Double ionization of noble gases by positron impact

The cross sections for double ionization and the ratios of double to single ionization, including Ps formation for Ne, Ar, Kr, and Xe are presented from threshold to 100 eV. Within the energy region 6.8 eV below the second ionization potential, i.e., the second Ore gap, we find Ne to have a double-ionization cross section indistinguishable from zero, which is consistent with prior measurements. However, the total double-ionization cross sections for Ar, Kr, and Xe just below the thresholds for direct double ionization are around 7{endash}24h{percent} of the maximum cross-section value for each atom, all of which are of the order of 10{sup {minus}21}hm{sup 2} and occur around 70{endash}90 eV. In contrast to what has previously been found for He and Ne, this is direct evidence of a significant amount of transfer ionization for the three heavier noble gases in the second Ore gap, which is consistent with previous measurements for Ar and Xe. {copyright} {ital 1999} {ital The American Physical Society}