Franck–Hertz experiment

It was presented on April 24, 1914, to the German Physical Society in a paper by James Franck and Gustav Hertz.

[1][2] Franck and Hertz had designed a vacuum tube for studying energetic electrons that flew through a thin vapor of mercury atoms.

[3][5] In a second paper presented in May 1914, Franck and Hertz reported on the light emission by the mercury atoms that had absorbed energy from collisions.

The relationship of energy and wavelength had also been predicted by Bohr because he had followed the structure laid out by Hendrik Lorentz at the 1911 Solvay Congress.

At Solvay, Hendrik Lorentz suggested after Einstein’s talk on quantum structure that the energy of a rotator be set equal to nhv.

[3] After a presentation of these results by Franck a few years later, Albert Einstein is said to have remarked, "It's so lovely it makes you cry.

"[10] On December 10, 1926, Franck and Hertz were awarded the 1925 Nobel Prize in Physics "for their discovery of the laws governing the impact of an electron upon an atom".

Franck and Hertz noted in their first paper that the 4.9 eV characteristic energy of their experiment corresponded well to one of the wavelengths of light emitted by mercury atoms in gas discharges.

[15] In a second paper, Franck and Hertz reported the optical emission from their tubes, which emitted light with a single prominent wavelength 254 nm.

[13] Franck and Hertz explained their experiment in terms of elastic and inelastic collisions between the electrons and the mercury atoms.

A short time later, the 4.9 eV of energy that was deposited into the mercury atom is released as ultraviolet light that has a wavelength of precisely 254 nm.

Further increases in the grid voltage restore enough energy to the electrons that suffered inelastic collisions that they can again reach the anode.

Electrons that have traveled roughly halfway from the cathode to the grid have already acquired enough energy to suffer a first inelastic collision.

[20] The fundamental assumption of the Bohr model concerns the possible binding energies of an electron to the nucleus of an atom.

[5] Franck and Hertz had proposed that the 4.9 V characteristic of their experiments was due to ionization of mercury atoms by collisions with the flying electrons emitted at the cathode.

In 1915 Bohr published a paper noting that the measurements of Franck and Hertz were more consistent with the assumption of quantum levels in his own model for atoms.

Writing following the end of World War I in 1918, Franck and Hertz had largely adopted the Bohr perspective for interpreting their experiment, which has become one of the experimental pillars of quantum mechanics.

[10][2] As Abraham Pais described it, "Now the beauty of Franck and Hertz's work lies not only in the measurement of the energy loss E2-E1 of the impinging electron, but they also observed that, when the energy of that electron exceeds 4.9 eV, mercury begins to emit ultraviolet light of a definite frequency ν as defined in the above formula.

This glow will move closer to the cathode with increasing accelerating potential, and indicates the locations where electrons have acquired the 18.7 eV required to excite a neon atom.

Photograph of a sealed glass cylinder. Wires penetrate the cylinder at its top, bottom, and side. Three wires lead to a cathode assembly; the top and side wires lead to a disk and a mesh that are close and parallel to each other. The wires are attached to feedthroughs on an aluminum panel in the background. — Photograph of a vacuum tube used for the Franck–Hertz experiment in instructional laboratories. There is a droplet of mercury inside the tube, although it is not visible in the photograph. C – cathode assembly; the cathode itself is hot, and glows orange. It emits electrons which pass through the metal mesh grid (G) and are collected as an electric current by the anode (A).

Wavelengths of light emitted by a mercury vapor discharge and by a Franck–Hertz tube in operation at 10 V. The Franck–Hertz tube primarily emits light with a wavelength near 254 nanometres; the discharge emits light at many wavelengths. Based on the original 1914 figure. ^{[

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Drawing showing three circles, each with a label "Hg" inside. The top circle is labeled "elastic collision". It is next to two arrows of equal length, one pointing towards the circle, and one pointing away. The middle circle is labeled "inelastic collision", and has a longer arrow pointing towards it, and a shorter arrow leading away. The lowest circle is labeled "light emission", and is next to a squiggly arrow that points away. — Elastic and inelastic collisions of electrons with mercury atoms. Electrons traveling slowly change direction after elastic collisions, but do not change their speed. Faster electrons lose most of their speed in inelastic collisions. The lost kinetic energy is deposited into the mercury atom. The atom subsequently emits light, and returns to its original state.

The drawing has a wide rectangle at the top labeled "vacuum levels". Underneath the rectangle and to the left is a vertical arrow that ends at the rectangle; the arrow is labeled "electron binding energy". In the middle is a long series of finely separated lines that are parallel to the bottom of the rectangle; these are labeled "classical energy levels". To the right is a series of four well-separated parallel lines; these are labeled "quantum energy levels". — The Bohr model of the atom assumed that an electron could be bound to an atomic nucleus only with one of a series of specific energies corresponding to quantum energy levels. Earlier, classical models for the binding of particles allowed any binding energy.

Franck-Hertz experiment with Neon resulting in glowing regions appearing