Using resonance ionization spectroscopy the first-ionization potential of actinium has been determined by analyzing different Rydberg series in two-color resonant laser excitation. Three individual Rydberg series were investigated, converging toward the ionic ground state and toward the first- and second-excited state of the actinium ion, respectively. A combined analysis of the convergence limits leads to a consistent value for the first-ionization potential of Ac of $43394.45\left(19\right){\text{cm}}^{-1}$, equivalent to $5.380226\left(24\right)$ eV.

• Received 19 December 2011

DOI:https://doi.org/10.1103/PhysRevA.85.012525