Differential positronium formation in positron-argon collisions

In an e⁺-Ar crossed-beam experiment with 75 eV incident positron energy we studied the angular dependence of the electron-transfer process Ar(e⁺, Ps) Ar⁺ from 0 degrees to 120 degrees with an angular resolution of +or-4 degrees . For comparison we also studied the angular distribution of secondary electrons from the impact-ionization process Ar(e⁺, e^-e⁺) Ar⁺. As anticipated, the angular distribution of the positronium is much more forward-peaked than that of the secondary electrons. For comparison of our relative Ps-formation cross section data with the theoretical angular dependence, we folded the theory of McAlinden and Waiters (1994) with the angular-resolution function of our experiment; the agreement is satisfactory. We also performed Ps measurements at 30 eV incident positron energy for which McAlinden and Waiters predicted a maximum of the differential cross section near 15 degrees . Folded with the angular-resolution function of the experiment, the theoretical curve retains a distinct minimum at 0 degrees which is not apparent in our data.