Precision Measurement of the Hydrogen-Deuterium1S2SIsotope Shift

Abstract
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 ⟨r2⟩d-⟨r2⟩p=3.820\,07(65)\,\,fm2, a more than twofold improvement compared to the former value.