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leading to a total spin F equal to 1=''F'' = 7/2 + 1/2 = 4; the other sub-level corresponds to anti-parallel electron and nuclear spin (i.e., pointing in opposite directions), leading to a total spin 1=''F'' = 7/2 - 1/2 = 3. In the caesium atom it so happens that the sub-level lowest in energy is the one with 1=''F'' = 3, while the 1=''F'' = 4 sub-level lies energetically slightly above. When the atom is irradiated with electromagnetic radiation having an energy corresponding to the energetic difference between the two sub-levels the radiation

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