Hyperfine structure has been observed in helium atoms in which one of the electrons has been replaced by an antiproton, providing a test of CPT symmetry
Since Niels Bohr presented his revolutionary model of the atom in 1913, physicists have come a long way in understanding the subtle spectroscopy of atoms. In the mid 1920s the quantized energy levels of orbiting electrons were found to be split into sublevels due to effects associated with the spin of the electron. The discovery of this “fine structure” was followed by that of hyperfine structure even smaller shifts in the electronic energy levels caused by the magnetic interaction between the electrons and the nucleus.
Now Eberhard Widmann and co-workers in the ASACUSA (Atomic Spectroscopy And Collisions Using Cold Antiprotons) collaboration, which includes researchers from Japan, Switzerland and Hungary, have observed hyperfine structure in an exotic form of helium (E Widmann et al. 2002 Phys. Rev. Lett. 89 243402).
In the March issue of Physics World, Yasushi Kino from the Department of Chemistry at Tohoku University in Japan explains these developments.
