Angle-differential Stokes parameters for spin-polarized electron-impact excitation of the Hg 6s6p 3P1 state at 25-eV scattering energy

Results of spin-resolved angle-differential Stokes parameters from electron-photon coincidence studies of electron-impact excitation of the 6s6p P-3(1) state of mercury, resulting in 254 nm radiation, are presented. Due to the intermediate-coupling nature of the excited state, the wave function of this state has a small singlet part. With increasing scattering energy, the influence of exchange scattering decreases, so that direct scattering via the singlet part can become relevant. Recent angle-integrated Stokes-parameter measurements indicated that exchange is still important for the 254 nm line (6s6p P-3(1) -> 6s(2) S-1(0)) up to at least 50 eV incident energy [Juttemann et al., Phys. Rev. A 79, 042712 (2009)]. At energies above 15 eV, however, cascade effects complicate a detailed comparison of these angle-integrated results with theoretical calculations. The angle-differential Stokes parameters presented here are unaffected by cascade effects due to the coincidence technique and thus allow for an analysis of the discrepancies observed by Juttemann et al. and also by Srivastava et al. [Phys. Rev. A 80, 022718 (2009)]. Comparison of the experimental data with theoretical predictions reveals that the description of the initial target state and the excited state in intermediate coupling have a significant influence on the overall agreement.