Direct measurements of the three Stokes parameters (polarization components) P 1 , P 2 and P 3 of the VUV Hg transition 6s6p 1 P 1 → 6s 2 1 S 0 (185 nm) have been carried out at electron impact energies of 15 eV, 50 eV and 100 eV. Within the experimental uncertainty, no influence of the electron spin was discovered for scattering angles θ 30 • . At 15 eV excitation energy and scattering angles θ 80 • , increasing spin effects become apparent. The experimental data are compared to theoretical predictions from a first-order full-relativistic distortedwave model, a five-state Breit-Pauli R-matrix (close-coupling) approach, and a convergent close-coupling model, in which relativistic effects are accounted for by adding non-relativistic amplitudes using known intermediate-coupling coefficients. At scattering angles θ 15 • , all of the theories reproduce the experimental data well, whereas the CCC model exhibits the best overall agreement with experiment at large scattering angles.
Results for the three Stokes parameters (polarization components) P 1 , P 2 and P 3 of the VUV Hg transition 6s6p 1 P 1 → 6s 2 1 S 0 (185 nm) obtained from electron-photon (e, eγ ) coincidence measurements at electron impact energies of 15 eV, 50 eV and 100 eV (Aussendorf et al 2006 J. Phys. B: At. Mol. Opt. Phys. 39 2403) were combined to obtain the total degree of polarization3 . In addition, the Stokes parameter P 4 was measured at an electron impact energy of 15 eV. The measured data are compared with predictions from a fullrelativistic distorted-wave calculation, a five-state semirelativistic Breit-Pauli R-matrix (close-coupling) model, and a convergent close-coupling ansatz, where relativistic effects are accounted for by recoupling of nonrelativistic results. In agreement with the theoretical predictions, no influence of the electron spin was observed for scattering angles θ 30• . At 15 eV excitation energy and scattering angles θ 40• , however, an increasing importance of spin effects is predicted.
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