On Precision Measurement of the Frequency of the 1¹ S–2³ S Forbidden Transition of a Helium Atom

The possibility is demonstrated of a precision frequency measurement for the 1¹S₀−2³S₁ forbidden transition (at 62.6 nm) of a helium atom using the stimulated Raman scattering method. The 1¹S₀ singlet state is the ground state of ⁴He, while the metastable 2³S₁ state has the lowest energy in the triplet part of the spectrum (⁴He*). The transition has a very small natural width, which allows us to consider it as a possible reference for creation of a frequency standard in the vacuum ultraviolet region.