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 43 394.45 (19) cm −1 , equivalent to 5.380 226 (24) eV.

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The in-source resonance ionization mass spectrometry technique was applied for quantification of ultratrace amounts of plutonium isotopes as a proof of principle study. In addition to an overall detection limit of 10(4) to 10(5) atoms, this method enables the unambiguous identification and individual quantification of the plutonium isotopes (238)Pu and (241)Pu which are of relevance for dating of radiogenic samples. Due to the element-selective ionization process, these isotopes can be measured even under a high surplus of isobaric contaminations from (238)U or (241)Am, which considerably simplifies chemical preparation. The technique was developed, tested, and characterized on a variety of synthetic and calibration samples and is presently applied to analyze environmental samples.

The ground-state hyperfine structure of neutral 229Th has been measured for the first time using high-resolution resonance ionization spectroscopy. The measurements were performed as a preparatory work aimed at the identification of the predicted low-lying 7.6 eV isomer in 229Th through an investigation of its hyperfine structure. The hyperfine coupling constants have been extracted for the atomic ground state as well as for three excited states at 26 096, 26 113 and 38 278 cm−1. Due to rather small splittings not all hyperfine components were completely resolved and therefore an extensive χ2-error analysis was performed to achieve reliable results. The ground-state transition to the excited state at 38 278 cm−1 was identified to be the most sensitive of the three transitions with regard to the future identification of the isomeric state.

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