Double-resonance method for the determination of level populations

The hyperfine transitions of atoms in the ground state are split when RF fields, whose frequencies correspond to the energy differences between the Zeeman sublevels, are applied. It is shown that the measurement of the intensities of these lines provides a very good determination of the populations of the different states. The calculation is made in the two extreme conditions of adiabatic or sudden application of the RE perturbation. The triplet splitting of the transition of a hydrogen maser used as an amplifier is studied experimentally under the sudden passage condition and this method is used to measure the efficiency of a state selector that improves the atomic beam composition of a H maser.