Principal quantum number dependence of polarisation fractions for electron impact excitation of the n¹P and n¹D states of helium

Abstract

The polarisation fractions of radiation emitted following electron impact excitation of the n'P ( n = 2-6) and n'D ( n =3-6) states of helium have been calculated using first-order many-body theory over the energy range from threshold to 500 eV. The predicted polarisation fractions are essentially independent of the principal quantum number ( n ) , in agreement with the experimental results of Standage for n ' P ( n = 2,3) and the Born results of Vriens and Carriere, but in apparent disagreement with the multichannel eikonal results.

Polarization of the 31D state of helium excited by electron impact

“…The FOMBT calculations of Csanak and Cartwright (1989) indicate that the n'D-2'P polarizations are essentially independent of the principal quantum number n. Although the experimental data are not in good agreement with the FOMBT results, it is of interest to make comparisons between the present 3'D data and previous experimental values for higher values of n.…”

“…It is clear that none of the calculations agree with experiment, all predicting too small values at energies beyond 30 eV. The recent first order many body theory calculation (FOMBT) of Csanak and Cartwright (1989) shows remarkable agreement with the first Born approximation calculations of Vriens and Carrikre (1970). Mansky and Flannery (1990) attribute this agreement to the neglect of channel coupling effects in both calculations.…”

“…Recent theoretical studies (Csanak andCartwright 1989, Mansky andFlannery 1990), of the polarization fraction for the 3'D-Z'P (667.8 nm) transition in helium following electron impact excitation of the 3'D state have aroused interest. This interest is intensified by the disagreement not only between the calculations, but also between both theories and the only experimental measurement (Moustafa Moussa et a/ 1969) over an extended range of incident electron energies.…”

Experiments: Ionizing Plasma

“…Alignment is considered on the levels n 1 P, n 1 D, n 3 S, n 3 P, and n 3 D with n 7. Two sets of alignment production cross sections Q 0,2 0 , (4.40b) or (5.41), are based on two theoretical calculations; one is calculated by the distorted wave approximation (DWA) [53] and the other by the convergent close-coupling (CCC) method [54].…”

“…Solid curve: Q 0,0 0 . Dashed curve: Q 0,2 0 by DWA [53]. Open circles and triangles are Q 0,0 0 and Q 0,2 0 , respectively, by CCC [54] (Quoted from [52], with permission from IOP Publishing.…”