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. © 1996 The American Physical Society.