Electron impact excitation of the 31D state of helium is studied using the electron-photon polarization correlation technique. Four Stokes parameters are determined at an incident electron energy of 29.6 eV and for scattering angles in the range 40-120 degrees . The data are interpreted in terms of the shape and dynamics of the excited state produced over the range of kinematically defined collisions. The lack of a theoretical model to adequately describe low-energy D state excitation is exposed.
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 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 3 1 D state at 30 eV and comparison is made between 111-state CCC calculations and the experimental data of Donnelly et al .
Ababart Experimental and theoretical stnudies of excitation of &e 3)D state of helium by 40 eV electrons are reported. Experimentally, electron-photon polarization correlations are measured in tk scattering plane defined by the incident and scattered electron momenta These are combined with previously reported linear and circular polarization measmmens perpendicular to the scattering plane. After allowing for the effects of fine srmcture depolarization, p;nameten describing the shape and dynamics of the excited state pmduced in the collision are determined. Thenretidy, the measured polarizations and excited state parameters are predicted using the convergent close coupling (a) theory. Excellent agreement is oMained between the experimental and theorelid results.
Electron impact excitation of the 3 1 D state of helium has been studied using the scattered electron-polarized photon correlation method. Results are presented at incident electron energies of 26.5, 29.6, 40.0, 45.0 and 60.0 eV for scattering angles up to 40 • . The data presented here complement the previous large angle data from this laboratory and taken together provide a complete mapping of the dimensionless parameters characterizing excitation of the 3 1 D state of helium over a wide range of both angle and energy. Strong variations in the parameters as a function of energy and angle are discussed. Good agreement is observed between the experimental data and recent theoretical predictions.
Abstract. Electron-photon polarisation correlations measured in the scattering plane defined by the incident and scattered electron momenta are reported for 40 eV electron impact excitation of the 3 ~D state of helium and for electron scattering angles in the range 40°_<0<120 ° . When combined with the results of our earlier polarisation analysis on radiation emitted perpendicular to the scattering plane, detailed information on the shape and dynamics of the exicted state is obtained. In the small angular range of overlap (40-60 ° ) there is excellent agreement with a pervious experimental study. The behaviour of the linear polarisation of the radiation emitted in the scattering plane is well reproduced by a multichannel eikonal theory for 0 =< 80 °. Otherwise theories totally fail to described the excitation process.
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