Penning and associative ionization of triplet metastable helium atoms

Potential energy curves for the ${}^1{\Sigma }_g^{+}$ , ${}^3{\Sigma }_u^{+}$ , and ${}^5{\Sigma }_g^{+}$ states which arise from two triplet metastable helium atoms (1s 2s ³S) have been calculated by a large configuration interaction expansion. From the potential curves for the ${}^1{\Sigma }_g^{+}$ and ${}^3{\Sigma }_u^{+}$ autoionizing states, cross section for Penning and associative ionization have been calculated; the total ionization cross section for thermal energy (0.026 eV) collisions, for example, is computed to be 94 Ų. In this low energy region the orbiting model is seen to be adequate for determining the total ionization cross section, but not the more specific cross sections.