Fast-beam measurements of the 10D-10Ffine-structure intervals in helium

Four fine-structure intervals separating the n=10, L=2 and 3 Rydberg states of helium have been measured using a fast-beam microwave-optical technique. These are found to agree both with previous measurements of these intervals and with high-precision calculations. The experimental results for the four intervals are 10 ${}_{}{}^1{}_{}{}^{}$ ${\mathit{D}}_2$–10 ${}_{}{}^{+}{}_{}{}^{}$ ${\mathit{F}}_3$, 10 918.826(37) MHz; 10 ${}_{}{}^3{}_{}{}^{}$ ${\mathit{D}}_1$–10 ${}_{}{}^3{}_{}{}^{}$ ${\mathit{F}}_2$, 15 760.667(17) MHz; 10 ${}_{}{}^3{}_{}{}^{}$ ${\mathit{D}}_2$–10 ${}_{}{}^{\mathrm{-}}{}_{}{}^{}$ ${\mathit{F}}_3$, 15 770.704(15) MHz; and 10 ${}_{}{}^3{}_{}{}^{}$ ${\mathit{D}}_3$–10 ${}_{}{}^3{}_{}{}^{}$ ${\mathit{F}}_4$, 15 781.991(11) MHz. The ${}_{}{}^{+}{}_{}{}^{}$F ${(}^{\mathrm{-}}$F) state is the mixed singlet-triplet state of higher (lower) energy. The result for the spin-averaged 10D-10F interval in helium is 14 560.650(13) MHz, which has an experimental uncertainty of better than one part per million. The good agreement with theory breaks the pattern of discrepancies between theory and experiment observed in measurements of the higher-L n=10 helium fine structure. Possible explanations for those discrepancies are discussed.