H. Kollmus

Three-Dimensional Imaging of Atomic Four-Body Processes

To understand the physical processes that occur in nature we need to obtain a solid concept about the ‘fundamental’ forces acting between pairs of elementary particles. It is also necessary to describe the temporal and spatial evolution of many mutually interacting particles under the influence of these forces. This latter step, known as the few-body problem, remains an important unsolved problem in physics. Experiments involving atomic collisions represent a useful testing ground for studying the few-body problem. For the single ionization of a helium atom by charged particle impact, kinematically complete experiments have been performed since 1969 (ref. 7). The theoretical analysis of such experiments was thought to yield a complete picture of the basic features of the collision process, at least for large collision energies. These conclusions are, however, almost exclusively based on studies of restricted electron-emission geometries. Here, we report three-dimensional images of the complete electron emission pattern for the single ionization of helium by the impact of C6+ ions of energy 100 MeV per a.m.u. (a four-body system) and observe features that have not been predicted by any published theoretical model. We propose a higher-order ionization mechanism, involving the interaction between the projectile and the target nucleus, to explain these features.

Triply differential single ionization cross sections in coplanar and non-coplanar geometry for fast heavy ion-atom collisions

We have performed a kinematically complete experiment and calculations on single ionization in 100 MeV/amu C6+ + He collisions. For electrons ejected into the scattering plane (defined by the initial and final projectile momentum vectors) our first- and higher-order calculations are in good agreement with the data. In the plane perpendicular to the scattering plane and containing the initial projectile axis a strong forward-backward asymmetry is observed. In this plane both the first-order and the higher-order calculations do not provide good agreement neither with the data nor amongst each other.

A high resolution 4π multi-electron spectrometer for soft electrons

Electrons emerging form ionizing collisions of any projectiles (ions, photons, electrons) with the particles in a supersonic jet target (atoms, clusters, molecules) are projected onto three large-area, multi-hit capable position sensitive detectors applying well controlled electric and solenoidal magnetic fields. Up to three electrons with an energy bandwidth for each between 0 eV ≤ Ee ≤ 1 keV are detected simultaneously with a solid angle close to 4π. Each momentum vector (three spatial components pei) is calculated from the measured hitting positions and time-of-flights with a resolution of Δ pei < 2 × 10−2 a.u.. Thus, the complete final many-electron momentum space (up to 9 momentum components) is visualized. The theoretical limits in resolution achievable with such a method is in the neV-regime. In addition, the recoil-ion momentum vector is determined simultaneously applying “conventional” recoil-ion momentum spectroscopy.

Dissociative ionization of H2 by fast protons: Three-body breakup and molecular-frame electron emission

Highly differential cross sections have been obtained for dissociative single ionization of H2 by 6 MeV proton impact by measuring the momentum vectors of the electron and the H+ fragment in coincidence. The investigation of the momentum balance of the fragments along the projectile beam provided detailed insight into the four-particle dynamics, even though the H atom is not detected. Within the axial recoil approximation, molecular-frame angular distributions of emitted electrons have been determined for molecules oriented perpendicular to the projectile beam. They are compared with the predictions of a CDW-EIS calculation.

Recoil-Ion Momentum Spectroscopy and “Reaction Microscopes”

The rapid and still ongoing development of recoil-ion momentum spectroscopy (RIMS) during the last ten years can undoubtedly be viewed as an experimental breakthrough for the investigation of any kind of atomic reaction dynamics. Whenever atoms or simple molecules interact with electrons, ions or photons, the concept of high—resolution recoil-ion measurements resulted in additional and complementary information compared to the traditional electron spectroscopy methods, and in some cases even kinematically complete data sets could be collected for the very first time. State—of—the—art high-resolution recoil-ion momentum spectrometers evolved through numerous technical developments like, e.g., the implementation of cold supersonicjet targets, the use of well defined electric extraction fields for recoil ions as well as for electrons and the rapid progress in charged particle detection techniques. Among them the use of supersonic jets to produce well localized and internally cold targets (COLd Target Recoil-Ion Momentum Spectroscopy COLTRIMS) can be viewed as the most important ingredient. They allowed one to achieve a recoil-ion momentum resolution far below 1 a.u. (atomic unit) that would be impossible with room-temperature targets due to the thermal motion (the momentum spread of room-temperature helium atoms is about 3.7 a. u.). Another decisive development was the invention of completely novel and extremely efficient electron—imaging concepts.

DIFFERENTIAL PROJECTILE ENERGY LOSS IN MULTIPLY IONIZING COLLISIONS

The momentum vectors of the recoil ion and up to three electrons were measured in coincidence with the projectiles which did not change charge state for 3.6 MeV amu-1 Au53+ Ne collisions. New techniques were applied to obtain differential energy-loss spectra for multiple-target ionization as a function of the recoil-ion charge state. A resolution unprecedented for this energy regime was achieved. The data are in good agreement with a classical trajectory Monte Carlo calculation. Our studies represent a first step for a method of modelling stopping powers on a microscopic level and are relevant for plasma applications.

Double ionization of helium in fast ion collisions: the role of momentum transfer

Double ionization of helium in the perturbative regime has been explored in a kinematically complete collision experiment using 100 MeV/u C 6+ ions. Different ionization mechanisms are identified by inspecting the angular distribution of the electrons as a function of the momentum transfer q to the target by the projectile. For q 1 :2 au, the faster electron resulting from a binary encounter with the projectile is emitted along the direction of momentum transfer, while the other electron is distributed uniformly. Experimental data are compared with various model calculations based on the Bethe-Born approximation with shake-off. Surprisingly, the effect of the final state interaction is found to depend decisively on the choice of the initial state wavefunction.

Double Ionization of Helium by Impact of Particles with Positive and Negative Charge

Double ionization of helium in collisions with 6 MeV protons (vP = 15.5 a.u.) has been explored in a kinematically complete experiment using a so‐called “reaction microscope”. For the first time fully differential cross sections for positively charged particle impact have been obtained and compared to previous (e,3e) results with 2 keV electrons (vP = 12 a.u.). Differences have been found in the angular distribution of the two emitted electrons, which originate from projectile charge effects that are not considered in the first Born approximation.