K. Wohrer

Charge-state distributions of few-electron ions deduced from atomic cross sections

A model for the evolution of charge-state distributions of fast heavy ions in solid targets is formulated, applying to the case when highly stripped ions are produced. Starting from atomic cross sections, the authors calculate the evolution of the individual populations in 1s, 2s and 2p subshells resulting from a dynamical competition between atomic processes, like electron capture, ionisation, excitation, and radiative decay processes. The final charge-state fractions are calculated by combining these individual populations. Numerical calculations are compared with experimental data for 7 MeV u-1 Fe and 35 MeV u-1 Kr ions in various targets.

Swift cluster–atom collisions: a progress report

Abstract Significant progresses in the field of swift cluster–atom collisions have been made, thanks to the development of new beam facilities. We report on experiments performed nearby the Tandem accelerator at Orsay (France) with beams of small carbon clusters C+n(n⩽5) of 2 MeV/C colliding with a single helium atom. New results concerning the ionization cross sections of the clusters are reported and compared to the predictions of the independent atom and electron model. New results concerning the fragmentation patterns of C+5 (branching ratios and dissociative energies) are presented. A strong observed multifragmentation is observed, and discussed.

Characteristics of single capture nl distributions and double capture probabilities in slow collisions of Al13+, Al12+ and Ne9+ ions with two-electron targets (He, H2)

Relative intensities of Lyman X-rays consecutive to one- or two-electron capture have been measured for collisions of 4 keV u-1 Al13+, Al12+, Ne9+ ions with He and H2 targets. A method is presented to extract separate information on single and double transfer processes. Preferential n states, proportion of p substrates and mean l values populated by single capture are measured, together with double capture probabilities. Comparison of experimental results with theoretical predictions for one-electron systems with near-crossing radii suggest that the location of these crossings may be more important than target internal structure to determine final l distributions. Finally, for the investigated systems with different ionic charges but near-crossing radii, double capture probabilities are found to be quite similar.

Model calculations of multi-ionization cross sections in high velocity ion-cluster collisions

Abstract A model is presented for predicting, on the basis of known ion-atom elementary processes, related cross sections in ion-cluster collisions. The method is particularly suitable in high velocity collisions for which individual impact parameter probabilities of electron capture, excitation and ionization processes, are better known. Application to multi-ionization of 25 MeV C + 60 in collisions with an H 2 target (quasi-perturbative regime) is given. The question of the stability of the C 60 n + projectiles, in connection with predicted multi-ionization cross sections in some specific sites, is discussed.

Excitation in swift heavy ion-atom collisions

This paper presents a short overview of the most recent experimental and theoretical studies of excitation cross sections of light atoms (H or He) and two-electron heavy ions by multicharged ions and neutral atoms respectively. In both cases, at intermediate velocity, the excitation cross sections tend to “saturate” when the exciting partner charge increases. Nevertheless, experimental results obtained for two-electron heavy ions may not be explained as is the case for the light atoms. Multiple processes involved in Ar16+ collisions have also been extracted and results show that target electrons seem to play an active role; these need to be taken into account in the theoretical treatment of single excitation.

Excitation of Ar16+ projectiles in intermediate velocity collisions with neutrals

Abstract Excitation cross sections 1s2 → 1snp of 13.6 MeV/u Ar16+ ions colliding with various target atoms (2 ≤ ZT ≤ 54) have been measured by looking at the projectile emission with a high-resolution X-ray spectrometer. This experimental technic allows us to distinguish all the processes involved in the collision such as, for example, the competitive capture ionization. The determination of intra-shell excitation ( 1s2s ⇎ 1s2p ) coming from the 1s2s(3S1) metastable state has also been deduced.

Recent questions about the excitation mechanism at intermediate velocity

Abstract For both fundamental reasons and potential applications, the process of K-excitation by heavy ions at intermediate velocity has been re-investigated experimentally and theoretically and the main results are described here. The nature and the origin of the so-called “saturation effect” at high exciting charge is not yet elucidated. The importance of K-excitation is outlined in connection with problems of radiation damage.

Role of angular correlations and total angular momentum on partial autoionisation widths of O6+ (3,3) and (4,4) states

Partial widths for autoionisation to 2l and 3l sublevels have been calculated for O6+ (3,3) and (4,4) states by a configuration-interaction method. The Gamma (2s)/ Gamma (2p) branching ratios of states under the N=3 threshold are shown to exhibit a characteristic dependence upon the quantum numbers K and L used in the doubly excited-state basis classification. An interpretation is provided by an analysis of predominant configurations in these states.