Absolute measurements with an accuracy of 22% and theoretical results in a distorted-wave Born approximation (DWBA) are reported for the triple-differential cross section for 26.6-eV electron-impact ionization of helium. An apparatus is used that allows all scattering angles to be independently varied for both coplanar and noncoplanar geometries. The measurements are compared with a DWBA calculation that includes exchange distortion in the calculation of the distorted waves, as well as with earlier calculations by Crothers [J. Phys. B 19, 463 (1986)] and Pan and Starace [Phys. Rev. Lett. 67, 185 (1991)]. Emphasis is placed on understanding the mechanisms for near-threshold ionization.
T. Rösel et al., "Absolute Triple Differential Cross Section for Ionization of Helium Near Threshold," Physical Review A - Atomic, Molecular, and Optical Physics, vol. 46, no. 5, pp. 2539-2552, American Physical Society (APS), Sep 1992.
The definitive version is available at http://dx.doi.org/10.1103/PhysRevA.46.2539
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