The light gets through more easily as it shines
Imagine shining a suitable polarized beam through a collection of atoms and finding that they gradually absorb less of it. The atoms can still be there. Repeated absorption and emission have moved many of them into an internal state that this particular light hardly excites. The beam has changed the sample it is passing through.
This is optical pumping. Pumping means redistributing atoms among internal states. The number occupying a state is its population. With the right transition and light, the redistribution builds a useful imbalance that can persist in the ground state, the atom’s lowest internal energy level.
Read the evidence: A. Kastler (1957), Optical Methods of Atomic Orientation and of Magnetic Resonance, JOSA 47, 460–465 — author abstractR. A. Bernheim (1965), Optical Pumping: An Introduction, chapters 2–3 — Berkeley-hosted excerpt
Each scattering cycle gives an atom another chance to change state
A ground state can contain several magnetic sublevels, distinguished by the projection of the atom’s internal angular momentum. Polarization describes the motion of the light’s electric field and helps determine which transitions the light can drive. Atoms in different sublevels therefore need not absorb a given beam equally.
An absorbing atom takes in a photon, reaches an excited state and later emits another photon. It may return to a different ground sublevel. Repeating this cycle can move atoms out of strongly absorbing states and collect them in a weakly absorbing one. That last state is dark to the chosen light.
Which state is dark depends on the full set of transitions and the polarization. Other branches, imperfect polarization and unwanted excitation can keep atoms participating in the cycle. The incoming polarized beam and the outgoing light carry angular momentum as well as the atoms themselves.
Read the evidence: A. Kastler (1957), Optical Methods of Atomic Orientation and of Magnetic Resonance, JOSA 47, 460–465 — author abstractR. A. Bernheim (1965), Optical Pumping: An Introduction, chapters 2–3 — Berkeley-hosted excerpt
Collisions keep undoing the preparation
Atoms can collide with each other or with the walls and lose the imbalance that the light has built. This return toward the unprepared distribution is relaxation. The observed populations reflect a competition: the light redistributes them one way, while relaxation mixes them back.
Switching the pumping light off lets the prepared imbalance decay. Turning it back on rebuilds it. Following absorption or fluorescence during these changes makes both preparation and relaxation accessible to an optical detector. Suppressing relaxation was an experimental challenge alongside choosing the right levels and light.
Read the evidence: R. A. Bernheim (1965), Optical Pumping: An Introduction, chapters 2–3 — Berkeley-hosted excerptLaboratoire Kastler Brossel, Birth of Double Resonance and Optical Pumping — institutional history
A radiofrequency field can disturb what the light prepared
A magnetic field gives neighboring sublevels different energies. Apply an oscillating field at the frequency corresponding to their separation, and it can transfer atoms between them. This is magnetic resonance. If that transfer refills a state that absorbs the pumping light, an optical signal changes as the radiofrequency is swept through resonance.
One interaction drives the change and another reads it: this is double resonance. In the 1952 Brossel–Bitter experiment, polarized light prepared unequal populations in the excited state of mercury, and magnetic resonance changed the polarization of the emitted light. Ground-state pumping used the related idea of storing an imbalance in longer-lived lower states.
Read the evidence: J. Brossel and F. Bitter (1952), A New Double Resonance Method for Investigating Atomic Energy Levels, Phys. Rev. 86, 308–316 — abstractA. Kastler (1957), Optical Methods of Atomic Orientation and of Magnetic Resonance, JOSA 47, 460–465 — author abstract
From Kastler’s proposal to a sodium-beam observation
The laboratory’s historical account places Kastler’s ground-state pumping proposal in 1950 and Brossel’s sodium atomic-beam observation in 1952. It describes failed attempts before the successful observation and the importance of irradiation and relaxation conditions. Later vapor-cell techniques made a further practical advance.
Kastler’s 1966 Nobel Prize recognized optical methods for studying radiofrequency resonances in atoms. Light had become a way to organize internal populations and read their response with high sensitivity. Optical pumping can prepare an internal state while leaving substantial center-of-mass motion; laser cooling adds the separate task of reducing that motion.
Read the evidence: Laboratoire Kastler Brossel, Birth of Double Resonance and Optical Pumping — institutional historyA. Kastler (1957), Optical Methods of Atomic Orientation and of Magnetic Resonance, JOSA 47, 460–465 — author abstractAlfred Kastler — official 1966 award facts