Precision measurement of the forbidden 21S0 – 23S1 transition frequency in a helium atom

We demonstrate the possibility of measuring the forbidden 21S0 – 23S1 transition frequency (λ = 1557 nm) of a helium atom by the method of stimulated Raman scattering through the intermediate 23P1 level. Singlet (21S0) and triplet (23S1) states have long lifetimes of 20 ms and 8000 s, respectively. The transition is important for the spectroscopy of the helium atom because it relates the singlet and triplet parts of the spectrum.