Abstract
Differential cross section measurements of the ionization of metastable He* atoms in collisions with ground state He have been performed in the energy range E=50-199 eV. The most prominent peak in the energy loss distributions of the product ions occurs near the ionization threshold ( Delta E approximately=5eV), implying the ejection of very low energy electrons. The angular distribution of this prominent feature is very strongly peaked at 0 degrees , displaying no apparent angular threshold, and its intensity increases substantially with increasing collision energy. This behaviour is consistent with a mechanism in which a small fraction of the incident flux follows the diabetic 3 Sigma g+ potential of He(23S)+He. Ionization occurs as this state crosses the continuum state He2+(2 Sigma u)+e, near the bottom of its potential well.
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