On the precise measurement of the frequency transition 1S–2S of the hydrogen atom

An experiment is suggested for the precise measurement of the frequency transition 1S–2S of the hydrogen atom with a relative precision of 10^-12 based on the use of a narrow resonance of two-photon absorption in a standing wave field. The relative width of the absorption line is determined by the second-order Doppler effect and has the same order as the relative width of Mössbauer transitions with absorption of γ-quanta. The results of this experiment can be included in the system of fundamental constants. A possibility for creating a new standard of the atomic time unit with a reproducibility of ～ 10^-15 has been examined.