Accurate oscillator strengths for neutral helium

With use of variational wave functions, oscillator strengths have been calculated for all the allowed transitions between the states $n{}_{}{}^1{}_{}{}^{}S,n{}_{}{}^1{}_{}{}^{}P,n{}_{}{}^1{}_{}{}^{}D,n{}_{}{}^3{}_{}{}^{}S,n{}_{}{}^3{}_{}{}^{}P$, and $n{}_{}{}^3{}_{}{}^{}D$,$n\le 9$, in neutral helium, with an estimated accuracy of better than 1% for most of the transitions and better than 0.1% for about a third of the transitions. The error estimate is based on numerical convergence as the number of expansion terms in the wave function is increased. By use of rapidly converging trial functions consisting of two sets of Hylleraas-type basis functions differing in the values of nonlinear parameters, the expansion lengths required are kept moderate (at most 140 terms). A discussion is given about the completeness of the Hylleraas-type basis.