Polarized electron-impact ionization of metastable helium

The singlet (${\sigma }_s$) and triplet (${\sigma }_t$) parts of the total ionization cross section were determined in the energy range from threshold to five times the threshold value with a crossed beam arrangement of polarized electrons and spin-polarized He($2^3$S) atoms. The measured spin asymmetry, $A_I$=(${\sigma }_s$-${\sigma }_t$)/(${\sigma }_s$+3${\sigma }_t$), is compared with results for the lithium atom. Both asymmetries are quite similar. Comparison with a distorted-wave Born approximation shows strong disagreement in the vicinity of the threshold.