  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 $z$.
