ELECTRON-IMPACT EXCITATION OF THE 3D STATES OF HELIUM - COMPARISON BETWEEN EXPERIMENT AND THEORY AT 30 EV

Experimental and theoretical values of the Stokes parameters P-1, P-2, P-3, P-4 are reported for electron impact excitation of the 3(3)D sta te of helium at 30 eV. The theoretical results are from a 111-state co nvergent close-coupling (CCC) method. Excellent agreement is obtained between theory and experiment for the Stokes parameters and for the de rived parameters describing the shape and dynamics of the excited stat e. By considering the excitation as the incoherent sum of two coherent processes, an ambiguity in constructing the excited state from the ex perimental dipole radiation pattern is removed. This provides an attra ctive way of providing a complete description of the excitation compar ed with a triple coincidence experiment involving the scattered electr on and photons from the 3D-2P-1S transitions. This analysis is also ca rried out for the 3(1)D state at 30 eV and comparison is made between 111-state CCC calculations and the experimental data of Donnelly et al .