Electric-field singlet-triplet anticrossings of He I

With regard to experimental investigations of electric-field anticrossings, the Stark splitting of 1snl levels of He I is evaluated for n = 3,4 and 5. The electric-field positions and the energy separations of closest approach of the anticrossings are calculated. Using the Liouville matrix formalism, a theory of anticrossing signals is formulated allowing a systematic analysis of signals arising from anticrossings of twofold degenerate Stark sublevels. The signal amplitudes are calculated as functions of the excitation parameters. The investigations show that these anticrossings signals are well suited for determining relative cross sections for the excitation of singlet and triplet substates by atom-atom and ion-atom collisions. (Author)