Fluorescence polarization of helium negative-ion resonances excited by polarized electron impact

We have investigated helium (1s3d) D-3 -> (1s2p) P-3 ( 588 nm) fluorescence produced by electron impact excitation in the vicinity of the (2s22p) P-2 and (2s2p2) D-2 negative-ion resonances at 57.2 and 58.3 eV, respectively. In contrast to previous work, we use spin-polarized incident electrons and report the relative Stokes parameters P-1, P-2 and P-3 in the 55-60 eV region. Our failure to see discernable resonance effects in P-2 indicates that even though the lifetime of these resonances is significant (similar to 10 fs), magnetic forces acting on the temporarily captured electron are small. Resonant structures in the values of P-1 and P-3 are observed because the polarization contributions of resonant states are generally different than those from direct excitation of the 3 D-3 state.