L. Spielberger

Fast position and time-resolved read-out of micro-channelplates with the delay-line technique for single-particle and photon-detection

Based on delay-line read-out methods of micro-channelplate (MCP) stacks we develop imaging system for single particle and photon spectroscopy. A complete system consists of an open MCP-detector with helical wire anode, specially designed front-end electronics and a stand alone PC-based TDC-system. We achieve a position resolution better than 0.1 mm and excellent linearity for open dimensions up to 100 mm, multi-hit operation, and detection rates up to 20 kiloEvents/sec in an event-listing mode or over 1 MegaCount/sec in a histogram mode. Both modes allow 2D position and time-of-flight (TOF) spectroscopy with approximately 1 nanosec TOF resolution. Furthermore, we currently test a delay-line anode on printed circuit that operates with image charge pick-up from a high-resistive collecting anode. With an image charge detection method this 3D-imaging technique can be applied to commercial sealed MCP single-photon detectors. While a simple high-resistive collection anode is placed inside the tube, a position sensitive pick-up electrode can be mounted next to it outside the vacuum wall.

Photo double ionization of He by circular and linear polarized single-photon absorption

We have measured absolute fully differential cross sections for photo double ionization of helium by circularly and linearly polarized light 20 eV above threshold. The data have been obtained by measuring in coincidence the momentum vector of the He 2+ ion and one of the electrons, covering 4π solid angle for all particles. We give an overview over the momentum distribution in the three-body continuum and show fivefold differential cross sections. We find a swirl in the electron momentum space for double ionization by circularly polarized light. The present data supersede earlier data from our group (V mergel et al 1998 Phys. Rev. Lett. 80 5301).

Cross-section ratio of double to single ionization of helium by Compton scattering of 40-100-keV x rays

We have measured the ratio of cross sections for double to single ionization of helium by Compton scattering, R{sub C}={sigma}{sub C}{sup ++}/{sigma}{sub C}{sup +}, at photon energies of 40, 80, and 100 keV using cold target recoil-ion momentum spectroscopy. Comparison with calculations involving highly correlated initial states and approximate final states with and without final-state correlations, represented by 3C and 2C wave functions respectively, shows that the influence of final-state correlations persists to very high photon energies. A comparison with recent charged-particle data is made. {copyright} {ital 1999} {ital The American Physical Society}

Microstructured electrode arrays: a source of high-pressure nonthermal plasma

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Recoil ion momentum spectroscopy a ‘momentum microscope’ to view atomic collision dynamics

Recoil ion momentum spectroscopy is a powerful tool for investigating the dynamics of ion, electron or photon impact reactions with atoms or molecules. It allows to measure the three dimensional momentum vector of the ion from those reactions with high resolution and 4 π solid angle. It can be easily combined with novel 4 π electron momentum analysers and for coincident detection of the projectile. This technique gives a complete image of the square of the correlated many body final state wave function in momentum space (i.e. fully differential cross sections) for the various reactions. The application to photo double ionization of helium by linear and circular polarized light is discussed.

From Atoms to Molecules

In the present chapter, we discuss direct photo double ionization by singlephoton absorption (Sect. 14.2) and Compton scattering (Sect. 14.3). We do not discuss the closely related phenomenon of multiple ionization by two-step processes such as photoionization followed by single- or multiple-Auger decay. We concentrate on the two most fundamental two-electron target systems: the helium atom (Sects. 14.2 and 14.3) and molecular hydrogen (deuterium) (Sect. 14.4). The subject of photo double ionization of helium is now a mature field in which an impressive experimental and theoretical breakthrough has been achieved in the previous 10 years. The theoretical progress is described in Part III of this book, we therefore restrict ourselves here to a phenomenological description and intuitive interpretation of the physical phenomena. For the problem of two-electron processes in molecules, in contrast, the major challenges for experimentalist and theoretician still lie ahead.

Recoil Ion Momentum Spectroscopy Momentum Space Images of Atomic Reactions

Recoil-ion momentum spectroscopy is a powerful tool for investigating the dynamics of ion, electron or photon impact reactions with atoms or molecules. It allows to measure the three-dimensional momentum vector of the ion from those reactions with high resolution and 4 π solid angle. In many cases already the recoil-ion momentum distribution alone unveils directly the physical processes dominating the reaction. The most detailed information, however is gained by combining the recoil-ion momentum measurement with the coincident detection of momentum vector of one or more emitted electrons or a measurement of the momentum exchange of the projectile. By such many particle momentum imaging one obtains a fully differential cross section of the reaction, i.e. for each registered event one measures the momenta of all particles and the full final state momentum space is covered in one experiment. Thus the experiment yields the square of the final state wave function of the reaction in momentum space. Such multidimensional data arrays can be sorted in many different ways after the actual experiment. Examples for ion impact ionization are discussed.

Electron Impact Ionization of Helium [(e,2e) & (e,3e)] Investigated with Cold Target Recoil-Ion Momentum Spectroscopy

One of the most fundamental tasks of nowadays atomic physics is the investigation of the dynamics in many-body systems. Its understanding is fundamental for a detailed treatment of practically all complex systems ranging from the simplest three-body problem, a charged projectile colliding with a hydrogen atom, via large molecules up to macroscopic structures of daily live. These many-body effects in systems governed by the Coulomb force (often called correlations) play a dominant role in atomic multiple ionizing reactions. Thus, already a study of double ionization can serve as a tool to investigate the specific many-body properties of bound and continuum wavefunctions as well as of the collision process itself.

Experimental separation of photoabsorption and Compton scattering contributions to He single and double ionization

We have experimentally separated the contributions of photoabsorption and Compton scattering to He single and double ionization for high‐energy photon impact by measuring the full momentum vector of the recoiling He1+,2+ ions. For recoil ions following photoabsorption large momenta and a distinct dipole emission pattern are observed. The ions produced by Compton scattering show small momenta. For the ratio of double to single ionization we find (1.22±.06)% at 8.8+1.5−1.65 keV for Compton scattering and (1.72±.12)% at 7.0+2.1−1.6 keV for photoabsorption. We compare our data with recent theories.

Thomas Process and Wave Function Imaging in P-He Transfer Ionization Investigated by Coltrims

One of the fundamental capture mechanisms for fast ion atom collisions was proposed by L.H. Thomas in 19271 based on a classical treatment and observed by Palinkas et al,2 for the first time. This Thomas process can be understood as two consecutive binary collisions, first by the projectile with one of the target electrons and, second, between this electron and either the target nucleus (e-N-Thomas-scattering) or another target electron (e-e-Thomas-scattering). The e-e-Thomas-scattering offers a unique possibility to investigate dynamic electron-electron- (e-e-) correlation in atomic collision processes. For fast proton impact the perturbation of the target (in our case helium) is small and the electrons are quickly removed from the bound state by the e-e-Thomas-scattering. The e-e-Thomas-scattering always leads to a double ionization of the target, while the capture can either be to a bound (transfer ion-ization, TI) or a continuum state of the projectile3. Thus this process will leave the nucleus behind with its momentum distribution from the initial ground state, which mirrors the sum momentum of the 2 electrons. Furthermore the absolute probability for that process yields information on the spatial distribution of the two electrons.