Two-Center Effect on Low-Energy Electron Emission in Collisions of 1-MeV/u Bare Ions with Atomic Hydrogen, Molecular Hydrogen, and Helium: II. H\u3csub\u3e2\u3c/sub\u3e and He

We have studied the energy and angular distributions of low-energy electron emission in collisions of bare carbon ions of 1-MeV/u energy with He and H2 targets. The double-differential cross sections (DDCS’s) are measured for electrons with energies between 0.5 and 300 eV emitted within an angular range of 15° to 160°. The large forward-backward asymmetry observed in the angular distributions is explained in terms of the two-center effect. Single differential cross sections (SDCS’s) and total cross sections are also derived by integrating the DDCS’s over emission angles and energies. The data are compared with different theoretical calculations based on the first Born, CDW (continuum-distorted-wave), and CDW-EIS (eikonal-initial-state) approximations. The angular distributions of DDCS’s and SDCS’s are shown to deviate largely from the predictions of the B1 calculations, and are in much better agreement with both the continuum distorted-wave models. The CDW approximation provides a better agreement with the data compared to the CDW-EIS approximation, especially at higher electron energies. The total ionization cross sections for all three targets are shown to follow a scaling rule approximately

K-shell processes in heavy-ion collisions in solids and the local plasma approximation

We have investigated K-shell vacancy production due to ionization and electron transfer processes, in collisions of highly charged oxygen ions with various solid targets such as Cl, K, Ti, Fe, and Cu at energies between 1.5 and 6.0 MeV/u. The K-shell ionization cross sections were derived from the measured K x-ray cross sections. An ab initio theoretical model based on the local plasma approximation ͑LPA͒, which is an extension of the dielectric formalism to consider core electrons, provides an explanation of the measured data only qualitatively. In case of asymmetric collisions (Z p /Z t Ͻ0.35, Z p , Z t being the atomic numbers of the projectile and target, respectively͒ and at higher energies, the LPA model explains the data to some extent but deviates for more symmetric collision systems. On the other hand, a perturbed-stationary-state ͑PSS͒ calculation ͑ECPSSR͒, including the corrective terms due to energy ͑E͒ loss, Coulomb ͑C͒ deflection, and relativistic ͑R͒ wave functions designed for ion-atom collisions agree quite well with the data for different combinations of target and projectile elements. In addition, we have also measured the K͑target͒-K͑projectile͒ electron transfer cross sections and compared them with a model based on perturbed-stationary-state approximation

Two-Center Effect on Low-Energy Electron Emission in Collisions of 1-MeV/u Bare Ions with Atomic Hydrogen, Molecular Hydrogen, and Helium: II. H\u3csub\u3e2\u3c/sub\u3e and He

We have studied the energy and angular distributions of low-energy electron emission in collisions of bare carbon ions of 1-MeV/u energy with He and H2 targets. The double-differential cross sections (DDCS’s) are measured for electrons with energies between 0.5 and 300 eV emitted within an angular range of 15° to 160°. The large forward-backward asymmetry observed in the angular distributions is explained in terms of the two-center effect. Single differential cross sections (SDCS’s) and total cross sections are also derived by integrating the DDCS’s over emission angles and energies. The data are compared with different theoretical calculations based on the first Born, CDW (continuum-distorted-wave), and CDW-EIS (eikonal-initial-state) approximations. The angular distributions of DDCS’s and SDCS’s are shown to deviate largely from the predictions of the B1 calculations, and are in much better agreement with both the continuum distorted-wave models. The CDW approximation provides a better agreement with the data compared to the CDW-EIS approximation, especially at higher electron energies. The total ionization cross sections for all three targets are shown to follow a scaling rule approximately

Doubly Differential Final-State Momentum Distributions of the Ionization Products in Collision of Bare Ions with Hydrogen

In this paper recoil-ion production cross sections are presented differential in recoil-ion longitudinal momentum and electron emission angle in ionization of atomic and molecular hydrogen by bare ion projectiles. A new formulation for constructing these double differential distributions from the measured electron double differential cross sections is used. A novel feature is the separation of two different branches of the recoil-ion longitudinal momentum distribution corresponding to soft and hard collision mechanisms of ionization. The single differential distributions have also been derived. © 1996 The American Physical Society

Two-Center Effect on Low-Energy Electron Emission in Collisions of 1-MeV/u Bare Ions with Atomic Hydrogen, Molecular Hydrogen, and Helium. I. Atomic Hydrogen

We have investigated ionization mechanisms in fast ion-atom collisions by measuring the low-energy electron emission cross sections in a pure three-body collision involving bare carbon ions (v=6.35 a.u.) colliding with atomic hydrogen targets. The measurements have also been extended to molecular hydrogen and helium targets. In this paper we provide the energy and angular distributions of double differential cross sections of low-energy electron emission for atomic hydrogen targets. The Slevin rf source with a high degree of dissociation was used to produce the atomic H target. It is found that the two-center effect has a major influence on the observed large forward-backward angular asymmetry. A detailed comparison is presented with calculations based on the continuum distorted-wave (CDW) and CDW-EIS (eikonal initial-state) approximations. Both the continuum distorted-wave calculations provide a very good understanding of the data, whereas the first Born calculation predicts almost symmetric forward-backward distributions that do not agree with the data. The two-center effect is slightly better represented by the CDW calculations compared to the CDW-EIS calculation. The total cross sections are, however, in good agreement with the theories used. The results for molecular hydrogen and helium will be discussed in the following paper

Ionization Dynamics in Fast Ion-Atom Collisions. II. Final-State Momentum Distributions of the Ionization Products in Collisions of He with Bare Carbon Ions

We have used the energy and angular distributions of the low-energy electron emission cross sections from the preceding paper [Lokesh C. Tribedi et al., Phys. Rev. A 58, 3619 (1998)] to derive the doubly differential final-state longitudinal momentum distributions of the electrons, recoil ions, and projectiles in ion-atom ionization for [Formula Presented] The complementary nature of the electron spectroscopy and the recoil-ion momentum spectroscopy have been investigated using a formulation based on three-body kinematics to explore the ionization dynamics in detail. The influence of the three-body ionization as well as the binary-encounter processes on the recoil-ion (and projectile) longitudinal momentum distributions has been investigated. The separation of the soft- and hard-collision branches of recoil-ion distributions is an important feature of the present technique. The present method also allows one to determine cross sections for very large electron momenta. The single- differential distributions are also derived by numerical integration of the double-differential distributions. The first Born approximation, the continuum distorted wave eikonal initial state, and the classical trajectory Monte Carlo calculations are used to explain the data. © 1998 The American Physical Society

Impact ionization of molecular oxygen by 3.5-MeV/u bare carbon ions

We have measured the absolute double-differential cross sections (DDCSs) for electron emission in ionization of O2 molecules under the impact of 3.5-MeV/u C6+ ions. The data were collected between 10 and 600 eV, in an angular range of 30◦ to 150◦. The single-differential cross sections (SDCSs) in emission angle and electron energy are deduced from the electron DDCS spectra. Also, the total cross section has been obtained from the SDCS spectra. The DDCS spectra as well as the SDCS spectra are compared with continuum distorted-wave eikonal initial-state calculations which employ molecular wave functions built as linear combinations of atomic orbitals. The DDCS ratio i.e. σO2/2σO, derived by dividing the experimental DDCS for molecular oxygen with the theoretical DDCS for atomic oxygen, does not show any primary or secondary oscillations arising from Young-type interference, which is apparently in contrast to what has been observed earlier for H2 and in agreement with the model calculation. Similarly, the forward-backward angular asymmetry increases monotonically with the velocity of the emitted electrons. However, the results on the DDCSs, SDCSs, the asymmetry parameter, and the nonexistence of oscillations are in qualitative agreement with the predictions of the model usedOne of the authors (F.M.) acknowledges the financial support from the MICINN Projects No. FIS2010-15127 and No. CSD 2007- 00010. C.A.T., R.D.R., and F.M. acknowledge the Programa de Cooperación Interuniversitaria e Investigación Científica entre España e Iberoamérica AECID Project No. A2/039631/1