The lowest-lying odd-parity energy levels of the atomic system of actinium have been predicted by theory for a long time, but were never experimentally observed. A team of scientists from the group of Prof. Budker at HIM and the LARISSA group finally revealed these "missing" lines in a laser spectroscopic experiment.
The extracted properties of the atomic transitions were compared to state-of-the-art atomic structure calculations and were found to be in perfect agreement. The collaboration of experimenalists and theorists from China, Germany, USA and Russia published the results in Physical Review Letters.