G. Hofmann

Indirect processes in the electron-impact ionization of Li-like ions

Detailed measurements of electron-impact ionization cross sections have been made in the vicinity of the excitation-autoionization thresholds of Li-like B2+, C3+, N4+, O5+ and F6+ ions. With an energy spread of 0.4 to 1% of the electron energy and statistical uncertainties as low as 0.1% we could clearly resolve thresholds for excitation of individual terms in 1s2s2l configurations. Numerous resonance features were found which are due to dielectronic capture of the incident electron with subsequent two-electron emission. In particular, dielectronic capture processes involving Δn=2 excitations of a 1s electron provide the dominant resonance contributions to the measured cross sections. Rydberg series 1s2snln′l′ of resonances withn=3 (Δn=2) andn=4 (Δn=3) are resolved up to principle quantum numbersn′ equal to 6 or 7.

Investigations of the new Giessen 10 GHz electron-cyclotron-resonance ion source

Abstract A 10 GHz electron-cyclotron-resonance (ECR) ion source has been designed and built to extend our studies of the collisions of highly charged ions with electrons. The magnetic system of the ion source consists of three solenoid coils for the axial field component and a hexapole permanent magnet for the radial field component. A 10 GHz microwave system supplies up to 2.5 kW of microwave power to the plasma. The ion source can be operated at a high voltage potential of 20 kV and yields total ion currents of 0.2–2.5 mA. Ion beams produced with argon, oxygen and xenon were analyzed by a dipole magnet. Substantial currents up to O7+, Ar11+ and Xe27+ were measured. Furthermore, newly recognized properties of the extracted ion beams were obtained from electron-impact ionization studies. A strong correlation between the electron-impact ionization cross section in the supplied microwave power level resulted in a significantly higher ionization cross section of the reaction O5+ + e− → O6+ +2e−. Multiply charged ions from ECR ion sources would appear to have a “memory” of the conditions of their production.

Electron-impact single ionization of low-charged titanium ions

The electron-impact single ionization of low-charged Ti ions was studied both experimentally and theoretically. Absolute cross sections for ions in charge states q = 1 to 6 except 3 were measured using a crossed-beams technique as well as calculated with the configuration-average distorted-wave method, including both direct ionization and excitation-autoionization contributions. For charge states q = 4, 5 and 6, the measurements are in good agreement with the calculations considering that large components of the incident ion beam were in metastable configurations. The cross sections for single ionization of and were determined in part with a high-resolution energy-scan method. Besides contributions of excitation-autoionization mechanisms, a remarkable number of structures arising from resonant-excitation double-autoionization (REDA) processes could be observed.

A high-current electron gun for crossed-beams electron-ion collision studies at keV energies

Abstract For crossed-beams electron-ion collision studies involving highly charged ions, we have developed an electron gun delivering a high-density electron beam of 2 mm ⊘ at energies up to 6.5 keV and currents up to 430 mA. In order to decrease the spread of the electron energy distribution, the electron beam space charge in the collision region with the ion beam is compensated by means of trapped ions resulting from electron impact ionization of residual gas particles. First measurements are showing good agreement of the obtained absolute cross sections for the single ionization of Xe 8+ and Fe 9+ ions with available experimental data. Employing a fast energy-scanning technique, energy-sharp resonance structures in the cross section for the triple ionization of Ba 2+ have been resolved. From these measurements, the spread of the electron energy distribution at 800 eV can be estimated to 3.2 eV FWHM.

Doubly autoionizing capture resonances ine + Mg+ collisions

Fine details in the cross section for electron impact ionization of Mg+ ions (configuration 1s2 2s2 2p6 3s) have been measured with an energy resolution of 0.3 eV. Structure on top of a smooth direct-ionization “background” reflects the presence of indirect ionization mechanisms. Such contributions in a Na-like ion involvenon-resonant excitation of a 2p electron to singly autoionizing intermediate states subsequently decaying into the channel of net single ionization of the parent ion. We observe even stronger indirect contributions fromresonant excitation of a 2p electron with simultaneous dielectronic capture of the projectile electron into doubly autoionizing states which decay by successive emission of two electrons.

Signal enhancement in electron-ion crossed-beams experiments

Abstract The collision rate in colliding-beams experiments increases with the particle density in the interaction region. High-density electron beams have been used in measurements of electron-impact ionization of ions for which ratios of signal to noise of more than 100:1 have been obtained. However, space-charge potentials in the high-intensity electron beam may affect the ion beam and the electron energy distribution. We have investigated these influences and demonstrated that space charge compensation of the electron beam by slow positive ions is a feasible method for accurate cross section measurements.

Electron-impact multiple ionization of singly and multiply charged tungsten ions

Cross sections for electron-impact double ionization of Wq+ ions in charge states q=1-6 as well as triple ionization in charge states q=1-4 have been measured using the crossed-beams technique. A new semiempirical formula for multiple ionization of three or more target electrons is in very good agreement with the measurements. Together with previously obtained experimental data for single ionization of tungsten ions, the measured cross sections are used to study the influence of multiple ionization on the Wq+ charge state evolution if tungsten atoms are exposed to an electron flux at 700 eV energy. It is shown that the charge state evolution is significantly influenced by multiple-ionization processes.

Resonances in the electron-impact single and multiple ionization of ions

Cross sections for electron-impact single ionization of Lilike ionsB2+,C3+,N4+,O5+ andF6+ and for n-fold ionization ofLi+ (n=1),Na+ (n=1),K+ (n=1,2,3),Rb+ (n=1,2,3),Cs+ (n=1,2,3,4) andBa+ (n=1,2,3,4) have been measured with extremely high precision (lowest limit of statistical uncertainties as low as 0.01%). Detailed studies of indirect ionization mechanisms proceeding via multiply-excited intermediate states could be carried out.

Single and multiple ionization of Niq+ ions by electron impact

Abstract Employing the crossed-beams technique, absolute cross sections for electron impact single ionization of Niq+ ions in charge states q = 2–5 have been measured for energies from threshold to 1000 eV. The Ni2+ and Ni3+ results show a significant ionization signal below the ground state threshold due to substantial components of metastable ions in the parent ion beams. In the case of Ni4+ and Ni5+, no significant contributions of metastable ions to the measured cross sections have been found. The results are in reasonable agreement with theoretical calculations. For the first time we have measured absolute cross sections for electron impact double ionization of Niq+ ions in charge states q = 2, 3, 4.

Recombination of highly charged ions with free electrons

Recombination of highly charged ions and free electrons is studied in interacting‐beam experiments. Beside direct recombination into bound states by radiative capture a variety of resonant recombination phenomena is observed. Resonant recombination produces a highly excited, usually short lived electron‐ion compound which can stabilize by the emission of photons and/or electrons. Depending on this emission, the final charge state of the ion can be one less than the parent charge state, but it can also be higher and thus a net single or multiple ionization of the ion is observed after the initial recombination.