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 have been previously investigated by numerous researchers. Observation of the resonant channels in the He $3\,^3\!D\rightarrow2\,^3\!P$ 587.5 nm transition is possible because of cascade contributions. Previous measurements of the intensity and linear polarization fraction of this line due to excitation by unpolarized electrons are available [1-3]. We report the integrated Stokes parameters P$_1$, P$_2$, and P$_3$ for excitation by spin-polarized electrons. 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. [1] A. Defrance, J. Phys. B \textbf{13}, 1229 (1980). [2] H. Batelaan \textit{et al}., J. Phys. B \textbf{24}, 5151 (1991). [3] D. Cvejanovi\'{c} \textit{et al}., J. Phys. B \textbf{33}, 2265 (2000).