Electron impact coherence parameter

Instead of X and x it is informative to choose the electron impact coherence parameters, which describe the excited-state charge cloud shown schematically in fig. 8.1. In the absence of spin-analysis they are (P/) describing the relative difference between the length and width w, (y) describing the alignment and L ) describing the orientation. In terms of the Stokes parameters the electron impact coherence parameters are... 

Fig. 8.12. Differential cross section for the 1 S, 2 S and 2 P states of helium and electron impact coherence parameters (8.40) for the 2 P state at Eo=50 eV. Experimental data for differential cross sections are 1 S, Register, Trajmar and Srivastava (1980) 2 S,2 P, Cartwright et al. (1992). Experimental data for electron impact coherence parameters are crosses, McAdams et al. (1980) squares, Beijers et al. (1987) plus signs, Eminyan et al. (1974). Solid curves, coupled channels optical (equivalent local) (McCarthy et al., 1991) broken curve, distorted-wave Born (Cartwright et al, 1992). From McCarthy et al (1991).

The calculation describes the differential cross sections and 2 P electron impact coherence parameters quite well. For the 2 P differential cross section it is contrasted with a variant of the distorted-wave Born approximation, first-order many-body theory, where the distorted waves are both calculated in the initial-state Hartree—Fock potential. 

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