Fine structure of Rydberg states. II.n=8 and 9DandFstates andn=16, 17, and 18PandDstates ofHe4

We report high-precision measurements of fine-structure intervals in Rydberg states of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ as follows: $8{}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}-8\mathrm{}{}_{}{}^{1,3}{}_{}{}^{}F_3^{}{}_{}{}^{}$ and $9{}_{}{}^3{}_{}{}^{}D_{2,3}^{}{}_{}{}^{}-9\mathrm{}{}_{}{}^{1,3}{}_{}{}^{}F_3^{}{}_{}{}^{}$ near 21 GHz and $n{}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}-n{}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}$ for $n=16, 17, \mathrm{and} 18$ near 23, 19, and 16 GHz, respectively. A microwave-optical-resonance method and electron-bombardment excitation were used. We state and discuss formulas for predicting as yet unobserved members of $n{}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}-n{}_{}{}^{1,3}{}_{}{}^{}F_3^{}{}_{}{}^{}$ and $n{}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}-n{}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}$ series. We give an accurate result for the $9{}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}-9\mathrm{}{}_{}{}^3{}_{}{}^{}D_2^{}{}_{}{}^{}$ interval obtained indirectly from our work. New results on singlet-triplet mixing coefficients in $F$ states and on coefficients in a Ritz series expansion of quantum defects are also given.