Electron impact excitation of the 3D states of helium: comparison between experiment and theory at 30 eV

Abstract

Experimental and theoretical values of the Stokes parameters are reported for electron impact excitation of the state of helium at 30 eV. The theoretical results are from a 111-state convergent close-coupling (CCC) method. Excellent agreement is obtained between theory and experiment for the Stokes parameters and for the derived parameters describing the shape and dynamics of the excited state. By considering the excitation as the incoherent sum of two coherent processes, an ambiguity in constructing the excited state from the experimental dipole radiation pattern is removed. This provides an attractive way of providing a complete description of the excitation compared with a triple coincidence experiment involving the scattered electron and photons from the 3D - 2P - 1S transitions. This analysis is also carried out for the state at 30 eV and comparison is made between 111-state CCC calculations and the experimental data of Donnelly et al.