= Rabi molecular beam magnetic resonance experiment
{c}
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.<The Molecular Beam Resonance Method for Measuring Nuclear Magnetic Moments by Isidor Isaac Rabi and others>[Rabi, Millman, Kusch and Zacharias, Physical Review 55 (1939), pp. 526–533; Figs. 1–5; Apparatus, Procedure and Results; p. 526 references to the 1938 reports]{ref}
\Image[/rabi-molecular-beam-selector.png]
{title=Functional beam selector: unchanged states refocus; RF transitions cause detector loss}
{description=Original functional diagram of the <Rabi molecular beam magnetic resonance experiment>; paths are schematic and not to scale.}
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.<Our History by Stanford University Physics Department>[Physics Department, Our History, The Middle 1930s through the 1960s, paragraphs on nuclear induction in condensed matter and spectroscopy]{ref}<Harvard’s Nobel Laureates in Physics by Harvard University>[Harvard’s Nobel Laureates in Physics, Edward M. Purcell section, nuclear magnetic resonance absorption]{ref}
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