F. Nez et al., TOWARDS A FREQUENCY-MEASUREMENT OF THE RYDBERG CONSTANT USING THE 2S-8S AND 2S-8D TRANSITIONS OF HYDROGEN, IEEE transactions on instrumentation and measurement, 42(2), 1993, pp. 217-221
A very precise measurement of the Rydberg constant is being performed
using a direct-frequency comparison of the 2S-8S and 2S-8D two-photon
transitions in atomic hydrogen with the difference of two optical stan
dards connected to a frequency chain.