Hyperfine interval of the metatable state of atomic

The authors report on measurements of the hyperfine splitting of the n=2 S metastable state of atomic hydrogen. Measurements are done using a thermal beam of atomic hydrogen and the hyperfine transition is driven using radio-frequency fields in a Ramsey separated-oscillatory-field configuration. Comparison of the measurement of this interval with the extremely precise measurements of the ground-state hyperfine interval allows for tests of higher-order corrections (such as relativistic and nuclear-structure corrections) to the hydrogenic hyperfine intervals.