On This Day: Quantum experiment "transformed our understanding of the world" - Apr. 24, 1914
(edited from Wikipedia)
"
Franck-Hertz experiment
The Franck–Hertz experiment was the first electrical measurement to clearly show the quantum nature of atoms, and thus "transformed our understanding of the world". It was presented on April 24, 1914, to the German Physical Society in a paper by James Franck and Gustav Hertz.
Franck and Hertz had designed a vacuum tube for studying energetic electrons that flew through a thin vapor of mercury atoms. They discovered that, when an electron collided with a mercury atom, it could lose only a specific quantity (4.9 electron volts) of its kinetic energy before flying away. This energy loss corresponds to decelerating the electron from a speed of about 1.3 million meters per second to zero. A faster electron does not decelerate completely after a collision, but loses precisely the same amount of its kinetic energy. Slower electrons merely bounce off mercury atoms without losing any significant speed or kinetic energy.
[Quantum energy levels]
These experimental results proved to be consistent with the Bohr model for atoms that had been proposed the previous year by Niels Bohr. The Bohr model was a precursor of quantum mechanics and of the electron shell model of atoms. Its key feature was that an electron inside an atom occupies one of the atom's "quantum energy levels".
Before the collision, an electron inside the mercury atom occupies its lowest available energy level. After the collision, the electron inside occupies a higher energy level with 4.9 electron volts (eV) more energy. This means that the electron is more loosely bound to the mercury atom. There were no intermediate levels or possibilities in Bohr's quantum model. This feature was "revolutionary" because it was inconsistent with the expectation that an electron could be bound to an atom's nucleus by any amount of energy.
[Mercury atoms]
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. They showed that the wavelength of this ultraviolet light corresponded exactly to the 4.9 eV of energy that the flying electron had lost. 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. Therefore, Bohr had followed the instructions given in 1911 and copied the formula proposed by Lorentz and others into his 1913 atomic model. Lorentz had been correct. The quantization of the atoms matched his formula incorporated into the Bohr model.
["... it makes you cry"]
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."
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".
"
https://en.wikipedia.org/wiki/Franck%E2%80%93Hertz_experiment
(edited from article)
"
The Franck-Hertz Experiment
In 1914, James Franck and Gustav Hertz performed an experiment which demonstrated the existence of excited states in mercury atoms, helping to confirm the quantum theory which predicted that electrons occupied only discrete, quantized energy states. Electrons were accelerated by a voltage toward a positively charged grid in a glass envelope filled with mercury vapor. Past the grid was a collection plate held at a small negative voltage with respect to the grid. The values of accelerating voltage where the current dropped gave a measure of the energy necessary to force an electron to an excited state.
Electrons are accelerated in the Franck-Hertz apparatus and the collected current rises with accelerated voltage. As the Franck-Hertz data shows, when the accelerating voltage reaches 4.9 volts, the current sharply drops, indicating the sharp onset of a new phenomenon which takes enough energy away from the electrons that they cannot reach the collector. This drop is attributed to inelastic collisions between the accelerated electrons and atomic electrons in the mercury atoms. The sudden onset suggests that the mercury electrons cannot accept energy until it reaches the threshold for elevating them to an excited state. This 4.9 volt excited state corresponds to a strong line in the ultraviolet emission spectrum of mercury at 254 nm (a 4.9eV photon). Drops in the collected current occur at multiples of 4.9 volts since an accelerated electron which has 4.9 eV of energy removed in a collision can be re-accelerated to produce other such collisions at multiples of 4.9 volts. This experiment was strong confirmation of the idea of quantized atomic energy levels.
"
http://hyperphysics.phy-astr.gsu.edu/hbase/FrHz.html
(edited from article)
"
The Franck-Hertz Experiment
[Bohr model]
In 1913 Niels Bohr introduced his model of the hydrogen atom. One of the predictions was that the electrons occupied only certain energy levels. This was in agreement with the observed spectrum, but physicists were eager to find another experiment that would also show this result.
[Experiment described]
In 1914 James Franck and Gustav Hertz (nephew of Heinrich) performed an experiment on a vacuum tube with a small amount of mercury enclosed. The tube was heated in an oven in order to vaporize the mercury, and then a series of voltages was applied to the tube. A small voltage was used to heat a filament for use as an electron source. Three more voltages were used to establish electric fields inside the tube.
The first field is a small field, it used in order sweep the electrons away from the filament. It is observed that when filaments eject electrons they become slightly positive, and the region around the filament becomes slightly negative due to the cloud of electrons. If a small field isn't put in place to draw the electrons away from the filament, it becomes hard to draw out more electrons. The second field is an accelerating field, this is what gives the electrons the bulk of their kinetic energy. This is usually called the grid voltage because it is established by a grid that the electrons can penetrate. Once the electrons go through the grid there is a reverse field that acts to retard electrons from the counter. If there is only vacuum in the tube then the grid voltage will accelerate electrons to the counter, and if the retarding voltage is less than the grid voltage a current will be detected.
The main key here is that electrons are drawn off of a filament, and are excited by a potential. They speed up, gaining kinetic energy. They must pass through a dilute gas, and when they collide with the gas atoms they give up energy in discrete packets. There is also a retarding voltage to make sure that only electrons with enough kinetic energy get counted as part of the current.
In the Franck-Hertz experiment the low-pressure mercury vapor affects the detected current. At low grid voltages the electrons gain kinetic energy. They collide with mercury atoms, but these are elastic collisions, and since electrons have such a smaller mass than mercury the electrons retain most of their kinetic energy. As the voltage increases, so does the current. However, once the electrons gain kinetic energy equal to the excitation energy of mercury they can have inelastic collisions - the kinetic energy of the electron exciting the mercury atom. If an electron with exactly the excitation energy had an inelastic collision, it would have a final velocity of zero. This is why the experiment also features a retarding field. Only electrons with a kinetic energy that can overcome the retarding field get counted in the current. As the grid voltage is increased to the excitation voltage, the current will drop because many of the filament electrons have lost their kinetic energy to inelastic collisions and cannot overcome the retarding field. If the observed current is plotted against voltage, the there will be a series of peaks and valleys. The peak-to-peak (or valley-to-valley) spacing will correspond to the excitation energy of the vapor (the multiple valleys are due to multiple inelastic collisions).
"
https://foothill.edu/psme/marasco/4dlabs/4dlab8.html
---------------------------------------------------------
On This Day: Romans double-up on temples to Venus to promote harmony and cooperation - Apr. 23, 181 BC
https://www.democraticunderground.com/1016376365
On This Day: German Army releases 168 tons of chlorine gas - Apr. 22, 1915
https://www.democraticunderground.com/1016376343
On This Day: U.S. blockades Cuba, leading to control of former Spanish colonies worldwide - Apr. 21, 1898
https://www.democraticunderground.com/1016376278
On This Day: Louis Pasteur disproves spontaneous generation, and invents pasteurization - Apr. 20, 1862
https://www.democraticunderground.com/1016376268
On This Day: Oklahoma City bombing continues to inspire extremists - Apr. 19, 1995
https://www.democraticunderground.com/1016376164
