= Lamb-Retherford experiment
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= Lamb shift experiment
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{synonym}
In 1947, <Willis Eugene Lamb Jr.> and <Robert Curtis Retherford> dissociated <hydrogen> in a <tungsten> oven, crossed its atomic jet with <Electrons> for <collisional excitation>, and detected surviving $2S$ atoms by electron ejection from metal. An electrometer and galvanometer read the current. They interrupted fixed-frequency <microwaves> while sweeping the static <magnetic field>; a flip coil calibrated the field. Their <Zeeman splitting> comparison placed $2S_{1/2}$ about $1000$ $\mathrm{MHz}$ above $2P_{1/2}$; the plotted $1000$ $\mathrm{MHz}$ displacement was explicitly not a best fit.<Fine Structure of the Hydrogen Atom by a Microwave Method by Willis Eugene Lamb Jr. and Robert Curtis Retherford>[Fine Structure of the Hydrogen Atom by a Microwave Method, pp. 241–243, Figs. 1–2; inspected APS full text]{ref}
The 1955 lecture's apparatus cross-section identifies movable slits, an electron bombarder, a radiofrequency transmission line with beam slots, a d.c. quenching electrode, metal target, electron collector, and magnet poles. A $2S\to2P$ transition followed by rapid <spontaneous emission> leaves a ground-state atom unable to eject detector electrons. The scan holds radiofrequency and amplitude fixed. <Natural linewidth> and unresolved <hyperfine structure> broaden resonances, whose analysis yields zero-field separations.<Fine Structure of the Hydrogen Atom by Willis Eugene Lamb Jr.>[Fine Structure of the Hydrogen Atom, delivered December 12, 1955; Columbia-hosted Nobel Lectures scan, printed pp. 288–290, Figs. 2–7]{ref}
Static <electric field> quenching supplied a control. The 1947 report identifies <natural linewidth>, <hyperfine structure>, excessive microwave intensity, and field inhomogeneity as width contributions, and flags imperfect magnetic calibration. It does not specify an absolute microwave-frequency calibration procedure.<Fine Structure of the Hydrogen Atom by a Microwave Method by Willis Eugene Lamb Jr. and Robert Curtis Retherford>[pp. 242–243, Fig. 1 caption and discussion of proposed refinements]{ref}
\Image[/lamb-retherford-metastable-beam-readout.png]
{title=Metastable beam preparation and electron-current readout}
{description=Original functional schematic of the Lamb-Retherford measurement principle, based on the apparatus descriptions above. The level sketch isolates the zero-field $2S_{1/2}$ and $2P_{1/2}$ pair; the historical field scans also addressed other Zeeman branches. Geometry and energy gaps are not to scale.}
The surname is <Robert Curtis Retherford>'s: “Retherford” and “Rutherford” name different people. The historical <Lamb-Retherford experiment> should retain the former spelling.<Retherford, Robert Curtis by Istituto della Enciclopedia Italiana>[Retherford, Robert Curtis, professional biography and account of the 1947 experiment]{ref}
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