Threshold behaviour of electron excitation and polarization functions in helium

The apparent discrepancy between many previous measurements of polarization and the theoretical predictions at threshold is discussed briefly. The problems of measurement are outlined and a data-logging system which enables long integration times to be combined with a rapid coverage of a suitable range of electron energy is described. The results of the experiment show that light from the 41D–21P, 31P–21S and 33P–23S transitions is polarized to an extent compatible with the theoretical threshold predictions, but that the polarization falls rapidly above threshold. The polarization is found to remain finite as the electron energy is reduced. Light from the 43S–23P transition is unpolarized at all energies.