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We present a method to calculate the natural line width and energy dependentline shape due to fluorescence decay of core excited atoms within a fullrelativistic multi-reference configuration interaction theory. The atomicabsorption lines show a deviation from a Lorentzian line-shape due to energydependent matrix elements of the localized electronic state coupling to thephoton field. This gives rise to spectral lines with small but visibleasymmetry. One generally finds an excess of spectral weight at the high energyshoulder of the atomic absorption line. We present the example of nuclear decayof $^{55}$Fe by electron capture of an inner-shell core electron. We show thatthe amount of ionizing radiation in the energy window between 50 and 200 keV isaround one order of magnitude larger due to the energy dependent fluorescenceyield lifetime compared to the value one would obtain if one assumes a constantfluorescence decay rate. This yields a total change of energy deposited intoionizing radiation of about 1\textperthousand. Our calculations are in goodagreement with previous calculations and experimental observations where datais available. Our results can be further validated by high precisionmeasurements of electron capture nuclear decay spectra using recently developedexperimental methods.

Non-Lorentzian atomic natural line-shape of core level multiplets: Access high energy x-ray photons in electron capture nuclear decay