A new version of the periodic table of elements has predicted hundreds of highly charged ions that could be used to create the next generation of optical atomic clocks.
The periodic table, first invented by Dmitri Mendeleev in 1869, groups the 118 known chemical elements according to their chemical properties. Because elements in the same parts of the periodic table share similar characteristics, this ordering enabled chemists to identify gaps in the table decades ago, and has since helped them discover the elements to fill them.
The table works very well for chemists in general, but for some physicists, who are more interested in finding and using high-energy ions, it doesn鈥檛 do the job they want. Such particles are used in X-ray lasers, in tumour therapy, in plasmas, to test fundamental physics theories and in optical clocks.
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鈥淲e wanted to search for highly charged ions for atomic clocks, to make them much more stable and much more accurate, says at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany.
Atoms consist of a nucleus containing protons and neutrons, with electrons arranged in shells and subshells outside the nucleus. In an atom, there are equal numbers of positively charged protons and negatively charged electrons. But atoms can gain or lose electrons, forming charged ions. An atom that loses many electrons becomes a highly charged ion.
The original periodic table is ordered according to the number of protons there are in an atom of each element. Instead, Lyu and his colleagues arranged their table according to the number of electrons in ions. Once the atom of one element has lost one or more electrons, it can have the same number of electrons as an atom of another element. This means that each cell of the table can contain ions of multiple elements sharing the same configuration of electrons, says Lyu.
It results in a table in which each row represents an electron shell and each column represents a subshell. The layout has allowed Lyu and his colleagues to predict what are known as forbidden transitions.
If an atom absorbs energy 鈥� for example, by colliding with another atom 鈥� electrons can move from one shell or subshell to another. According to quantum theory, some of these transitions are much more likely than others, depending on the shells where electrons start and finish.
But there are also rare, unusual transitions that aren鈥檛 strictly impossible, just highly improbable and slow to occur. These are known as forbidden transitions and because they take longer, they are very stable, which makes them ideal for designing optical atomic clocks.
Lyu and his colleagues have used their table to predict the existence of 700 highly charged ions that could be used for these transitions to make more accurate optical atomic clocks.
Mark Leach, a chemist who keeps and runs the Meta-Synthesis chemistry consultancy service, says all those forbidden transitions could be out there.
Now that the transitions have been theoretically predicted, says Lyu, you could tune the energy of an electron beam to collide with atoms to generate the high-energy ion you want and hold it in this forbidden state with lasers.
The ion could then be measured experimentally with spectroscopy to learn more about the energetic structure of the electrons spinning around the nucleus, and employed to construct even more accurate atomic clocks, he says.
Such timepieces could aid navigation for spacecraft far from Earth, help coordinate satellites, test Albert Einstein鈥檚 theory of relativity and run quantum communications networks.
鈥淭his is very far from the main idea of the periodic table. It鈥檚 a configuration of highly ionised elements,鈥� says at the Max Planck Institute for Mathematics in the Sciences in Leipzig, Germany. 鈥淏ut they have found interesting and forbidden transitions, which opens a new path for improving atomic clocks, which is really important.鈥�
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