February 20, 2017
As a child in the 1950s, I got to see a lot of movies in the back seat of my parents' car at the drive-in theater. Our local drive-in was perhaps unique in having the rear of the giant outdoor projection screen configured as a decorative waterfall, lighted with colored lights as was Niagara Falls. One film I attended, but likely didn't understand, was the 1954 Three Coins in the Fountain, known also for the Frank Sinatra song of the same name.
Fountains figure mightily in the Biblical account of Noah's flood, in which "...all the fountains of the great deep burst forth..." (Genesis 7:11). The Cassini spacecraft, in a 2005 flyby of Saturn's moon, Enceladus, discovered huge (up to 200 kilometer) fountains of water vapor and ice particles.
Closer to home, the ammonia fountain is a simple chemistry experiment that demonstrates the high solubility of ammonia gas in water. Introduction of a small quantity of water into an ammonia-filled flask causes formation of a partial vacuum that draws more water into the flask from a lower reservoir, and that causes an increased suction of more water to dramatic effect.
An impressive fountain arises from the introduction of Mentos candy into a bottle of Diet Coke to cause a Diet Coke and Mentos eruption. This fountain is interesting, since the mechanism for bubble formation is physical, and not chemical. The Mentos introduces nucleation sites for bubbles of carbon dioxide gas that exists in the carbonated beverage.
While the ammonia and Diet Coke fountains illustrate chemical and physical laws, they're not that useful technologically. There is one fountain, the atomic fountain, that forms the basis of extremely accurate clocks. If we force atoms to flow upwards in a fountain, they will eventually fall downwards under the influence of gravity. As the atoms fall, they are weightless, and this reduces the uncertainty in a particular measurement.
The Ramsey method was invented by physicist, Norman Foster Ramsey Jr. (1915-2011), who was awarded the 1989 Nobel Prize in Physics for this research. This method is a technique to synchronize an oscillator to an electron transition in a cloud of atoms. Application of this method is complex, but it allows an accurate measurement of the percentage of atoms that were induced to transition at an applied microwave frequency. In this way, the difference between the applied frequency and the transition frequency can be determined, and the applied frequency can be adjusted to equal the transition frequency.
Accuracy in the frequency measurement is enhanced when the atoms in the cloud are motionless. At the peak of a fountain, the movement of atoms is small, so an atomic fountain allows creation of a highly accurate atomic clock. One such clock is the NIST-F1 at the US National Bureau of Standards and Technology (see photo). This clock, active since 1999, measures a hyperfine transition in a fountain of cesium-133 atoms. The second is defined as 9,192,631,770 periods of this transition.
While generating atomic fountains may seem complicated, an atomic fountain is far easier to create than a fountain of molecules. First, cesium and its companion frequency standard element, rubidium, are singly-charged species. Second, as spherical atoms, there's no complication from rotational forces. The impetus for creation of a molecular fountain is that spectrographic and interferometric resolution in experiments is ultimately limited by how long a particle can be examined. Putting molecules in a fountain allows for long observations and better resolution.[4-5] Of what use is such enhanced resolution? It can yield highly accurate measurements of the proton-to-electron mass ratio; and, perhaps, reveal a permanent electric dipole moment of the electron.
The first molecular fountain, of ammonia molecules, was created by physicists at Vrije Universiteit (Amsterdam, The Netherlands).[4-6] While atomic fountains reduce the velocity of their atoms by laser cooling, this technique won't work for molecules because of their internal energy structure. In the molecular fountain, the molecules are cooled using time-dependent inhomogeneous electric fields applied by electrodes. As shown in the figure, an electrode structure called a Stark decelerator removes energy from the molecular beam to slow it down.
In the molecular fountain experiments, the applied electric fields were able to decelerate the molecules to speeds between 1.4 and 1.9 meters per second. After they peaked in the fountain, they were observed as they fall back under gravity.[4-5] The corresponding transverse temperature of the molecules at that time was less than, or equal to, 10 μK, and the longitudinal temperature was less than, or equal to, 1 μK.[4-5]
The molecular free-fall period was up to 266 milliseconds, which allowed sub-hertz frequency mesurements.[4-5] This measurement time is more than a hundred times larger than that provided by other techniques. The molecules were ionized using a laser beam and detected to differentiate them from molecules that are present in the background gas. At this time, just a single molecule can be examined every five fountain launches, so less than a molecule per second can be examined. Future improvements are expected.
|The NIST-F1 cesium atomic clock in 1999. A companion to NIST-F1, NIST-F2, was activated in April, 2013.|
While there are still some problems that need correction, NIST-F2 has a goal of about a second's error in 300 million years.
(National Institute of Standards and Technology, Physics Laboratory, Time and Frequency Division, photograph, via Wikimedia Commons.)
- You can also tell my age by my use of "lighted," and not "lit," as the past tense of light, an explanation of which can be found at the Grammar Girl Web Site.
- Three Coins in the Fountain (1954, Jean Negulesco, Director) on the Internet Movie Database.
- Scott Milam, "Ammonia Fountain Demo," YouTube Video, January 27, 2015.
- Cunfeng Cheng, Aernout P. P. van der Poel, Paul Jansen, Marina Quintero-Pérez, Thomas E. Wall, Wim Ubachs, and Hendrick L. Bethlem, "Molecular Fountain," Phys. Rev. Lett., vol. 117, no. 25 (December 13, 2016), document 253201, DOI:https://doi.org/10.1103/PhysRevLett.117.253201.
- Cunfeng Cheng, Aernout P. P. van der Poel, Paul Jansen, Marina Quintero-Pérez, Thomas E. Wall, Wim Ubachs, and Hendrick L. Bethlem, "Molecular Fountain," arXiv, November 11, 2016.
- Noah J. Fitch, "Viewpoint: What Goes Up Must Come Down," Physics, vol. 156 (January 9, 2017).
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