First determination of the ionization potential of actinium and first observation of optical transitions in ferminm

Abstract

For the determination of the first ionization potential of actinium, 227Ac was electrodeposited on a Ta backing and covered with –1 μm Zr. From this filament, Ac atoms were evaporated at ≥ 1250 °C. By resonant excitation with UV light of 388.67 nm and subsequent excitation with light of ca. 568 nm, Ac was ionized in an external electrical field. By determining the ionization thresholds as a function of the electrical field strength and by extrapolation to zero field strength, the first ionization potential of 43398(3) cm−1 = 5.3807(3) eV was measured. About 1 ng of 255Fm, half life 20.1 h, was prepared at ORNL by milking from 255Es produced in the High Flux Isotope Reactor and shipped to Mainz. Here, a fraction of 2.7 · 1010 atoms was electrodeposited on a Ta backing and covered with ˜ 1 μm Ti. From this filament, Fm atoms were evaporated at ˜ 1000 °C in the buffer gas of an optical cell and resonantly excited and ionized with two beams of an excimer-dye-laser combination. The ions were identified by mass-selective detection employing the Ion-Guide-Quadrupole-Mass Separation (IGQMS) technique. Two resonant transitions were found at energies of 25099.8 cm−1 and 25111.8 cm−1. © 2014 Taylor and Francis Group, LLC.