Newly identified energy levels and calculated transition rates in astatine atom, the rarest element on Earth

- A detailed investigation of the atomic structure of At I is presented.
- Spectroscopic designations are given for some experimentally observed energy levels.
- Newly identified energy levels in 6p47p, 6p49p and 6p410p configurations are reported.
- Level energies along the 6p4np and 6p4nd Rydberg series are predicted.
- The influence of intravalence, core-valence and core-core electron correlation is discussed.

A theoretical investigation of the atomic structure and radiative parameters involving the lowest states within the 6p5, 6p47s and 6p47p configurations of neutral astatine is reported for the first time in the present paper. Using different physical models based on the pseudo-relativistic Hartree-Fock approach, the influence of intravalence, core-valence and core-core electron correlation on the atomic parameters is discussed in detail. This work allowed us to establish the intermediate-coupling composition and to clearly fix the spectroscopic designation of six excited levels within the 6p47s and 6p47p configurations which had been located in previous experimental studies. In addition new tentative identifications of four levels belonging to the 6p49p and 6p410p configurations are reported in this work together with predicted level energies along the 6p4np and 6p4nd Rydberg series up to n = 50.