Near-Threshold Ionization of He andH2by Positron Impact

The single ionization cross sections for He and ${\mathrm{H}}_2$ by positron impact have been measured in the first few eV above threshold and found to exhibit a different energy dependence from the corresponding electron results. If the data, between 1 and 3 eV above threshold, are fitted by a power law, exponents of $1.99\pm 0.19$ and $1.70\pm 0.11$ are obtained for He and ${\mathrm{H}}_2$, respectively. This agrees qualitatively with extensions of the Wannier theory in that the exponent is larger than for electron impact. The quantitative disagreement with the predicted value of 2.65 might indicate that the range of validity of this theory is smaller than expected.