R W McCullough

State-selective electron capture by C2+, C3+, N2+ and Ar2+ ions in rare gases

Energy loss/gain spectroscopy is used to study state-selective one-electron capture by 1.6-6 keV C2+ in He, Ne and Ar, by 0.8-8 keV N2+ in He and Ne, by 0.14-5 keV Ar2+ in He and Ne, and by 3-18 keV C3+ in He. The relative contributions of the various collision product channels are determined. Moderately exothermic processes involving curve crossings at moderate internuclear separations are shown to be dominant and a strong influence of metastable ions in the primary beam is observed for some reactions. Serious discrepancies with some previous measurements are noted. The main collision channels observed are in accord with the Wigner total electron spin conservation rule.

Dissociation of hydrogen in high-frequency discharges

The collision processes which influence the yield of hydrogen atoms from a microwave discharge source have been considered. For hydrogen gas pressures in the range 0.02-1.5 Torr it is shown that H atom production is determined mainly by direct electron impact dissociation of H2 and by an excitation transfer process involving collisions of H (n=2) excited atoms with H2, leading to the formation of the repulsive b3 Sigma u state. Atom loss by three-body recombination is found to be small compared with wall recombination. A study of the balance between atom production and loss allows the dependence of the dissociation fraction on source pressure and input microwave power to be predicted. These predictions are found to be in satisfactory accord with the measured characteristics of a 2.45 GHz microwave discharge source previously developed in the authors laboratory.

Electron capture by He2+ ions in collisions with H and H2 at impact energies below 10 keV

Total cross sections for the process He2++H(1s) to He+( Sigma n,l)+H+ involving capture into all final bound states of He+ have been determined for 3He2+ ions in the range 1.5-9.0 keV using a furnace-target technique previously employed for similar measurements at higher impact energies. Measured cross sections are considerably smaller than those measured previously by Fit et al. (1962) and are in better accord with recent theoretical estimates. A likely explanation for the discrepancy is given in terms of the corresponding cross sections for electron capture in He2+-H2 collisions which have also been determined.

State-selective electron capture by slow C2+ and C3+ ions in atomic hydrogen

Translational energy spectroscopy in a furnace target configuration has been used to identify the main collision channels in the process of one-electron capture by C2+ and C3+ ions in atomic hydrogen within the energy range 0.6-18 keV. For C2+ impact, collision channels involving both ground and metastable primary ions are unambiguously identified. In the case of C3+ impact, where only ground-state primary ions are present, the observed energy change spectra have been used to derive individual cross sections for capture into specific states of C2+. Most of the results are shown to be in reasonable agreement with the theoretical predictions of Beinstock et al. (1982) based on a full quantal analysis of the C3+-H(1s) system. The measurements provide the first detailed assessment of theory for such a process.

One-electron capture by slow doubly charged ions in h and H2

Cross sections for one-electron capture by Ba2+, Ti2+, Mg2+, Cd2+, Zn2+, Kr2+ and B2+ in both H and H2 have been determined in the energy range 0.8-40 keV. A tungsten-tube furnace was used to provide a target of highly dissociated hydrogen. Cross sections in H for moderately exothermic processes, which involve pseudo-crossings of potential energy curves at moderate internuclear separations were found to be very large at low velocities. Cross sections for other processes decrease rapidly with decreasing velocity. No clear relationship between cross sections in H and those in H2 is apparent. Measured cross sections are believed to pertain to ground-state primary ions.

Electron capture by 0.1-13 keV C+, N+ and O+ ions in H and H2

Cross sections for one-electron capture by C+, N+ and O+ ions in H and H2 have been investigated within the energy range 0.1-13 keV from measurements using a tungsten tube furnace to provide a target of highly dissociated hydrogen. Measured cross sections are believed to pertain to ground-state ions except in the case of O+-H collisions where accurate measurements were precluded by the presence of metastable species in the O+ ion beam.

Ionization of sodium and potassium vapour by 20-100 keV H+ and He+ ions

A modulated crossed-beam technique has been used to study the ionization of potassium and sodium vapour by 20-100 keV H+ and He+ ions. Absolute cross sections sigma e for electron production have been determined from the gross yield of electrons. Mass spectrometric measurements of the relative abundance of secondary ion species, together with measurements (in a subsidiary experiment) of the total one-electron capture cross section sigma 10 have been used to determine cross sections for Na+, K+, and K2+ formation. Proton impact ionization cross sections are shown to be in generally poor accord with theoretical estimates based on both the classical impulse and Born approximations.

State-selective capture by slow Ar4+, Ar5+ and Ar6+ recoil ions in H, H2 and He

Translational energy spectroscopy has been used to study the distribution of excited product ion channels in one-electron capture by Ar4+, Ar5+ and Ar6+ in H, H2 and He at q*1 keV and q*2 keV. Primary ions with the required small energy spread have been obtained from a recoil ion source, but otherwise the apparatus was the same as that developed in the authors previous work. In the absence of detailed theoretical predictions, they have calculated reaction windows based on the dependence of the single-crossing Landau-Zener cross section on the crossing distance. They have also carried out multichannel Landau-Zener calculations to obtain cross sections for capture into specific states. The calculations are shown to be in reasonable accord with the observed energy change spectra.

State-selective one-electric capture by 8 keV He2+ ions in collisions with oxygen atoms

The authors have successfully demonstrated the feasibility of using translational energy spectroscopy with partially dissociated oxygen within an iridium tube furnace to provide, for the first time, data on state-selective one-electron capture in collisions of 8 keV He2+ ions with ground state atomic oxygen. Near-resonant electron capture into the He+ (n=2) states is the dominant collisional channel. Cross sections for the main excited product channels in one-electron capture in collisions of He2+ with both O and O2 have been estimated from an analysis of the observed energy change spectra; the total cross section for one-electron capture by He2+ in O has also been determined.

State-selective electron capture by slow S3+ recoil ions in H, H2 and He

Translational energy spectroscopy has been used to study electron capture by slow S3+ ions from a recoil ion sources. These ions are important in astrophysical plasmas. Cross sections for state-selective electron capture by S3+ ions in H, H2 and He have been determined at energies within the range 2.4-9.0 keV. The main excited product channels have been identified and their relative importance has been assessed. In the case of H and He a multichannel Landau-Zener model has been used to predict cross sections which have been compared with experiment. The presence of S3+(4P) metastable as well as S3+(2P degrees ) ground-state ions in the primary beam has been clearly demonstrated. Electron capture by ground-state ions is dominated by 3p electron capture. For H and H2 the majority of channels also involve 3s to 3p excitation of the projectile core.