Electron-photon coincidence experiment for measuring the polarisation of the 63P1-61S0resonance line of Hg after electron excitation with ΔML=0

The polarisation of 2154 nm photons emitted from Hg atoms which are excited by electrons scattered in the forward direction has been measured for collision energies between 5.5 and 15 eV. From the results the relative contribution of exchange excitation could be determined. Exchange is found to be the dominant excitation mechanism from threshold up to 8 eV collision energy. The results are also expressed by the alignment parameter lambda and good agreement is found with a previous measurement using incident polarised electrons. The results are explained in terms of intermediate coupling.