Results of spin-resolved angle-differential Stokes parameters from electron-photon coincidence studies of electron-impact excitation of the $6s6p\hspace{0.5em}{}^3{P}_1$ 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 ($6s6p\hspace{0.5em}{}^3{P}_1\to 6s^2\hspace{0.5em}{}^1{S}_0$) up to at least $50$ eV incident energy [Jüttemann 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 Jüttemann 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.

• Received 24 September 2009

DOI:https://doi.org/10.1103/PhysRevA.81.012705