Polarization measurements on a magnetic quadrupole line in Ne-like barium

We have measured the absolute polarization of the 2${\mathit{p}}^{61}$${\mathit{S}}_{0}$-2${\mathit{p}}^{5}$${(}^{2}$${\mathit{P}}_{3/2}^{\mathit{o}}$)3s[3/2${]}_{2}^{\mathit{o}}$ magnetic quadrupole transition in Ne-like barium, excited in an electron-beam ion trap at a variety of energies. We find strong evidence for the existence of resonant excitation processes that are not explained by our collisional-radiative calculations even when the polarization arising from impact excitation is included. At energies well away from where the resonances occur, the agreement between experiment and theory is good. \textcopyright{} 1996 The American Physical Society.