Precision Measurement of the Hydrogen-Deuterium 1S-2S Isotope Shift

Measuring the hydrogen-deuterium isotope shift via two-photon spectroscopy of the 1S - 2S transition, we obtain 670 994 334 606(15) Hz. This is a 10-times improvement over the previous best measurement [A. Huber et al., Phys. Rev. Lett. 80, 468 (1998)] confirming its frequency value. A calculation of the difference of the mean square charge radii of deuterium and hydrogen results in < r(2)>(d) - < r(2)>(p) = 3.820 07(65) fm(2), a more than twofol( improvement compared to the former value.