A Zeeman sublevel is a magnetic-field-resolved component of an angular-momentum level, labeled by its projection quantum number . In the weak-field regime, for the relevant electronic angular momentum, with appropriate modifications for hyperfine levels.
For angular momentum one, the values give three shifts. The states can be degenerate at zero field yet acquire different energies in a field. Light polarization and selection rules control which sublevels a laser addresses. Strong fields can change the angular-momentum coupling, so one cannot extrapolate every weak-field formula indefinitely.
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