Near-threshold study of the polarization of He resonance radiation using an energy-selected electron beam

The linear polarization of resonance vacuum-ultraviolet vuv radiation emitted from collisionally excited helium atoms has been measured in the threshold region below 25 eV. The apparatus consisted of a hemispherically analyzed electron beam interacting with a gas jet; the normally emitted radiation was analyzed using reflection optics. The threshold polarization was measured to be unity, in agreement with arguments based on the conservation of angular momentum. The measured vuv excitation functions and corresponding polarization curve were compared to 11-state R-matrix calculations of the excitation of the 2¹P state and excellent agreement was observed up to the n=3 threshold. Negative-ion resonances and cascade had pronounced effects on the observed sublevel excitation functions and on the polarization. {} 1996 The American Physical Society.