DIFFERENTIAL IONIZATION IN E(+)-AR SCATTERING

Differential cross sections of positron scattering from argon atoms le ading to singly and doubly charged ions were measured in the angular r egion from 0 degrees to 120 degrees. These reactions include single an d double ionization, transfer, as Ps formation is referred to here, an d transfer ionization. The latter is the process of simultaneously eje cting an electron and forming a positronium atom with another electron . The measurement of the time correlation between the detected ion and the scattered particle distinguishes among the reactions, resulting i n Ar+ and Ar++, respectively. Measurements were carried out at 30 eV, and, mostly, at 75 eV with an estimated resolution of 2 eV FWHM. The a ngular acceptance ranged from +/-4 degrees to +/-2 degrees. Cross sect ions for single ionization are strongly forward peaked and Ps formatio n has the most narrow FWHM of +/-12 degrees (75 eV incident positron e nergy) of all. A maximum in the Ps-formation cross section away from 0 degrees, predicted for low energies, was not observed. A very intrigu ing structure was observed in the transfer-ionization cross section at 45 degrees.