DIFFERENTIAL POSITRONIUM FORMATION IN POSITRON-ARGON COLLISIONS

In an e(+)-Ar crossed-beam experiment with 75 eV incident positron ene rgy we studied the angular dependence of the electron-transfer process Ar (e(+), Ps) Ar+ from 0 degrees to 120 degrees with an angular resol ution of +/-4 degrees. For comparison we also studied the angular dist ribution of secondary electrons from the impact-ionization process Ar( e(+), e(-)e(+)) Ar+. As anticipated, the angular distribution of the p ositronium is much more forward-peaked than that of the secondary elec trons. For comparison of our relative Ps-formation cross section data with the theoretical angular dependence, we folded the theory of McAli nden and Waiters with the angular-resolution function of our experimen t; the agreement is satisfactory. We also performed Ps measurements at 30 eV incident positron energy for which McAlinden and Waiters predic ted a maximum of the differential cross section near 15 degrees. Folde d with the angular-resolution function of the experiment, the theoreti cal curve retains a distinct minimum at 0 degrees which is not apparen t in our data.