Precision measurement of the isotope shift of the 1S-2Stransition of atomic hydrogen and deuterium

Using the extreme resolution of a cold hydrogen beam spectrometer and a novel sideband technique, the isotope shift of the 1S-2S transition of hydrogen and deuterium has been directly measured to an accuracy of 3.7×${10}^{\mathrm{-}8}$. Our experimental result of 670 994 337(22) kHz is compared with the theoretical prediction of 670 994 414(22) kHz that can be calculated, if we assume the value of 1.9627(38) fm for the structure radius of the deuteron and 0.0189(4) ${\mathrm{fm}}^2$ for the neutron-electron correction. The 2.5σ difference between the predicted and the measured values may be interpreted in terms of polarizability of the deuterium nucleus.