New data on positron collisions with atomic hydrogen leading to ionization were taken by using the high-intensity positron beam at Brookhaven National Laboratory (BNL). The measured cross section values for positron-impact ionization, which supersede earlier ones, are in good agreement with other experimental and most theoretical results, while the new data for positronium formation, formerly normalized to impact-ionization data, now disagree with recent theoretical predictions.

Our previous differential cross section measurements on argon were extended to impact energies of 120 eV and krypton was used as an alternative target gas. Some distinctive features were observed in the cross sections of the transfer and transfer-ionization reaction channel which await a theoretical explanation. The narrow feature at 45 • , observed for the transfer-ionization reaction Ar(e + , Ps e − )Ar 2+ at 75 eV, does not persist at 90 eV and could not be observed with krypton. Therefore, a connection to Thomas scattering (45 • is the Thomas angle for Ps formation) is not supported.

In an &-AI crossed-beam experiment with 75 eV incident positron energy we sNdied the angular dependence of the electron-transfer ploeess Ar (e+, Ps) Art fmm 0 ' to 120°with an angular resolution of i4". For comparison we also studied the angular distribution of semndary electrons from the impact-ionimion process Me+. e-e+) Ar+. As anticipated. lk angular distribution of the positronium is much more forward-peaked than t b l of the seconduy eleeunns. For comparison of our relative Ps-formation cross seclion dam with the lheoretical angular dependence, we folded the theory of McAlinden and Walters with the angular-resolution function of our experiment; the agreement is satisfactory. We also performed Ps measurements at 30 eV incident pasitron enezgy for which McAlinden and Walten predicted a marimum of the differential a s s section near 15". Folded with the angular-resolution function of the experiment, the theoretical curve retains a distinct minimum at 0" which is not apparent in our data.The fact that positronium (Fs) formation in positron-atodmolecule scattering is strongly peaked in the forward direction was predicted theoretically (Mandal etal 1979) and observed by Brown (1986) and Laricchia et al (1986, 1987a) as well as Tang and Surko (1993), in experiments with extended gas targets. In order to measure differential Ps formation with higher angular resolution and up to larger angles, crossed-beam experiments are necessary. This is the first crossed-beam experiment on Ps formation.

In an e+-Ar crossed-beam experiment with positrons of 75 eV energy, we studied differential positronium (Ps) formation in the vicinity of the Thomas angle 0&h(Ps) = 45, searching for a Thomas resonance. Our experiment distinguishes two reactions: (1) the transfer Ar(e+, Ps)Ar+ and (2) the tvansfer ionization Ar(e, e Ps)Ar++. The angular dependence of transfer shows no statistically significant structure.Unexpectedly, however, we discovered a distinct minimum in the angular dependence of transfer ionization right at 45 .

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