Determination of the first ionization potential of actinium

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 430.16em0ex394.450.16em0ex(19)0.28em0excm^-1, equivalent to 5.3800.16em0ex2260.16em0ex(24) eV.