Circular polarization measurements of Lyman- alpha radiation from the decay of the 2²P_j states of hydrogen excited by 54.4 eV electrons

Coincidence measurements are reported for the circular polarization of Lyman- alpha radiation resulting from the decay of 2²P_j states excited by electron impact at an incident energy of 54.4 eV in a range of scattering angles from 5'<or= theta _e<or=40'. The data were obtained using a recently developed efficient polarization analyser for this spectral line and the statistical significance of the data is much better than for previously reported measurements. The data are analysed to yield values for the coherence parameter P⁺ and show that spin exchange interactions play a significant, if unexpected, role in the dynamic range investigated.