Electron-Impact Broadening of Isolated Lines of Neutral Atoms in a Plasma. I

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Abstract
This paper deals with the generalized impact theory for electron broadening of isolated spectral lines emitted by neutral atoms in a plasma. The existing theory is improved by quantitative evaluation of the second-order (quadrupole) term in the multipole expansion including broadening of the lower level and interference effects, by approximate inclusion of the "back reaction" of atom on perturber in inelastic collisions through symmetrization with respect to initial and final perturber states, and by corrections to the broadening functions in the classical-path approximation. While the quadrupole contribution is normally a small correction, it is shown that it may, in principle, dominate the weak collision broadening in some situations even when the multipole series converges; however, strong collisions will always dominate in this case. Numerical results of the improved theory are given for the half-widths at half-maximum of some helium i lines, and the effects of the corrections are evaluated.