ions has been studied by the binary collision lattice simulation code Cosipo. The sputtering yields, angular distributions, energy distributions and the space distributions of the original positions of the sputtered Cu atoms have been calculated. The results are discussed within the framework of cascade generations and surface structure.     

Potentials and scattering cross sections for collisions of He atoms with adsorbed CO

Ab initio calculations of the total scattering cross section for the collision system He → $\text{CO}\text{Pt(111)}$ within the renormalized distorted wave Born approximation are reported. The interaction potential for this atom-adsorbate system consists of the usual two-body gas phase-like potential as well as two additional substrate mediated van der Waals contributions, all with similar long range behaviour. Comparison of the calculated cross sections for various incident velocities and angles with available experimental data is made without using any adjustable parameters in the potential to fit the data, and the importance of including the anisotropic and substrate-mediated forces is emphasized.     

Total differential scattering cross section of Ar+-Ar at 15 to 400 keV

We report on the measurement of the total differential scattering cross section of Ar⁺-Ar at laboratory energies between 15 and 400 keV. Using an ab initio relativistic molecular program which calculates the interatomic potential energy curve with high accuracy, we are able to reproduce the detailed structure found in the experiment.     

Pulse duration effects on laser-assisted electron transfer cross section for He2+ ions colliding with atomic hydrogen

The physics of a two-component cold Fermi gas is now frequently addressed in laboratories. Usually this is done for large samples of tens to hundreds of thousands of particles. However, it is now possible to produce few-body systems (1–100 particles) in very tight traps where the shell structure of the external potential becomes important. A system of two-species fermionic cold atoms with an attractive zero-range interaction is analogous to a simple model of nucleus in which neutrons and protons interact only through a residual pairing interaction. In this article, we discuss how the problem of a two-component atomic Fermi gas in a tight external trap can be mapped to the nuclear shell-model so that readily available many-body techniques in nuclear physics, such as the Shell-Model Monte Carlo (SMMC) method, can be directly applied to the study of these systems. We demonstrate an application of the SMMC method by estimating the pairing correlations in a small two-component Fermi system with moderate-to-strong short-range two-body interactions in a three-dimensional harmonic external trapping potential.     

He4-He4 interaction potential from low energy total cross section measurements

Total relative and absolute cross sections for He⁴-He⁴ have been measured between 110 and 1100 m/s velocity of the primary beam at scattering chamber temperatures of 5.0 and 77 K. Our measurements are sensitive to the low energy repulsive and the attractive part of the potential. The resulting potential is able to describe scattering measurements of other authors; it is also in very good agreement with ab initio calculations of Bertoncini and Wahl.     

Inclusive measurements of pionic double charge exchange on 3He at low energies

Using the CHAOS spectrometer at TRIUMF pion momentum spectra and angular distributions were measured for the ³He(π⁻,π⁺) reaction at incident energies from 65 to 120 MeV and total cross sections were deduced. The data are compared to Monte Carlo simulations for the conventional reaction process as well as to predictions for the production of the hypothetical πNN resonance d′ with and without collision damping.     

Lifetime measurements for metastable He like ions

We present work in progress on the development of a new technique for measurements of μs lifetimes of metastable excited states of He like ions. The key element in this new technique is an electrostatic storage device for ions with keV energies. This revised version was published online in August 2006 with corrections to the Cover Date.     

Total cross section prediction of the collisions of positrons and electrons with alkali atoms using Gradient Tree Boosting

A relatively new computational technique, namely gradient tree boosting (GTB), is presented for modeling the total cross sections of the scattering of positrons and electrons by alkali atoms in the low and intermediate energy regions. The calculations have been performed in the framework of gradient tree boosting (GTB). The GTB has been running based on the experimental data of the total collisional cross sections to produce the total cross sections for each alkali atom as a function of the incident energy of the projectile as well as the atomic number and the static dipole polarizability of the atom. Moreover our GTB model is used to predict the experimental data for total collisional cross sections that are not used in the training session. The calculated and predicted total collisional cross sections are compared with the experimental data. We find that the GTB technique shows a good match to the experimental data. To our knowledge, this is the first application of the GTB technique to the data of positron and electron collisions with alkali atoms at low and intermediate energies.