Continuous Absorption Coefficients for Non-Hydrogenic Atoms

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Abstract
Continuous absorption coefficients for hydrogenic atomic systems are calculated using an asymptotic expansion of the Gaunt factor and the Boltzmann relation for the relative populations of the atomic energy levels. The hydrogen results are then extended to give similar coefficients for neutral oxygen, nitrogen and magnesium by taking into account separately all non-hydrogenic nl levels using the general formula of Burgess and Seaton for the atomic photo-ionization cross sections. A comparison with the experimental results of Boldt for oxygen and nitrogen in the wavelength region 4300−6300 A shows good general agreement. The measurement of the 3 3 P cross section in magnesium by Bötticher gives a much greater rate of fall-off with increasing frequency than is given by the general formula. The continuous absorption by the ground state of aluminium is found to have a large threshold cross section which is of interest in connection with the sudden change in the solar continuum at 2085 A.