On precision measurement of the frequency of the $1^1S-2^3S$ forbidden transition of a helium atom

The possibility is demonstrated of a precision frequency measurement for the $1^1S_0-2^3S_1$ forbidden transition (at 62.6 nm) of a helium atom using the stimulated Raman scattering method. The $1^1S_0$ singlet state is the ground state of ${}^4\mathrm{He}$, while the metastable $2^3S_1$ state has the lowest energy in the triplet part of the spectrum $(^{4}\mathrm{He}^{*})$. 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.