Measurement of the muonium 1S‐2S transition frequency

Resonant ionization spectroscopy has been employed for measuring the 12S1/2−22S1/2 frequency difference in the hydrogen‐like muonium atom to 2 455 529 002(33)(46) MHz. The 1S‐2S two‐photon transition was induced Doppler‐free using two counter‐propagating laser beams. The 2S state was photo‐ionized by a third photon from the same laser field. The measurement agrees with QED theory within two standard deviations. The mass of the positive muon can be extracted from the isotope shifts in this transition to hydrogen and deuterium to 105.658 80(29)(43) MeV/c2.