Fluorescence Polarization of Helium Negative Ion Resonances Excited by Polarized Electron Impact

The helium triply-excited negative ion $2s^22p\,^2\!P$ and $2s2p^2\,^2\!D$ resonance states can autoionize to populate the singly excited $3\,^3\!D$ level. Optical observation of interference from these resonant channels in the $3\,^3\!D\rightarrow2\,^3\!P$ 587.5 nm transition is possible and measurements of the intensity and linear polarization fraction of this line are available [1-4]. In contrast to previous work, we utilize spin-polarized electrons and report the integrated Stokes parameters P$_1$, P$_2$, and P$_3$ in the 55-60 eV region. Our null result for P$_2$ indicates that even though these resonances are long lived ($\sim$200 ps), magnetic forces acting on the temporarily-captured electron are negligible. Values of P$_3$ show no statistically-significant variation from their asymptotic non-resonant levels. \newline \newline [1] A. Defrance, J. Phys. B \textbf{13}, 1229 (1980). \newline [2] D. Cvejanovi\'{c} \textit{et al}., J. Phys. B \textbf{33}, 2265 (2000). \newline [3] H. Batelaan \textit{et al}., J. Phys. B \textbf{24}, 5151 (1991). \newline [4] P. J. M. van der Burgt \textit{et al}., J. Phys. B \textbf{19}, 2015 (1986).