Transition Probabilities for he I.

Simple, screening-type wave functions have been utilized to derive general expres- sions for the line strengths in the 2s-np and 2p-nd series of He i. The resulting strengths calculated for these series compare favorably with those computed by 11yi- leraas for the transitions up to n = 6. A computation of the radiational damping con- stants shows that the He I triplet damping factors are of the order of one hundred times smaller than the singlet values, a result that arises from the metastability of the lowest triplet level and from the short wave length of the fundamental singlet line. The expressions for the discrete f-values have been extended to the continuum by the device of letting n -~ ~K, where K is a continuous quantum number, according to the method outlined by Menzel and Pekeris. The resulting expressions provide for the determination of the absorption coefficients for bound-free transitions from the 2S and 2~ levels. Thef-sum rule of Kuhn and Thomas-Reiche predicts that the sum of thef-values of all transitions from a singly excited level of He I should equal unity. The rule appears to be satisfactorily obeyed for all but the 2 ~S level, for which the sum is 0.849