Precision spectroscopy of helium atom

Quantum electrodynamic theory (QED) of the simplest bound-three-body system is now stringently tested by precise spectroscopic frequency measurements in He­ lium. Alternatively, comparison between He measurements and theory could be used for accurate determination of some fundamental quantities, as for example, the fine structure constant a, and the differences of nuclear charge radii between two 3He and 4He. In the following, we review our precise spectroscopic measure­ ments on the 1083 nm Helium transition, connecting the triplet 28 and 2P states. Frequency differences among the 1083 nm measured frequencies give precise val­ ues of the fine structure (FS) and hyperfine structure (HFS) splittings of the 23 level in 4 He and 3He respectively, and of the isotope shift (IS) between these two isotopes. Implications of these results in the a determination as well as for nuclear charge radii and structure are discussed.