Profile of the two-photon absorption line due to the 1S–2S transition in atomic hydrogen

An analysis is made of the profile of a two-photon absorption line of hydrogen free of the first-order Doppler broadening. The possibility of eliminating the quadratic Doppler effect with the aid of a narrow peak of the second derivative of the line profile is considered. It is shown that for a realistic temperature stability it should be possible to achieve a reproducibility of the order of 10^–15.