J. Hellhund

Electron-impact single and double ionization of W17 +

A crossed-beam setup was used to measure cross sections for electron-impact single and double ionization of W17 + ions. Absolute data and high-resolution scan spectra were obtained at collision energies ranging from threshold up to 1000eV. A comparison of the experimental results with theoretical calculations for direct ionization suggests substantial contributions of excitation–autoionization processes to electron-impact single ionization of W17 +.

Photoionization of Ne Atoms and Ne+ Ions Near the K Edge: PrecisionSpectroscopy and Absolute Cross-sections

Author(s): Muller, A; Bernhardt, D; Borovik, A; Buhr, T; Hellhund, J; Holste, K; Kilcoyne, ALD; Klumpp, S; Martins, M; Ricz, S; Seltmann, J; Viefhaus, J; Schippers, S | Abstract:

Single-photon single ionization of W+ ions: experiment and theory

Experimental and theoretical results are reported for photoionization of Ta-like (W+) tungsten ions. Absolute cross sections were measured in the energy range 16–245 eV employing the photon–ion merged-beam setup at the advanced light source in Berkeley. Detailed photon-energy scans at 100 meV bandwidth were performed in the 16–108 eV range. In addition, the cross section was scanned at 50 meV resolution in regions where fine resonance structures could be observed. Theoretical results were obtained from a Dirac–Coulomb R-matrix approach. Photoionization cross section calculations were performed for singly ionized atomic tungsten ions in their J = 1/2, ground level and the associated excited metastable levels with J = 3/2, 5/2, 7/2 and 9/2. Since the ion beams used in the experiments must be expected to contain long-lived excited states also from excited configurations, additional cross-section calculations were performed for the second-lowest term, J = 5/2, and for the 4F term, with J = 3/2, 5/2, 7/2 and 9/2. Given the complexity of the electronic structure of W+ the calculations reproduce the main features of the experimental cross section quite well.

Stepwise contraction of the nf Rydberg shells in the 3d photoionization of multiply- charged xenon ions

Triple photoionization of Xe3+, Xe4+ and Xe5+ ions has been studied in the energy range 670-750 eV, including the 3d ionization threshold. The photon-ion merged-beam technique was used at a synchrotron light source to measure the absolute photoionization cross sections. These cross sections exhibit a progressively larger number of sharp resonances as the ion charge state is increased. This clearly visualizes the re-ordering of the

Single and multiple photoionization of Wq+ tungsten ions in charge states q=1,2,..,5: experiment and theory

epsilon

Near- K -edge single, double, and triple photoionization of

f continuum into a regular series of (bound) Rydberg orbitals as the ionic core becomes more attractive. The energies and strengths of the resonances are extracted from the experimental data and are further analyzed by relativistic atomic-structure calculations.

C^{+}

Absolute cross sections for single and multiple photoionization of Wq+ ions have been measured in a photon-ion merged beam setup employing synchrotron radiation from the Advanced Light Source. The experimental data are compared to large-scale close-coupling calculations within the Dirac-Coulomb R-matrix (DARC) approximation.

ions

Author(s): Muller, A; Borovik, A; Buhr, T; Hellhund, J; Holste, K; Kilcoyne, ALD; Klumpp, S; Martins, M; Ricz, S; Viefhaus, J; Schippers, S | Abstract:

Observation of four-electron Auger processes

Multiple ionization of ions subsequent to absorption of a single photon has been studied employing a photon-ion merged-beam setup at the PETRA III synchrotron radiation facility of DESY in Hamburg. Absolute cross sections for single, double and triple ionization of C+ ions were measured with emphasis on specific well defined terms of K-shell excited C+. In particular, the terms C+ (1s2s22p2 2D,2P) were excited from the ground level of C+. Subsequent autoionization processes resulted in the production of C2+, C3+ and C4+ ions. The associated decay mechanisms are single-Auger, double-Auger and triple-Auger decay. The observation of C4+ products arising from C+(1s2s22p2 2D,2P) unambiguously confirmed the existence of triple-Auger decay, i.e., a process in which 4 electrons interact with one another such that one fills the K-shell vacancy and the others are simultaneously ejected. The experiment yields branching ratios for the Auger decay channels as well as individual decay rates for autoionization and radiative stabilization of the C+(1s2s22p2 2D,2P) terms.

Cross-section measurements with interacting beams

Interacting beams were first employed to determine absolute cross sections for atomic processes a half-century ago. Significant advances in many technologies have improved the precision and sensitivity of such measurements to the degree that the high quality of those early experiments seems remarkable today. Despite their early successes and continuing advances, interacting beams experiments and the determination of absolute cross sections and rate coefficients from them remain a challenge. This paper concentrates on one large-scale technical advance, the synchrotron light source that has facilitated interacting-beams measurements of cross sections for photoionization of atomic and molecular ions. Recent results will be presented for photoionization of fullerene and endofullerene molecular ions.