Core effects on ionization potentials in thallium

Ionization potentials (IP’s) are evaluated for various excited states of Tl using the relativistic coupled cluster (CCCD) theory in the even-parity pair channel approximation (CCSD-EPC). An average accuracy below half a percent is reached. The effect of deep core electrons on the core-valence correlations is investigated. It is found that electrons in the third subshell $\mathrm{}(n=3\mathrm{})\mathrm{}$ modify the IP’s of the $6p$ orbitals by $100\hspace{0.5em}{\mathrm{cm}}^{-1}.$ By comparison with calculations made in the linearized CCSD (LCCSD) approximation it is demonstrated that nonlinear contributions are mandatory to reach an accuracy below half a percent for the ${6p}_{1/2}$ orbital.