Experimental determination of the Rydberg constant using two-photon spectroscopy of atomic hydrogen

Doppler-free two-photon absorption was used to observe the 2S–8D and 2S–10D transitions in atomic hydrogen and deuterium with a relative width of ~ 1.8 X 10^{– 9}. A comparison between the wavelengths of these transitions and that of an iodine-cell-stabilized He–Ne laser yielded a new value ofthe Rydberg constant R_∞ = 109737.31569 ± 0.00006 cm^{– 1}.