Direct measurement of P+ for electron impact excitation of H(2p) at 54.4

We report results of direct measurements of the reduced Stokes parameters P-bar1, P-bar2 and P-bar3 for electron impact excitation of H(2p) at 54.4 eV over the scattering range 10-40 deg. These three parameters have been measured simultaneously for the first time using a VUV double-rotation polarization analyser consisting of a MgF2 retarder followed by a SiO2 reflection linear polarizer. As expected, our measurements for P-bar1 and P-bar2 are in good agreement with theoretical calculations and previous experimental data. Our data for P-bar3 differ significantly from previous experimental measurements and theoretical calculations. Consequently, we find that the coherence parameter P+ deviates significantly from unity at 30 deg. If correct, this signifies that spin-exchange scattering may be more important than has previously been thought