PhysicsLAB Resource Lesson
Famous Discoveries: The Franck-Hertz Experiment

James Franck and Gustav Hertz received the 1925 Nobel Prize in Physics for their "discovery of the laws governing the impact of an electron upon an atom." 
In their experiments with mercury during 1912-1914, these two scientists confirmed Bohr's hypothesis of discrete atomic energy states and the resulting spectral lines atoms radiate. Bohr had suggested that monochromatic light was emitted only when electrons gave up a discreet amount of energy equal to the difference between two energy states. The experimentation of Franck-Hertz showed that the kinetic energy of cathode rays (electrons) could be used to "boost" steady-state orbital electrons to higher energy states. The energy required was documented as a lose of kinetic energy in the electron beam.
Their experimental numerical results were not only in agreement with Bohr's theory but also corresponded to know wavelengths in mercury's spectrum. In their work, they cited the 1908 work of Walter Ritz which stated that all of an atom's radiated frequencies (spectral lines) were equal to either the sum or the difference of two other emitted frequencies as well as Philipp Lenard's 1905 Nobel Prize winning work with cathode ray tube design and properties of electron beams.
Energized electrons (cathode rays) were released from a filament and sent through a tube filled with low pressure mercury. Mercury atoms (atomic number 80) are monatomic with 78 of their electrons closely bound to their nuclei. Only the two outer-most electrons are "free" to interact with the cathode rays. When low-energy electrons (0-4.5 eV) in the cathode rays struck the ground-state Mercury atoms, the collisions appeared to be elastic - that is, the electron bounced off the Mercury atoms with very little loss of energy. This was measured by the fact that the stopping potential almost exactly equaled the original accelerating potential for the electron beam.

Image courtesy of Kansas State University
However, when electrons were accelerated to energies between 5 and 6 eV, the stopping potentially radically changed to only 0.1 to 1.1 eV. The electrons consistently lost 4.9 eV of energy during their collisions with the Mercury atoms. The collisions were no longer elastic, they were now inelastic and a high percentage of the original kinetic energy was absorbed as potential energy by the Mercury atoms. They concluded that the Mercury atoms were entering a new "stable state" with the addition of the 4.9 eV of energy transferred from the electrons. Their results confirmed that ground state Mercury atoms could only accept 4.9 eV of energy - no more, no less.
If the voltage between the filament and the grid is increased, the amount of current would increase; but the current would still exhibit consistent "drop-offs" at intervals of 4.9 eV. It is as if the electrons, upon losing some their initial energy upon colliding with one Mercury atom (4.9 eV), are accelerated by the field and upon attaining 4.9 eV or more, can once again exchange energy with a subsequent Mercury atom.

Image courtesy of Kyushu University
Upon learning of Bohr's hypothesis, Franck and Hertz tested the radiation from heated Mercury gas and confirmed that there was indeed an emission line corresponding to 4.9 eV which had a frequency of 1.183 x 1015 hz, corresponding to a wavelength of 254 nm. The kinetic energy lost in collisions between the electrons in the cathode ray and the ground-state Mercury atoms exactly matched the emission spectra of the heated gas.
Bibliography: - Franck-Hertz Experiment
Hyperphysics - The Franck-Hertz Experiment
Hyushu University - 4-3: The Bohr Model of Atoms
Kansas State University - Visual Quantum Mechanics - The Nobel Prize in Physics 1905 Presentation Speech - The Nobel Prize in Physics 1925 Presentation Speech
Space Research Institute - (Q-3) Atomic Energy Levels
University of California at San Diego - Experiment 5: The Franck-Hertz Experiment

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