Laser spectroscopy on the radioactive element promethium

Promethium (Z = 61) is an exclusively radioactive element with short half-lives of up to 17 years. Consequently, Pm sample amounts that can be safely handled in laboratories are small and data on atomic transitions is scarce. Apart from the heavy actinides and transactinides, Pm is the last element where the first ionization potential (IP) has not been directly measured until now. Here we present the results from resonance ionization spectroscopy of ¹⁴⁷Pm (t_1/2 = 2.6 y) in a hot cavity laser ion source. More than 1000 new optical transitions were recorded in the spectral ranges from 415 - 470 nm and 800 - 910 nm using pulsed Ti:sapphire lasers. Although a straightforward analysis of Rydberg convergences was prevented by complex spectra for high excitation energies, the IP could be determined with a precision of better than 1 cm^-1 by measuring the electric field ionization threshold for several weakly bound states. Finally the hyperfine structure of two subsequent transitions in a newly developed RIS scheme was measured with experimental linewidths of ∼ 120 MHz as preparation for the extraction of nuclear structure parameters in on-line spectroscopy experiments on short-lived Pm isotopes.