R Becker

Absolute ionisation cross sections for electrons incident on O+, Ne+, Xe+ and Ari+(i=1,...,5) ions

Absolute ionisation cross sections for electrons incident on O+, Ne+, Xe+ and Ar1+,..., Ar5+ ions have been measured at electron energies Ee between the ionisation threshold Ei and 830 eV. The measurements have been performed with crossed electron and ion beams. The comparison of the data for O+ and Ne+ with results of Aitken and Harrison (see ibid., vol.4, p.1176, 1971) and Dolder et al. (see Rep. Prog. Phys., vol.39, p.693, 1976), respectively, shows agreement within the combined experimental errors. The measured cross sections for ionisation of Ari+ ions can be reproduced within +or-20% of the empirical formula sigma i,i+1=1.4*10-13((ln(Ee/Ei))/(EeEi))(eV)2 cm2 for the charge states i=1 up to i=5.

Experimental cross sections for electron impact ionisation of hydrogen-like Li2+ ions

Absolute measurements have been made of cross sections for the ionisation of Li2+ ions by electrons with energies which range from threshold (122.45 eV) to 1000 eV. The results are in good agreement with available distorted-wave exchange calculations. The scaling behaviour of ionisation cross sections along the hydrogen isoelectronic sequence is discussed.

Multiple ionisation of multiply charged xenon ions by electron impact

Single-ionisation cross sections sigma q,q+1 for electrons incident on Xeq+ ions (q=1, 2, 3, 4) have been measured for electron energies up to 700 eV by employing a crossed beams technique. The results for Xe3+ ions agree with previous experimental data of Gregory et al. (1983). The obtained cross sections increasingly exceed distorted wave with exchange (DWX) calculations of Younger (1980) for direct ionisation by factors of up to about five when the charge state of the parent ions is increased from q=1 to q=4. The cross sections sigma 3,4 and sigma 4,5 show an unexpected enhancement at electron energies around 100 eV. These features are attributed to indirect ionisation processes, which probably also cause another unusual result: for the first time the authors found a case where ionisation of an atomic ion becomes easier when its charge state is increased, namely, for electron energies above 250 eV sigma 4,5 is greater than sigma 3,4.

An improved crossed-beams technique for the measurement of absolute cross sections for electron impact ionisations of ions and its application to Ar+ ions

Based on the animated-beam method of Brouillard and co-workers (1983) an improved experimental technique has been developed to measure cross sections for electron impact ionisation of ions. In a crossed-beams arrangement the overlap of the electron and the ion beam is periodically varied by a mechanical displacement of the complete electron gun system. Simultaneously with the ionisation signal, the parent ion beam current and the velocity of the gun movement are registered. As a test of the new method cross sections for single ionisation of Ar+ ions are measured from threshold up to 850 eV electron energy. The results are in excellent agreement with the crossed-beams data of Woodruff et al.

A dense electron target for the study of electron-ion collisions

Abstract A new high current (0.5 A), high density (0.3 A cm 2 ) electron gun has been designed for crossed beams electron-ion-experiments to yield high signal to background counting rates in the energy range 10–1000 eV. Comprehensive computer runs were performed to assure a uniform electrostatic potential in spite of the high electronic space charge. Trapped ions can be used to compensate this space charge, because their contribution to the counting rate is well discriminated below the threshold of the process to be studied. The overlap integral is determined by sweeping the whole electron gun perpendicular to both beams across the ion beam, which is possible, because the interaction region potential extends in perpendicular direction of both beams and sweeping therefore does not affect ion beam transmission. The accuracy and the reproducibility of cross section measurements with this new gun is demonstrated by a comparison with data from Harrisons group, exhibiting “perfect” agreement.

A new technique for the measurement of ionization cross sections with crossed electron and ion beams

Abstract A new experimental technique has been developed and used to measure cross sections for electron impact ionization of ions. Each single cross section is determined independently absolute by moving the electron gun mechanically across the ion beam with simultaneous registration of the ionization signal and the actual beam current. Different from the method of Brouillard et al. it is not necessary then to provide constant speed of the motion. Typical relative uncertainties of the measured cross sections are of the order of ±2% and absolute total errors are usually within ±6.5%. Very good agreement of ionization cross sections σ1,2 measured with our new technique for Ar+ ions is found with data from Harrisons group. Cross sections for single and multiple ionization of singly and multiply charged ions of Ar, Kr, Sb and Bi have been measured from threshold to 1000 eV.

Electron impact double ionisation of Ar+ and Ar4+ ions

Employing an improved crossed-beam technique described in the preceding paper, cross sections sigma q,q+2 for electron impact double ionisation of Arq+ ions (q=1 and q=4) were measured for electron energies from threshold to about 700 eV. The improved experimental accuracy allowed contributions of L-shell ionisation-autoionisation to sigma 1,3 to be identified for Ar+. With increasing q the relative importance of the L-shell contribution to sigma q,q+2 strongly increases. As a result the cross section sigma 4,6 for Ar4+ is by far dominated by the L-shell contribution at energies above 350 eV. Calculations on the basis of semiempirical and semiclassical theories are made to compare the relative strengths of direct double ionisation and inner-shell ionisation with subsequent electron emission.

Radiative recombination of highly charged ions: Enhanced rates at low energies

In a single‐pass merged‐beams experiment employing a dense cold electron target recombination of highly charged ions is studied. Unexpected high recombination rates are observed at low energies Ecm in the electron‐ion center‐of‐mass frame. In particlar, theoretical estimates for radiative recombination are dramatically exceeded by the experimental recombination rates at Ecm=0 eV for U28+ and for Au25+ ions. Considerable rate enhancement is also observed for Ar15+. This points to a general phenomenon which has to be interpreted as a consequence of high electron densities, low electron beam temperatures, high ion charge states and presence of strong magnetic fields.

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.

Ionisation of CO2+ ions by electron impact

Absolute ionisation cross sections for electrons incident on CO2+ ions have been measured at electron energies from threshold up to 830eV. The measurements have been performed with crossed electron and ion beams. By extrapolation of the cross section a threshold energy of (23.8+or-0.5)eV is determined. The cross section maximum is nearly 4*10-17 cm2 at an electron energy of about 150eV.