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Isidor Rabi developed molecular-beam magnetic resonance for measuring nuclear magnetic moments. Two magnetic field gradient regions focus a selected beam onto a detector, while an intervening uniform magnetic field sets the resonance frequency. An oscillating transverse magnetic field drives transitions between magnetic quantum states. On resonance, the changed states no longer follow the focused path, reducing the detected intensity. Measuring the Larmor frequency in a known magnetic field thereby replaces a small static deflection with a more precise frequency comparison.^[ref]

Identity source: www.nobelprize.org/prizes/physics/1944/rabi/facts/

An experimental atomic physicist who coauthored the molecular-beam method for measuring nuclear magnetic moments. Instead of determining a moment solely from a beam's static deflection, the experiment found the Larmor frequency in a known magnetic field. A resonance-induced loss of focused beam intensity made this frequency measurable with high precision.^[ref]

Magnetic resonance is a resonantly driven transition between states whose energy separation depends on a magnetic field. A static field splits magnetic substates through the Zeeman effect, while an oscillating transverse field supplies energy near the transition frequency. For a simple spin with gyromagnetic ratio $\gamma$, the angular resonance frequency is ${\omega }_0=|\gamma |B_0$.

In the Rabi molecular beam magnetic resonance experiment, inhomogeneous magnets selected and refocused molecular trajectories. A radio-frequency field changed magnetic substates at resonance, altering the transmitted beam intensity. Scanning the frequency therefore measured magnetic moments through a frequency-dependent signal.^[ref]

In 1938, Isidor Isaac Rabi, Jerrold Zacharias, Sidney Millman and Polykarp Kusch introduced molecular-beam magnetic resonance. Their 1939 account describes a heated oven, vacuum slits, inhomogeneous A/B magnetic fields with opposite gradients, and a uniform static C magnetic field. Conductors supplied transverse oscillatory radiofrequency (RF) fields. Transitions spoiled refocusing, reducing the tungsten-filament detector signal. Alignment compared A-only and B-only attenuation; C-field sweeps at fixed RF frequency limited oscillator pickup. Matching frequency/field ratios across compounds checked nuclear assignments. Magnetic field calibration limited absolute magnetic moments; weak intervening fields caused unwanted transitions.^[ref]

The readout has a useful logical structure: A and the apertures admit trajectories correlated with a magnetic moment projection; B tests whether that projection has survived C. The two deflectors compensate for unchanged states, whereas a changed projection experiences a different force in B. A dip followed by recovery as the static magnetic field passes through resonance is therefore a population-sensitive signal. A comparison with RF absent or far from resonance is the natural baseline in this functional model; RF pickup, drift or background loss must not be mistaken for a state transition. A dip alone does not establish a pure initial quantum state or resolve an entire time-domain Rabi cycle.

This beam method measures transmitted particles after state-dependent deflection. The later bulk nuclear magnetic resonance methods of Felix Bloch and Edward Mills Purcell instead use a macroscopic sample and detect nuclear induction or RF absorption; NMR spectroscopy builds on that bulk setting. The common resonant coupling does not make their preparations or readouts interchangeable.^[ref][ref]

An experimental atomic physicist who coauthored the Rabi molecular beam magnetic resonance experiment. Two nonuniform-field magnets focused a beam onto a detector, while an oscillating field in the intervening uniform region drove magnetic transitions. At resonance, changed magnetic states disrupted the focusing and reduced the detected intensity, allowing nuclear magnetic properties to be measured through a frequency.^[ref]