Measurement of coherence parameters for the electron impact excitation of the 31D state of helium at small electron scattering angles

The authors have measured the Stokes parameters of the coincident decay radiation resulting from the electron impact excitation of the 3 1 D states of helium. This was done as a function of the electron scattering angle theta e in the small scattering angle range (15'-30') at energies of 45 and 60 eV. The results do not provide a preference for any of the theoretical models currently available. Measurements of the Stokes parameter P 3 (proportional to L perpendicular to ) tend to confirm the predictions of the distorted-wave Born approximation (DWBA). For the coherence parameters P l and gamma resulting from the measurements of P 1 and P 2 , the results for P l seem to be consistent with the dipole multichannel eikonal treatment (DMET) and with the first-order many-body theory (FOMBT), whereas the results for gamma are in disagreement with these theories.