Improved measurement of theHe4i3D1−3D3separation: Confirmation of predicted mass-polarization isotopic shift

From new measurements of the ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ $1s2p-1s3d$ lines, we have determined the $3{}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}-3\mathrm{}{}_{}{}^3{}_{}{}^{}D_2^{}{}_{}{}^{}$ separation to be 102 459(15) MHz. The corresponding nonrelativistic $1s3d$ singlet-triplet separation is 102 196(15) MHz, as compared with the value 102 116(5) MHz previously determined for the equivalent ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ separation. The difference of 80(16) MHz agrees with the predicted isotopic difference of these separations, the main contribution being a mass-polarization difference of 79 MHz as recently calculated by Drake. The experimental difference for $1s3d$ is consistent with the rough trend of such isotopic differences of $1snd$ singlet-triplet separations determined in previous experiments. We infer that mass-polarization shifts probably contribute significantly to the previously observed differences, at least up to $n\simeq 8$.