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The existence of a second stable state of the negative ion has been proved. It corresponds to the doubly excited (2s) (2p)3P state of He with energy -1.506 in Rydberg units. The energy of the state has been calculated both for He and H with neglect of the mutual polarization energy ot the two electrons. As to the distortion of the electron orbits also, mainly that of the s-electron has been taken into account. The binding energy of the second electron is in this approximation about 0.29 e.v. but it may possibly be considerably larger.
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Egil A. Hylleraas (1950) studied this question.
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