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The Franck-Hertz Experiment: 100 Years Ago and Now

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The Franck-Hertz Experiment: 100 Years Ago and Now The Franck-Hertz experiment: 100 years ago and now A tribute to two great German scientists Zoltán Donkó1, Péter Magyar2, Ihor Korolov1 1 Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Budapest, Hungary 2 Physics Faculty, Roland Eötvös University, Budapest, Hungary Franck-Hertz experiment anno (~1914) The Nobel Prize in Physics 1925 was awarded jointly to James Franck and Gustav Ludwig Hertz "for their discovery of the laws governing the impact of an electron upon an atom" Anode current Primary experimental result 4.9 V nobelprize.org Elv: ! # " Accelerating voltage ! " Verh. Dtsch. Phys. Ges. 16: 457–467 (1914). Franck-Hertz experiment anno (~1914) The Nobel Prize in Physics 1925 was awarded jointly to James Franck and Gustav Ludwig Hertz "for their discovery of the laws governing the impact of an electron upon an atom" nobelprize.org Verh. Dtsch. Phys. Ges. 16: 457–467 (1914). Franck-Hertz experiment anno (~1914) “The electrons in Hg vapor experience only elastic collisions up to a critical velocity” “We show a method using which the critical velocity (i.e. the accelerating voltage) can be determined to an accuracy of 0.1 V; its value is 4.9 V.” “We show that the energy of the ray with 4.9 V corresponds to the energy quantum of the resonance transition of Hg (λ = 253.6 nm)” ((( “Part of the energy goes into excitation and part goes into ionization” ))) Important experimental evidence for the quantized nature of the atomic energy levels. The Franck-Hertz experiment: 100 years ago and now Franck-Hertz experiment: published in 1914, Nobel prize in 1925 Why is it interesting today as well? “Simple” explanation (“The electrons ....”) → description based on kinetic theory (Robson, Sigeneger, ...) Modern experiments Various gases (Hg, He, Ne, Ar) Modern experiment + kinetic description (develop an experiment that can be modeled accurately ...) → P. Magyar, I. Korolov, Z. Donkó: "Photoelectric Franck-Hertz experiment and its kinetic analysis by Monte Carlo simulation" Phys. Rev. E 85, 056409 (2012). Franck-Hertz experiment: Boltzmann eq. analysis Solution of the Boltzmann equation (1D) Most accurate theoretical description so far - however, without new experiment Homogeneous Hg vapor density (!!) Grid : “loss frequency” (!!) ÜBERBLICK ATOMPHYSIK Ein Grundstein der Atomphysik Die gängige Lehrbuchinterpretation des Franck-Hertz-Experiments lässt viele Fragen offen. Robert E. Robson, Malte Hildebrandt und Ronald D. White Das Franck-Hertz-Experiment aus dem Jahre !"!# fehlt in keinem Lehrbuch, zeigt es doch anschaulich die quantisierten Eigenschaften der Atome und legt damit den Grundstein für die moderne Atomphysik. Allerdings versagt die traditionelle Interpretation, wenn mehrere angeregte Niveaus ins Spiel kommen. Nur eine Analyse aus dem Blickwinkel der elemen- AIP Emilio Segre Visual Archives Visual Segre Emilio AIP / taren kinetischen Gastheorie erlaubt es, den kor- rekten Zusammenhang zwischen mikroskopischen Vorgängen, die den Gesetzen der Quantenmechanik unterliegen, und den im Labor gemessenen makro- Collection Born Archives, Visual Segre Emilio AIP skopischen Größen herzustellen. A.B. Lagrelius & Westphal & Lagrelius A.B. ie bedeutenden Experimente des späten !". und frühen #$. Jahrhunderts zur Untersuchung von James Franck (!''# – !()%, links) und Wilhelms-Universität Berlin, wo sie ihr D elektrischen Strömen in Gasen leiteten eine neue Gustav Hertz (!''* – !(*&) begegneten bahnbrechendes Experiment durch- Epoche in der Geschichte der modernen Physik ein. sich !(!! am Physikinstitut der Friedrich- führten. Besonders bemerkenswert in diesem Zusammenhang sind die Experimente, die James Franck und Gustav Gegenüber der von Franck und Hertz verwendeten Hertz ab dem Jahr !"!! in Berlin durchführten. Bis zylindrischen Geometrie wird heute meist ein Aufbau !"!% untersuchten sie die Wechselwirkung langsamer mit ebener Elektrodenanordnung vorgezogen (Abb. "). Elektronen mit neutralen Gasatomen [!]. Ihr Ziel war Die Kathode emittiert Elektronen mit einer konstanten eine allgemeine „kinetische Theorie der Elektronen in Rate in den gasgefüllten Driftraum, wo sie beim Durch- Gasen“, da Franck Aussagen von J. S. Townsend zum laufen einer Potentialdifferenz zum Kontrollgitter in Elektronenstreuprozess be zweifelte. elastischen und inelastischen Stößen an den Gasatomen Franck und Hertz verbesserten die experimentellen streuen. Eine Bremsspannung zwischen Kontrollgitter Methoden, mit denen unter anderem Philipp Lenard und Anode erlaubt nur Elektronen ausreichender En- die Kathodenstrahlen in Gasen untersucht hat. Die ergie, die Anode zu erreichen und damit zum Anoden- bahnbrechende Untersuchung in Quecksilberdampf, strom IA beizutragen. Als Funktion der Spannung U os- !"!% veröffentlicht und üblicherweise als „das“ Franck- zilliert dieser Strom, wobei die Abstände ∆U zwischen Hertz-Experiment bezeichnet, interpretierten sie den Strommaxima üblicherweise direkt einem quanti- zunächst als Messung der Ionisierungsspannung von sierten atomaren Energieniveau zugeordnet werden. Quecksilber ["]. Erst nach weiteren Experimenten zur Lichtemission von Quecksilberdampf erkannten bei- KOMPAKT de, dass es sich um ein Anregungsniveau des Queck- I Das Franck-Hertz-Experiment illustriert mit dem os- silberatoms handelt [#]. Für ihre Arbeiten erhielten zillierenden Anodenstrom in anschaulicher Weise die Franck und Hertz den Nobelpreis für Physik des Jahres quantisierten Energieniveaus der Atome. Dabei bestim- !"#'. men im Allgemeinen nicht nur ein, sondern mehrere Das Experiment selbst ist heute als Standardpro- Energieniveaus den Abstand zwischen den Strom- Prof. Robert E. Rob- dukt im Fachhandel erhältlich, und Generationen von Maxima. son, James Cook Physikstudenten haben es im Praktikum durchgeführt, I Im Gegensatz zur traditionellen Lehrbucherklärung University, Towns- haben die Elektronen in der Driftregion eine breite ville, Australien, Dr. wahrscheinlich primär aus pädagogischen Gründen. Malte Hildebrandt, Energieverteilung und eine nahezu isotrope Geschwin- Die entsprechenden Erklärungen in den üblichen Paul Scherrer Insti- digkeitsverteilung. tut, CH-&#"# Villigen, Lehrbüchern (z. B. [$]) oder auf heutzutage unzähligen I Eine eingehende theoretische Analyse auf der Grundla- Schweiz und Prof. Internetseiten sind jedoch sehr vereinfacht, und die ge der kinetischen Theorie zeigt, wie das Franck-Hertz- Ronald D. White, wahre Vielfalt der involvierten Physik wurde bis vor Experiment mit dem zur Boltzmann-Gleichung gehö- James Cook Univer- sity, Townsville, Franck-Hertz experiment:renden Eigenwertproblem modern interpretation verbundenÜBERBLICK ist. einigen Jahren übersehen [%, &]. Australien © !"#$ Wiley-VCHATOMPHYSIK Verlag GmbH & Co. KGaA, Weinheim #%#&-'$('/#$/"("(-$( Physik Journal !" (#$!%) Nr. " !" Ein Grundstein der Atomphysik Die gängige Lehrbuchinterpretation des Franck-Hertz-Experiments lässt viele Fragen offen. Robert E. Robson, Malte Hildebrandt und Ronald D. White need kinetic theory to explain and understand observations Das Franck-Hertz-Experiment aus dem Jahre !"!# fehlt in keinem Lehrbuch, zeigt es doch anschaulich die quantisierten Eigenschaften der Atome und legt damit den Grundstein für die moderne Atomphysik. Allerdings versagt die traditionelle Interpretation, wenn mehrere angeregte Niveaus ins Spiel kommen. Nur eine Analyse aus dem Blickwinkel der elemen- AIP Emilio Segre Visual Archives Visual Segre Emilio AIP / taren kinetischen Gastheorie erlaubt es, den kor- rekten Zusammenhang zwischen mikroskopischen Vorgängen, die den Gesetzen der Quantenmechanik unterliegen, und den im Labor gemessenen makro- Collection Born Archives, Visual Segre Emilio AIP skopischen Größen herzustellen. A.B. Lagrelius & Westphal & Lagrelius A.B. ie bedeutenden Experimente des späten !". und frühen #$. Jahrhunderts zur Untersuchung von James Franck (!''# – !()%, links) und Wilhelms-Universität Berlin, wo sie ihr D elektrischen Strömen in Gasen leiteten eine neue Gustav Hertz (!''* – !(*&) begegneten bahnbrechendes Experiment durch- Epoche in der Geschichte der modernen Physik ein. sich !(!! am Physikinstitut der Friedrich- führten. Besonders bemerkenswert in diesem Zusammenhang sind die Experimente, die James Franck und Gustav Gegenüber der von Franck und Hertz verwendeten Hertz ab dem Jahr !"!! in Berlin durchführten. Bis zylindrischen Geometrie wird heute meist ein Aufbau !"!% untersuchten sie die Wechselwirkung langsamer mit ebener Elektrodenanordnung vorgezogen (Abb. "). Elektronen mit neutralen Gasatomen [!]. Ihr Ziel war Die Kathode emittiert Elektronen mit einer konstanten eine allgemeine „kinetische Theorie der Elektronen in Rate in den gasgefüllten Driftraum, wo sie beim Durch- Gasen“, da Franck Aussagen von J. S. Townsend zum laufen einer Potentialdifferenz zum Kontrollgitter in Elektronenstreuprozess be zweifelte. elastischen und inelastischen Stößen an den Gasatomen Franck und Hertz verbesserten die experimentellen streuen. Eine Bremsspannung zwischen Kontrollgitter Methoden, mit denen unter anderem Philipp Lenard und Anode erlaubt nur Elektronen ausreichender En- die Kathodenstrahlen in Gasen untersucht hat. Die ergie, die Anode zu erreichen und damit zum Anoden- bahnbrechende Untersuchung in Quecksilberdampf, strom IA beizutragen. Als Funktion der Spannung U os- !"!% veröffentlicht und üblicherweise als „das“ Franck- zilliert dieser Strom, wobei die Abstände ∆U zwischen Hertz-Experiment bezeichnet, interpretierten sie den Strommaxima üblicherweise direkt einem quanti- zunächst als Messung der Ionisierungsspannung
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