T van Zoest

Bio: T van Zoest is an academic researcher from University of Giessen. The author has contributed to research in topics: Ionization & Resonance (particle physics). The author has an hindex of 2, co-authored 2 publications receiving 48 citations.

Threshold truncation of a 'giant' dipole resonance in photoionization of Ti3+

Abstract: Photoionization of triply charged titanium ions was investigated using the merged photon–ion beam technique at a synchrotron light source. The experimental photon energy range 42.6–49.4 eV encompasses the threshold for the photoionization of Ti3+(3p6 3d2 D3/2) ground-state ions at 43.267 eV. Prominent resonance features due to 3p → 3d and 3p → 4s excitations are observed with the strongest one being the 'giant' 3p5(3d23F)2F dipole resonance which has a width of 1.5 eV. Since it is located only 0.2 eV above the ionization threshold a cut-off of this resonance is observed in photoionization. By employing the principle of detailed balance the results are compared with an earlier experimental study of the time-reversed Ti4+ photorecombination. The comparison clarifies the giant resonance's role in photorecombination and yields state-selective photoionization and photorecombinaton cross sections on an absolute scale.

Electron-impact ionization of Ti3+ ions

Abstract: Absolute cross sections for electron-impact single ionization of potassium-like titanium have been measured from threshold to 1000 eV using a crossed-beams experimental set-up. A 0.5 eV resolution energy-scan technique revealed structures in the cross section in the range from 40 eV to 150 eV due to dominant contributions of indirect ionization mechanisms. A large fraction of the cross section is due to excitation–autoionization proceeding via intermediate excited states 3p5(3d2 3F) 2F, 3p5(3d2 3P) 2P, 3p5(3d2 3F) 2D which subsequently decay by super-Coster–Kronig transitions. The results of the measurements are in satisfactory agreement with previous advanced theoretical calculations.

Ionization rate coefficients for the elements hydrogen through zinc

Abstract: Aims. The interpretation of astrophysical spectra depends directly on a knowledge of the ionization state of the emitting plasma. This is determined, in part, from collisional ionization rate coefficients. The most recent assessments of these were performed by Arnaud & Rothenflug (1985, A&AS, 60, 425) and Arnaud & Raymond (1992, ApJ, 398, 394). Since their work, new laboratory measurements of ionization cross sections have become available as well as the Flexible Atomic Code (FAC) which enables theoretical calculations of these rates. Our goal is to provide a complete set of ionization rate coefficients for the elements hydrogen through zinc. Methods. A scaling law, which assists the analysis of ionization cross sections and rate coefficients, has been developed following the approach of Burgess & Tully (1992, A&A, 254, 436). Essentially all available measured cross sections along each isoelectronic sequence have been examined and compared to cross sections calculated with the Flexible Atomic Code (FAC) and with other calculations. Two approaches has been taken to provide a complete set of ionization cross sections. In the first, fits to scaled measured ionization cross sections, particularly for neutral and singly ionized species, are performed. In the second, fits to scaled calculated direct ionization and excitation-autoionization cross sections are performed to provide the remainder of the set. The fits to the cross sections are then integrated over a Maxwellian velocity distribution to derive ionization rate coefficients. Results. A complete set of ground level ionization cross sections and rate coefficients has been developed through the combination of these two approaches. A tabulation of parameter fits to ground level ionization rate coefficients for all atoms and ions of the elements of H through Zn is provided.

Electron–ion collisions: Fundamental processes in the focus of applied research

Abstract: Electron–ion collisions are among the most important atomic processes in ionized gases. They comprise elastic scattering, excitation, ionization, and recombination. Theoretical approaches are numerous but calculating reliable results for applications is a task that has not generally been solved, although theory has progressed considerably in the last decade and can now provide reference data for electron collisions with few-electron systems. Experimental data on electron–ion collisions can be inferred from plasma observations, from ion trap measurements, from careful preparation and analysis of ion–atom collisions, and from colliding-beams experiments. The spectrum of equipment employed in the measurements ranges from table top size to large accelerator and storage-ring facilities. This article provides an overview of the processes involved in electron–ion collisions, their cross sections, the experimental approaches and typical results. The present status of the field is discussed and characterized by numerous examples illuminating the accomplishments as well as the limitations of experimental access to accurate detailed cross sections and rate coefficients, as they are required for understanding and modelling laboratory or astrophysical plasmas.

Photoionization cross sections of atomic ions from merged-beam experiments

Abstract: The recent development of experimental photoionization cross sections of atomic ions has been reviewed. Owing to the construction of intense undulator-based photon sources it has been possible during the last ten years to perform a large number of absolute cross-section measurements using the merged-beam method. Photoionization cross sections provide a critical test of theoretical calculations, with implications for the modelling of astrophysical and laboratory plasmas for which fundamental data of this kind are required.

Study of valence band structure of Fe doped anatase TiO2 thin films

Abstract: We study the structural and electronic properties of Fe doped (4–8 at. %) and undoped TiO2 thin films deposited by pulsed laser deposition on Si(111) substrate. The films grow in single phase anatase structure of TiO2 as revealed by x-ray diffraction and Raman spectroscopy measurements. The Fe doped films reveal room temperature magnetic hysteresis behavior. We have probed the electronic environment of Fe in TiO2 matrix and its coupling to the cations, using photoelectron spectroscopy measurements. Photoelectron spectroscopic studies reveal the ionic state of Fe in TiO2, precluding the formation of Fe metal clusters. Valence band spectra of these films suggest that it primarily consists of O-2p derived state, however, Fe derived state is also observed in Fe doped films. Resonance photoelectron spectroscopy studies indicate that Fe ions are hybridized with Ti3+ defect states.

Fusion-Related Ionization and Recombination Data for Tungsten Ions in Low to Moderately High Charge States

Abstract: Collisional processes and details of atomic structure of heavy many-electron atoms and ions are not yet understood in a fully satisfying manner. Experimental studies are required for guiding new theoretical approaches. In response to fusion-related needs for collisional and spectroscopic data on tungsten atoms in all charge states, a project has been initiated in which electron-impact and photon-induced ionization as well as photorecombination of Wq+ ions are studied. Cross sections and rate coefficients were determined for charge states q ranging from q = 1 to q = 5 for photoionization, for q = 1 up to q = 19 for electron-impact ionization and for q = 18 to q = 21 for electron-ion recombination. An overview, together with a critical assessment of the methods and results is provided.