E Salzborn

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.

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.

Measurements of charge transfer in H+-He+ collisions

A crossed-beams experiment has been set up to study charge changing collisions between ions. Absolute cross sections for the charge transfer reaction H++He+ to H0+He2+ have been measured in the centre-of-mass energy range 8-100 keV using a coincidence technique for the detection of the reaction products. The data are compared with previous experimental and theoretical results.

Charge transfer of 0.2-5.0 keV protons and hydrogen atoms in sodium-, potassium- and rubidium-vapour targets

Total cross sections sigma +0 and sigma +- for single- and double-electron capture by protons, and total cross sections sigma 0- and sigma 0+ for single-electron capture and single-electron loss by neutral hydrogen atoms in single collisions with atoms of sodium, potassium and rubidium have been measured in the energy range 0.2-5.0 keV. All electron capture cross sections show a maximum at an energy which depends on the energy defects of the reaction. The electron loss cross sections increase monotonically with increasing collision energy. The results are discussed and compared with previous measurements and theoretical calculations. Some of the cross sections show considerable discrepancies with previous measurements.

Electron impact double ionisation of Arq+ ions (q=1, 2, . . ., 7): two-electron processes compared with inner-shell contributions

Cross sections for electron impact double ionisation of Ar2+, Ar3+, Ar5+, Ar6+ and Ar7+ ions have been measured for electron energies ranging from the threshold to 1000 eV. Earlier experiments on Ar+ and Ar4+ have thus been complemented at a comparable level of accuracy. The obtained data set permits analysis of trends in direct doubling ionisation and inner-shell ionisation, with subsequent Auger processes, to be made along the Ar isonuclear sequence.

High-energy electron capture by fully stripped ions from He atoms -a QED approach

One-electron-capture cross sections sigma CT and transfer ionisation cross sections sigma TI are calculated for fully stripped ions colliding with He atoms at energies above 100 keV amu-1 in a relativistic covariant manner. Numerical results for Li3- on He are presented and compared with existing experimental data and numerical calculations. The energy distribution of the emitted electron due to transfer ionisation (TI), when electron capture occurs into the Li2+(1s) state, is discussed.

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.

Charge transfer in He+-He+ collisions

Cross sections for charge transfer in He+-He+ collisions have been measured in the centre-of-mass energy range 7-112 keV, using crossed beams and coincident detection of the reaction products. The results obtained are in excellent agreement with previous measurements by Peart et al in the CM energy range 19-57 keV. A two-state coupling calculation with asymptotically correct pseudomolecular states shows that the capture into the ground state of He is approximately only 50% of the total capture cross section at energies below the cross section maximum.

Mutual neutralisation in H+-H- collisions

Cross sections for mutual neutralisation in H+-H- collisions have been measured in the CM energy range 1-40 keV, using crossed beams and coincident detection of the reaction products. The results extend smoothly the latest measurements by Peart et al. (1985) and Szucs et al. (1984) towards higher energies and they are partly in excellent accord with recent theoretical predictions.

Transfer ionization in H++H

Absolute cross sections for transfer ionization in H/sup +/+H/sup -/ collisions have been measured in the center-of-mass energy range 50 eV--40 keV, with the use of merged- and crossed-beam techniques, together with coincident detection of the products. A mechanism is proposed for the process in the low-energy region. The cross section, calculated accordingly, is found to be in satisfactory agreement with the experimental data.