Variational calculations using Hylleraas coordinates have been performed for the first time for estimating the energies of 3dnf(~(1,3)D~o)state of helium for n=4,5,6.We predict absorption peaks at 12.219,12.647,and 12.857 eV for the ~3D~o series converging to N=3 ionization threshold of He~+ which can be expected to be observed in the experiment of single photon double excitation of lowest ~3P~e state of helium placed in synchrotron radiation.
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