Atomic transitions and the first ionization potential of promethium determined by laser spectroscopy

The atomic spectrum of neutral promethium has been studied extensively by laser resonance ionization spectroscopy.We report on more than 1000 atomic transitions in the blue and near infrared spectral ranges, most of them between high excited energy levels. As Rydberg convergences could not be assigned unambiguously in the dense spectrum at high excitation energies, the first ionization potential (IP) was determined via field ionization of weakly bound states within a static electric field. By applying the saddle-point model, a value of IP(Pm) = 45 020.8(3) cm−1 [5.58188(4) eV] was derived, which confirms previous expectations of 45 027(80) and 44 985(140) cm−1, which were obtained indirectly from lanthanide IP systematics.