Proton-impact excitation of HeI triplet levels

Abstract

We investigated the excitation of the λ(1s3d3D−1s2p3P)=588 nm line of atomic helium by proton and deuteron impact for projectile energies 10 keV≦Ep≦25 keV. In apparent contradiction to Wigner's spin conservation rule, the emission cross section does not vanish. By measuring the intensity of the impact radiation as a function of homogeneous magnetic and electric fields applied to the collision volume, it has been shown thatp- andd-impact excitation of the 1s3d3D level of HeI proceeds via 1snl states withl≧3, which populate the 33D states by cascade decays. The well-known strong singlet-triplet mixing of these 1snl states enables a population of triplet states in accord with Wigner's rule. Accordingly, we determine the excitation cross section of the 1s4f multiplet from the measured emission cross section of the 588 nm line. The field-dependent signals give evidence that predominantly substates with |mL|≦1 are excited.