Fine structure of Rydberg states:n=6 and 7DandFstates ofHe4

We have measured all the fine-structure intervals in the configurations $1s7d$ and $1s7f$ of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ as well as the intervals $6{}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}-6\mathrm{}{}_{}{}^1{}_{}{}^{}F_3^{}{}_{}{}^{}$ and $6{}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}-6\mathrm{}{}_{}{}^3{}_{}{}^{}F_3^{}{}_{}{}^{}$ by a microwave-optical-resonance method. The level of accuracy considerably exceeds that of available calculations. We discuss the results in relation to the breakdown of $\mathrm{LS}$ coupling, and we estimate the singlettriplet mixing coefficients and the corresponding level perturbations. We obtain an improved fitting formula useful for predicting the frequencies of $n{}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}-n{}_{}{}^{1,3}{}_{}{}^{}F_3^{}{}_{}{}^{}$ transitions.