Measurement of spin asymmetries in the electron impact ionisation of alkali atoms

Using a GaAsP polarised electron source and state-selected atomic beams of lithium, sodium and potassium we investigated the spin dependence of the total ionisation cross section in the energy range from threshold to ten times the threshold value.For Li and Na the measured spin asymmetries are very similar, having as a function of energy E a maximum of about 0.5 at twice the threshold energy I and a fall off towards zero at higher energies proportional to l n ( l / E ) .The potassium data are considerably lower in the region from threshold up to five times threshold energy, showing an asymmetry of about 0.25 there.With lithium the near-threshold range was probed with a narrow electron energy distribution ( A € = 0.17 eV).It was found that the spin asymmetry stays well below unity at threshold ( A , 20.45) and has little dependence on energy in this region.