Stepwise excitation and level-crossing spectroscopy of the tripletDstates of helium-4

Abstract

The fine-structure intervals of the lowest two triplet $D$ states of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ have been obtained by a new method: Ground-state atoms are excited by a discharge to the triplet metastable state, which then undergoes a two-step optical excitation to the triplet $D$ states, where level-crossing measurements are made. The experimental results for ${}_{}{}^3{}_{}{}^{}D_1^{}{}_{}{}^{}-{}_{}{}^3{}_{}{}^{}D_3^{}{}_{}{}^{}$ (MHz) are 1400.67 ± 0.29 and 591.25 ± 0.14 for the $3{}_{}{}^3{}_{}{}^{}D$ and $4{}_{}{}^3{}_{}{}^{}D$ states, respectively; for ${}_{}{}^3{}_{}{}^{}D_2^{}{}_{}{}^{}-{}_{}{}^3{}_{}{}^{}D_3^{}{}_{}{}^{}$ (MHz) they are 75.97 ± 0.23 and 36.15 ± 0.24 for $3{}_{}{}^3{}_{}{}^{}D$ and $4{}_{}{}^3{}_{}{}^{}D$. These values are much more precise than the previously available data, obtained by optical, particle-bombardment level crossing, or beam-foil quantum-beat spectroscopy, and are consistent with the previous data except for one case, where a previous measurement of $3{}_{}{}^3{}_{}{}^{}D_2^{}{}_{}{}^{}-3\mathrm{}{}_{}{}^3{}_{}{}^{}D_3^{}{}_{}{}^{}$ by electron bombardment level-crossing spectroscopy is believed to be in error. These new experimental values for the fine structures of the triplet $D$ states of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ clearly indicate the inadequateness of existing theoretical calculations of these fine structures.