Atomic physics gets its own ‘periodic table’ that covers highly charged ions

A new chart for highly charged ions (HCIs) has been proposed, aiming to replicate the conventional periodic table’s accessibility and patterns for the cutting edge of atomic physics. This table could help physicists that are looking to produce the next generation of atomic clocks.

The periodic table, created by Dmitri Mendeleev in 1869, is one of the most recognisable icons of chemistry, allowing scientists to understand an element’s properties at a glance. A key part of the modern table is the Aufbau, or ‘building up’ principle, showing how atomic orbitals are filled according to their relative energies, creating the s, p, d and f blocks. This gives the table its unique shape, and recognisable categories, such as the transition metals and the lanthanide and actinide series. Today, chemists use the table to predict the outcome of reactions, allowing them to create new structures, materials and medicines.

However, while the classic table is a useful guide for neutral atoms, its predictable patterns of elemental properties break down for HCIs. HCIs are particles where most of the outer shell of an atom’s electrons has been stripped off to create a cation. Although HCIs are rare, they are increasingly used in areas of physics such as studying astronomical plasmas, or developing x-ray lasers, anti-cancer therapies and optical clocks.

Now, a team from the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, has proposed a new ‘periodic table’ that will allow physicists to classify these HCIs and understand their behaviour. HCIs fill orbitals with electrons differently because of a stronger electromagnetic interaction with the nucleus, known as the Coulomb force. This means electrons move at higher speeds, resulting in relativistic effects that impart different properties to HCIs than other ions. However, this force is also predictable, allowing the group to identify a filling pattern.

Rather than arranging the table by the number of protons in the nucleus, which gives each element its atomic number, the HCI table is arranged according to the number of electrons, with a reference in each place on the table to indicate at which point relativistic effects overcome electron–electron interactions. The result is a new type of periodicity, which allowed the team to quickly spot patterns and trends in HCIs that have previously been tricky to determine.

For example, HCIs of d and f block elements have such complex structures that their ground states have been difficult to identify; however, the team found their table allowed them to quickly work out an HCI’s possible angular momentum, and use the pattern to determine ground states. The team hopes that the new periodicity will also allow physicists to spot patterns in excited states that might otherwise have been missed. The table, they suggest, also highlights so-called ‘forbidden transitions’, low-probability changes that seem to break simplified quantum mechanics rules but are essential for laser pulses and atomic clocks.

Just like its traditional counterpart, the HCI table could create an at-a-glance tool to allow physicists to predict the outcome of experiments and improve our understanding of plasmas, lasers and astronomic spectroscopy. This, in turn, could speed up research, such as in identifying ions for precise measurements of time.

If so, says Juris Meija, a senior research officer at the National Research Council of Canada, Ottawa, the HCI table could help with the ongoing redefinition of the second, which is based on the latest generation of atomic clocks. ‘Thus,’ Meija says, ‘the periodic table of highly charged ions might well prove to be a new guiding tool to seek even better optical clocks in the future.’