The spin-dependent ionisation asymmetry in sodium

The authors have measured the spin-dependent ionisation asymmetry in electron impact ionisation of sodium over the energy range 5.6 to 37 eV in a crossed beam experiment with polarised electrons and polarised sodium atoms. The observed ionisation asymmetry A=( sigma up arrow down arrow - sigma up arrow up arrow )/( sigma up arrow down arrow + sigma up arrow up arrow ) reaches a peak value of approximately 0.30 and shows relatively little variation in energy up to three times the ionisation energy. Then the ionisation asymmetry starts to decrease with increasing electron energy. From the ionisation asymmetry the authors obtain information about the ratio of the interference to the total ionisation cross section and also about the ratio of the triplet to the singlet ionisation cross section. A comparison of the authors' measurements is made with theoretical results of a Born approximation and predictions following from threshold laws.