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Abstract

We report measurements of electron-impact excitation cross sections for
the strong K-shell n = 2 -> 1 transitions in S xv, using the LLNL EBIT-I
electron beam ion trap, two crystal spectrometers, and the EBIT
Calorimeter Spectrometer. The cross sections are determined by direct
normalization to the well-known cross sections of radiative electron
capture, measured simultaneously. Using contemporaneous polarization
measurements with the two crystal spectrometers, whose dispersion planes
are oriented parallel and perpendicular to the electron beam direction,
the polarization of the direct excitation line emission is determined,
and in turn the isotropic total cross sections are extracted. We further
experimentally investigate various line-formation mechanisms, finding
that radiative cascades and collisional inner-shell ionization dominate
the degree of linear polarization and total line-emission cross sections
of the forbidden line, z.