Ionization of atoms and molecules by antiproton impact

During the decade that the LEAR accelerator at CERN has existed, the PS194 collaboration has obtained a substantial amount of experimental data for the ionization of atoms and molecules by antiproton impact. Here, we present a few of the most important and recent results, compare them to newly developed theory, and point out the progress that has been achieved for our understanding of ionization in few-body systems because of this information, as well as some remaining problems. PS194 collaboration at CERN This revised version was published online in August 2006 with corrections to the Cover Date.     

Fully correlated electronic dynamics for antiproton impact ionization of helium

We present total cross sections for single and double ionization of helium by antiproton impact over a wide range of impact energies from 10 keV/amu to 1 MeV/amu. A nonperturbative time-dependent close-coupling method is applied to fully treat the correlated dynamics of the ionized electrons. Excellent agreement is obtained between our calculations and experimental measurements of total single and double ionization cross sections at high impact energies, whereas for lower impact energies, some discrepancies with experiment are found. At an impact energy of 1 MeV we also find that the double-to-single ionization ratio is twice as large for antiproton impact as for proton impact, confirming a long-standing unexpected experimental measurement.     

The capture of slow antiprotons in helium, neon and argon

We consider the capture of antiprotons with energies less than roughly 27 eV by helium, neon and argon atoms. We use the adiabatic ‘hidden-crossing’ theory to calculate both the capture cross-sections and the angular distribution of the antiprotons. We find that important differences between helium and the other noble gases can be attributed to the properties of the ‘hidden’ crossings appropriate to each system. This revised version was published online in August 2006 with corrections to the Cover Date.     

Capture of slow antiprotons by helium atoms

A consistent quantum mechanical calculation of partial cross-sections leading to different final states of antiprotonic helium atom was performed. For the four-body scattering wave function, corresponding to the initial state, as well as for the antiprotonic helium wave function, appearing in the final state, adiabatic approximations were used. Further, symmetric and non-symmetric effective charge (SEC, NEC) approximations were introduced for the two-electron wave functions in the field of the two fixed charges of the He nucleus and the antiproton. Calculations were carried out for a wide range of antiprotonic helium states and incident energies of the antiproton below the first ionization threshold of the He atom. The origin of the rich low-energy structure of certain cross-sections is discussed in detail.     

Measurements of double differential cross sections in electron impact ionization of helium and argon

Double differential cross sections (angular distributions and energy loss spectra) have been measured of electrons after ionizing electron collisions with helium at primary energiesE ₀ between 25 eV and about 260 eV and with argon atE ₀=75, 150 and 200 eV. The spectra have been measured with an energy analyzing collector system of constant transmission. It was found that for high collision energies (E ₀≧ 80 eV) the outgoing electrons belong to one of the two energetically well separated groups, either thefast electrons, which are scattered mainly in forward direction or theslow electrons which are distributed isotropically into all angles. At low primary energiesE ₀ no separation into groups is possible. Several findings indicate the qualitative applicability of the binary collision model.     

Ionization of helium in positron collisions

A new Coulomb distorted-wave method with coupled-channel target functions is used to calculate total ionization cross-sections for helium in positron collisions. Besides Slater-like orbitals we use regular Coulomb wave packets in our configurational interaction basis to describe the target continuum. The incident positron energy was varied between the ionization threshold and 500 a.u. The results are in good agreement with experimental data and other theoretical calculations. Comparing to other sophisticated distorted wave methods our model is much easier to implement and gives accurate results. As a new feature we present ionization cross-sections where the He ⁺ ion remains in the 1s ground state or excited to the 2s or 2p state. As we know there are no experimental work done to determine such cross-sections. In the case of ionization followed by 2s or 2p excitation we compared our results with other calculations.Received: 18 February 2004, Published online: 15 April 2004PACS: 34.85. + x Positron scattering     

Detection of helium implanted into nickel by slow positrons

Slow/monoenergetic positrons were used to detect helium atoms implanted in nickel. Doppler broadening profiles of positron annihilation radiation as a function of incident positron energy were found to be quite sensitive to the helium implantation. The detection of helium implanted with a concentration of several ppm was proved to be possible by this technique.     

Ionization of atomic vapors and helium by collision with relativistic heavy ions

We have studied collisions of atomic vapors and helium with relativistic, highly charged ions having Z v c. Formulas for the cross-sections of ionization and excitation of discrete levels of the atomic vapor and the cross-section of single and double ionization of helium atoms have been obtained. Comparisons with experimental data of the single and double ionization of helium are presented.Tashkent University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 23–29, February, 1995.     

Generation of very slow polarized muons by moderation

At the Paul Scherrer Institute very slow, nearly 100% polarized, positive muons with an energy of \sim\mbox10 eV are produced by moderating a secondary beam of surface muons in a thin film of an appropriate condensed gases. These epithermal muons can be used as a source of a tertiary beam of tunable energy between \sim\mbox10 eV and \sim\mbox20 keV. Such a beam allows the μSR technique to be extended to the study of thin films and surfaces. In order to be able to perform time differential μSR experiments we have developed an ultra‐thin detector that registers the passage of keV muons and permits to trigger the experiment. The results achieved so far demonstrate that first investigations of thin film samples can be performed with the present set‐up. This revised version was published online in August 2006 with corrections to the Cover Date.     

Ionization of hydrogen atoms by electron impact

Cross sections for the ionization of hydrogen atoms by electron impact have been computed for incident energies from threshold up to 54 eV. A projection technique has been used in conjunction with a variational pseudostate calculation.     

Ionization of K-shell electrons by positron impact

We have investigated the universal scaling behavior for cross sections of the single K-shell ionization by positron impact. The calculations are performed within the framework of non-relativistic perturbation theory, taking into account the one-photon exchange. Special emphasis is laid on the near-threshold energy domain. The results obtained are applicable for wide family of atomic targets.     

Ionization of diatomic molecules by electron impact

An analytical expression for the amplitude of ionization of the hydrogen molecule by electron impact in the first Born approximation with a one-center Coulomb continuum wave function is derived. The case where the incident electron energy is much greater than the ejected electron energy is considered. The molecular wave functions were constructed in the approximation of linear combination of atomic orbitals with overlapping configurations. The role of the orthogonalization of the initial and final wave functions of the active electron of the target is elucidated. The triple differential ionization cross sections for the different orientations of the molecular axis and that averaged over all orientations are calculated. The secondary electron angular distribution is represented in the form of three-dimensional images. A comparison with the results of other theoretical calculations and experimental data is performed.     

Ionization of K-shell electrons by electron impact

We have investigated the universal scaling behavior for cross sections of the single K-shell ionization by electron impact. The calculations are performed within the framework of non-relativistic perturbation theory, taking into account the one-photon exchange diagrams. Special emphasis is laid on the near-threshold energy domain. The results obtained are applicable for wide family of atomic targets with moderate values of the nuclear charge Z.     

Ionization for Hf- and W-L-shell by electron impact

Electron-induced Hf-, W-L-shell partial, total production cross sections, mean ionization cross sections and Hf-L_3-shell ionization cross sections (at two energies) have been measured as functions of electron energies (from near threshold to 36keV). The influence of electrons reflected from the backing of the thin targets on measured results was corrected using a model to relate to the electron transport process. The mean paths of electron multi-scattered in the target itself (including forward and backward scattering) were calculated by means of Monte Carlo program (EGS4) and they were used to correct measured results. A comparison with both theoretical predictions was given.