[en] Recently, a new method of analysing electron correlations based on intensity interferometry has been applied to double ionization of He and Ne by fast ion impact [1]. The data reveal sensitively correlation effects while they appear to be very insensitive to the collision dynamics. In order to analyse the role of the initial state electron correlation a statistically defined correlation function based on intensity interferometry was calculated for the ground state of He. In a comparative study of model wave functions we demonstrate that correlation can be considered from a statistical point of view which offers a new tool to study correlation effects in many-particle systems. (orig.)

[en] Vector momentum distributions of Neⁿ⁺ (n=1,2,3) ions created by 30 fs, ∼1 PW/cm² laser pulses at 795 nm have been measured using Recoil-Ion Momentum Spectroscopy. Distinct maxima along the light polarization axis are observed at 4.0 a.u. and 7.5 a.u. for Ne²⁺ and Ne³⁺ production, respectively. Hence, mechanisms based on an instantaneous release of two (or more) electrons can be ruled out as dominant contribution to non-sequential strong-field multiple ionization. The positions of the maxima are in accord with kinematical constraints set by the classical ''rescattering model''. (orig.)

[en] Helium single ionization by 3.6 MeV/u Ni²⁴⁺ impact was explored in a kinematically complete experiment. Combining a high-resolution recoil-ion momentum spectrometer (ΔE_Rparallel∼±12 μeV) with a novel 4π open-quotes soft-electronclose quotes (0 eV ≤ E_e ≤ 50 eV) analyzer all 3 momentum components of both the emitted recoil ion and electron were recorded simultaneously. More than 90% of the soft electrons are ejected in forward direction in agreement with CTMC predictions. The backwards recoiling He¹⁺-ion compensates the electron momentum completely in the longitudinal (except of the Q-value) and partly in the transverse direction. Energy losses of the 208 MeV projectile as small as ΔE_p/E_p ∼ 10^-8 become detectable

[en] Full text: Using a reaction-microscope (COLTRIMS combined with an electron analyzer) multiple ionization of rare gas atoms (He, Ne, Ar) in strong ultra-short laser pulses (6 fs, 25 fs) has been studied with hitherto unachieved resolution and completeness. The beam of a Ti:Sa laser (800 nm) was crossed with an intrinsically cold atomic beam in the center of the spectrometer enabling to analyze the coincident atomic reaction fragments with respect to their initial momentum vectors. Though electron recollision is known to be the dominant mechanism in non-sequential double (multiple) ionization of atoms at intermediate intensities, disturbing differences in the behavior of different target atoms as well as inconsistencies between theory and experiment lead to the conclusion that the dynamics of electron recollision in the presence of the external laser field is by far not yet understood. Almost all experiments have been performed with target atoms heavier than He. Here we present fully differential ionization rates for He, the prototype system for theory. In addition, differential data on triple up to 7-fold ionization of Ne and At at laser peak intensities between 0.3 and 20.0 PW/cm² have been collected for the very first time. The results will be interpreted in terms of possible non-sequential and sequential multiple ionization mechanisms of atoms in strong fields. (author)

[en] Full text: Single ionization of atoms by fast ion impact has been studied extensively over decades. The measurement of fully differential cross sections (FDCS), however, was not possible until recently. Such FDCS data are known to be very sensitive to the collision dynamics and they can be considered as the ultimate test of single ionization theories. Results for ionization of He in collisions with various projectiles over a wide range of perturbation strengths Z/v (projectile charge to velocity ratio) from Z/v 0.1 up to 4.4 reveal significant discrepancies between theory and experiment. Surprisingly, this is even true in a regime where the 1st Born approximation was considered to be valid. The experimental results are discussed in terms of possible higher order ionization mechanisms which are not taken into account in theory. Double ionization of He is as a benchmark reaction for the investigation of electronic correlation in atomic systems. The first kinematically complete experiments for photon and electron impact ((gamma,2e) and (e,3e) experiments) succeeded in the mid and late nineties, respectively. Equivalent measurements for fast ion impact were not feasible up to now. Here, we report on the first fully differential measurement for double ionization of He by fast ion impact (6 MeV protons, v = 15.5 a.u.). The FDCS results are compared to previously collected (e,3e) data at 2 keV (v = 12 a.u.) in order to investigate possible differences in the break-up dynamics between positively and negatively charged projectiles

[en] Utilizing the different information from spatially separated segments of high-granular photon detectors the measured (LAB) energy of photons emitted by fast moving ions can be corrected individually for the Doppler effect according to the particular observation angles of each detector segment. By a redundant fitting procedure the center of mass photon energy can be determined with high precision. This new Doppler-shift assisted spectroscopy is explained for the case study of 277.4 MeV/u Pb⁸²⁺ ions colliding with a N₂-gas target at the heavy ion storage ring ESR. Spectroscopic information for hydrogenic Pb⁸¹⁺ ions is given for the ground-state transitions, for the Balmer transitions, as well as for the total K-binding energy. (orig.)

[en] The heavy ion synchrotron/storage ring facility at GSI, SIS/ESR, provides intense beams of cooled, highly-charged ions up to naked uranium (U⁹²⁺). By electron capture during ion-atom collisions in the gas target of the ESR or by recombination at ion-electron encounters in the ''electron cooler'' excited states are populated. The detailed structure of very heavy one-, two- and three-electron ions is studied. The different mechanisms leading to the excited states are described, as well as the new experimental tools now available for a detailed spectroscopy of these interesting systems. Special emphasis is given to X-ray transitions to the groundstates in H- and He-like systems. For the heaviest species the groundstate Lambshift can now be probed on an accuracy level of better than 10% using solid-state X-ray detectors. Applying dispersive X-ray analyzing techniques, this accuracy will certainly be improved in future. However, utilizing the dielectronic resonances for a spectroscopy, the structure in Li-like heavy ions can already be probed now on the sub eV level. (orig.)

[en] We have measured and calculated doubly differential electron-emission cross sections (DDCS's) for well-defined degrees of target ionisation in 3.6 MeV/amu Au⁵³⁺ + He collisions. The calculations are based on the continuum distorted wave with eikonal initial state approximation. We focus on DDCS's obtained in coincidence with double ionisation of the target, and investigate different models for the final two-electron continuum state. Whereas the results of an analysis within the independent particle model overestimate the experimental data badly, we find reduced DDCS's when electron correlation effects are included in a relatively simple fashion. The comparison between theoretical and experimental results indicates that the effective strength of the electron repulsion differs in close and distant collisions

[en] An electron beam ion trap is under construction at the University of Freiburg. It will be operated both as an ion trap and as an ion source, and its design specifications will allow production of hydrogenic ions from even the heaviest elements. It will be possible to extract and accelerate the ions to energies up to 350 kV/q. (orig.)

[en] The classical 'rescattering' model is used to give an interpretation of ion momentum distributions for multiple ionization of atoms in intense fs laser pulses. Kinematical constraints for the ion momenta derived from simple classical considerations reproduce the positions of the maxima observed in experimental momentum distributions of Ne²⁺ and Ne³⁺ ions. Based on these constraints we present a first prediction on the principle structure of differential ion momentum distributions for multiple ionization. We show that different ionization channels can be characterized by their kinematics, leading to specific ion momentum distributions. A qualitative analysis of the intensity dependence is also given. (author). Letter-to-the-editor