Anticrossing spectroscopy of then=6,7,8 D1,3states inHe3

The anticrossing spectra of the $n=6,7,8\mathrm{} {}_{}{}^1{}_{}{}^{}D-{}_{}{}^3{}_{}{}^{}D$ states of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ have been observed. From these measurements precise values of the electrostatic singlet-triplet separation of these states have been obtained and are found to differ systematically from the accepted values of the ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ singlet-triplet separation by approximately 34 MHz. We have investigated mass-dependent contributions to both the energy-level structure and the magnetic-tuning characteristics of the states of interest. We have found no mechanism that can explain such a large difference in the electrostatic-energy separation of the singlet and triplet $D$ states between the two helium isotopes.