A pulsed -sed beam technique incorporating time-of-flight specpwcopy (previously developed in this laboratory) has been used to study the single and double ionization of ground-state oxygen atoms produced by an indium tube f u m e source. Relative m s s sections (11 and (12 for single and double ionization have been determined in the respective energy ranges 14.1-2000 eV and 9&2000 eV which are wider than any previous measurements. These values have been normalized by reference lo the absolute values of a, previously measured by Brook er al at energies (50-997 ev) where the possible influence of an unknown admixture of long-lived excited atoms in their experiment should be small. However at energies below about 25 eV our values of at become much smaller than the values of Brook er al. Above 40 eV, the present ratios az/al are in satisfactory agreement wilh other measurements which extend to 400 eV. Our values of at have been compared with selected theoretical predictions.Some evidence of structure in the energy dependence of UI in the region of the cross section maximum is consistent wilh predicted contributions from inner-shell 2s ionization and from autoionization in addition to outer 2p elemon removal. W R Thompson er a[ Experimental approach General descriptionThe basic apparatus and measuring procedure was similar to that used in our previous studies

A crossed-beam technique incorporating time-of-flight analysis and coincidence counting of the collision products has been used to study one-electron capture by 6 - 100 keV protons in collisions with oxygen atoms. In these measurements, ground-state oxygen atoms were provided by an iridium tube furnace dissociation source. The measurements extend the energy range of previous experiments and, for the first time, provide separate cross sections for the simple charge transfer process (which is dominated by accidentally resonant one-electron capture) and for the transfer ionization process leading to formation. The present cross sections were obtained using a technique and normalization procedure different from all previous measurements. In particular, our measured values of are in good accord with previous 0.04 - 10 keV data by Stebbings et al based on a modulated crossed-beam technique but at variance with the considerably smaller values in the range 2.5 - 25 keV obtained by Williams et al using a furnace target method. While our values of decrease with increasing energy over the range considered, transfer ionization cross sections pass through a peak value at 34 keV where this process accounts for about 9% of the total one-electron capture cross section rising to 17.5% at 100 keV.

The technique of double translational energy spectroscopy has been used for the first time with an atomic hydrogen target. We have carried out measurements of state-selective oneelectron capture in 6 keV C 2+ -H(1s) collisions which avoid the ambiguities in interpretation of previous experiments and allow separate identification of the collision channels associated with either C 2+ (2s 2 ) 1 S ground-state or C 2+ (2s2p) 3 P o metastable primary ions. In a theoretical counterpart, partial cross sections for both reactions have been calculated using a semiclassical molecular approach. There is good agreement between experiment and calculations.

A crossed-beam technique incorporating time-of-flight analysis and coincidence counting of the collision products, recently used in this laboratory to study one-electron capture in collisions of H + ions with ground-state oxygen atoms, has been adapted to allow measurements of the corresponding cross sections for ionization for the first time. Cross sections for the single ionization of O atoms have been determined within the range 34-100 keV. These cross sections pass through a peak value at an energy of about 60 keV. At higher energies, the process of ionization rather than electron capture becomes the main source of O + production. O n+ formation for n > 2 was too small to detect in the energy range considered. It is also shown that although production of O 2+ through transfer ionization is quite significant, the yield of O 2+ from pure ionization is unimportant.

Double translational energy spectroscopy (DTES) has been used to study stateselective one-electron capture by pure ground-state N 2+ 2s 2 2p( 2 P) ions in collisions with He, Ne and Ar at energies within the range 1.75-8.0 keV. The main collision channels have been identified and their relative importance has been assessed quantitatively. These measurements avoid the ambiguities inherent in the interpretation of previous TES measurements carried out using primary N 2+ ion beams containing unknown fractions of metastable N 2+ 2s2p 2 ( 4 P) ions. In the absence of any detailed theoretical studies, we have calculated Landau-Zener reaction windows for these processes and shown that they accommodate the observed collision channels with limited success.