The He4 fine structure and He3 hyperfine structure as well as the isotope shift between He4 and He3 in the 1s2s S13 1snp (n=579) transitions have been measured using an atomic beam and an intracavity frequency-doubled coumarin ring dye laser near 270 nm. The observed 1snp P3 He4 fine structure is in agreement with (extrapolations of) existing experimental results and theoretical calculations. The 1snp He3 hyperfine structure has been analyzed using the He4 fine-structure parametrization and the Fermi-contact interaction of the screened 1s electron. For n>35 hyperfine components of the 1snp P1 levels are also excited as a consequence of hyperfine-induced singlet-triplet mixing. Calculated hyperfine splittings agree with experiment for n<20. Deviations up to 50 MHz are observed for n>20. These are shown to be due to hyperfine-induced n-mixing effects. The experimental results are excellently reproduced in a three-channel two-limit quantum-defect model without any adjustable parameter. The observed 1s2s S131snp P3 isotope shifts can be fully explained using recent mass polarization calculations for the 1s2s S13 and 1snp (n=38) P3 levels and confirm an isotope dependence of p1p2 for the 1s2s S13 level. © 1989 The American Physical Society.
Vassen, W., & Hogervorst, W. (1989). HIGH-RESOLUTION UV LASER SPECTROSCOPY OF THE 1S2S3S1-]1SNP(N=5-79) TRANSITIONS IN HE-3 AND HE-4. Physical Review A, 39(9), 4615-4627. https://doi.org/10.1103/PhysRevA.39.4615