Extended Classical Over-Barrier Model for Collisions of Highly Charged Ions with Conducting and Insulating Surfaces

We have extended the classical over-barrier model to simulate the neutralization dynamics of highly charged ions interacting under grazing incidence with conducting and insulating surfaces. Our calculations are based on simple model rates for resonant and Auger transitions. We include effects caused by the dielectric response of the target and, for insulators, localized surface charges. Characteristic deviations regarding the charge transfer processes from conducting and insulating targets to the ion are discussed. We find good agreement with previously published experimental data for the image energy gain of a variety of highly charged ions impinging on Au, Al, LiF and KI crystals.Comment: 32 pages http://pikp28.uni-muenster.de/~ducree

Ab-initio validation of a simple heuristic expression for the sequential-double-ionization contribution to the double ionization of helium by ultrashort XUV pulses

Citation: Liu, A., & Thumm, U. (2015). Ab-initio validation of a simple heuristic expression for the sequential-double-ionization contribution to the double ionization of helium by ultrashort XUV pulses. 635(9). doi:10.1088/1742-6596/635/9/092039We study two-photon double ionization of helium by short XUV pulses by numerically solving the time-dependent Schrodinger equation in full dimensionality within a finite-element discrete-variable-representation scheme. Based on the emission asymmetries in joint photoelectron angular distributions, we identify sequential and non-sequential contributions to two-photon double ionization for ultrashort pulses whose spectrum overlaps the sequential (? > 54.4 eV) and non-sequential (39.5 eV < ? < 54.4 eV) double-ionization regimes. © Published under licence by IOP Publishing Ltd

Criterion for Distinguishing Sequential from Nonsequential Contributions to the Double Ionization of Helium in Ultrashort Extreme-Ultraviolet Pulses

Citation: Liu, A. H., & Thumm, U. (2015). Criterion for Distinguishing Sequential from Nonsequential Contributions to the Double Ionization of Helium in Ultrashort Extreme-Ultraviolet Pulses. Physical Review Letters, 115(18), 5. doi:10.1103/PhysRevLett.115.183002We quantify sequential and nonsequential contributions in two-photon double ionization of helium atoms by intense ultrashort extreme-ultraviolet pulses with central photon energies (h) over bar omega(ctr) near the sequential double-ionization threshold. If the spectrum of such pulses overlaps both the sequential ((h) over bar omega > 54.4 eV) and nonsequential ((h) over bar omega omega(ctr) = 50 eV pulses with a sine-squared temporal profile, we find that the sequential double-ionization contribution is the largest at a pulse length of 650 as, due to competing temporal and spectral constraints. In addition, we validate a simple heuristic expression for the sequential double-ionization contribution in comparison with ab initio calculations

Laser-assisted XUV double ionization of helium: Energy-sharing dependence of joint angular distributions

Citation: Liu, A. H., & Thumm, U. (2015). Laser-assisted XUV double ionization of helium: Energy-sharing dependence of joint angular distributions. Physical Review A, 91(4), 9. doi:10.1103/PhysRevA.91.043416By numerically solving the time-dependent Schrodinger equation in full dimensionality, we discuss the dependance of joint photoelectron angular distributions on the energy sharing of the emitted electrons for the double ionization of helium atoms by ultrashort pulses of extreme ultraviolet (XUV) radiation in coplanar emission geometry with and without the presence of a comparatively weak infrared (IR) laser pulse. For IR-laser-assisted single-XUV-photon double ionization, our joint angular distributions show that the IR-laser field enhances back-to-back electron emission and induces a characteristic splitting in the angular distribution for electrons that are emitted symmetrically relative to the identical linear polarization directions of the XUV and IR pulse. These IR-pulse-induced changes in photoelectron angular distributions are (i) imposed by different symmetry constraints for XUV-pulse-only and laser-assisted XUV double ionization, (ii) robust over a large range of energy sharings between the emitted electrons, and (iii) consistent with the transfer of discrete IR-photon momenta to both photoelectrons from the assisting IR-laser field. While selection-rule forbidden at equal energy sharing, for increasingly unequal energy sharing we find back-to-back emission to become more likely and to compete with symmetric emission

NASA Utilization of the International Space Station and the Vision for Space Exploration

In response to the U.S. President's Vision for Space Exploration (January 14, 2004), NASA has revised its utilization plans for ISS to focus on (1) research on astronaut health and the development of countermeasures that will protect our crews from the space environment during long duration voyages, (2) ISS as a test bed for research and technology developments that will insure vehicle systems and operational practices are ready for future exploration missions, (3) developing and validating operational practices and procedures for long-duration space missions. In addition, NASA will continue a small amount of fundamental research in life and microgravity sciences. There have been significant research accomplishments that are important for achieving the Exploration Vision. Some of these have been formal research payloads, while others have come from research based on the operation of International Space Station (ISS). We will review a selection of these experiments and results, as well as outline some of ongoing and upcoming research. The ISS represents the only microgravity opportunity to perform on-orbit long-duration studies of human health and performance and technologies relevant for future long-duration missions planned during the next 25 years. Even as NASA focuses on developing the Orion spacecraft and return to the moon (2015-2020), research on and operation of the ISS is fundamental to the success of NASA s Exploration Vision

Inspiring the Next Generation: Student Experiments and Educational Activities on the International Space Station, 2000-2006

One important objective of NASA has always been to inspire the next generation. NASA and human space flight have a unique ability to capture the imaginations of both students and teachers. The presence of humans onboard the International Space Station (ISS) for more than five years now has provided a foundation for numerous educational activities aimed at capturing the interest and motivating study in the sciences, technology, engineering, and mathematics. Yet even before the Expedition 1 crew arrived at station in November 2000, experiments with student participation were being conducted onboard ISS in support of NASA missions. One of NASA's protein crystal growth experiments had been delivered to station by the shuttle Atlantis during STS-106 in September 2000 and was returned to Earth six weeks later aboard the shuttle Discovery during the STS-92 mission. From very early on it was recognized that students would have a strong interest in the ISS, and that this would provide a unique opportunity for them to get involved and participate in science and engineering projects on ISS. It should be noted that participation is not limited to U.S. students but involves the 16 International Partner countries and various other countries under special commercial agre

Zu einer Theorie biographischen Lernens

Der Autor schlägt vor, biographisches Lernen (theoretisch) in vier Arten aufzuschlüsseln. Er nennt diese Arten: "Erfahrungslernen", "strukturelles Lernen", "traumatisches Lernen" und "pädagogisches Lernen". Diese verschiedenen Arten des biographischen Lernens stehen in einem bestimmten Zusammenhang, der allerdings bislang nur teilweise erforscht und bekannt ist. Besonders wichtig und gleichzeitig besonders schwierig zu erforschen ist der Zusammenhang von stochastischen und nichtstochastischen Entwicklungsprozessen. Der Autor plädiert dafür, diesen vielschichtigen und komplizierten Prozeß analog zur Evolution - und damit auch zur Phylogenese - zu interpretieren. (DIPF/Orig.