Measuring ionization potential one atom at a time

Calculating the electronic structure of an atom with 100 or more electrons is hard, mainly because of relativistic effects: Exact solutions aren’t computationally feasible, and it’s not known what approximate methods offer the best trade-off between speed and accuracy in such highly relativistic systems. Experimental studies can be even more difficult: Elements beyond fermium (atomic number Z = 100) can be produced just one atom at a time, and no atomic energy levels are confidently known for any of them. Now Yuichiro Nagame (Japan Atomic Energy Agency) and his colleagues have taken a step toward filling that knowledge void by measuring the ionization potential—the energy required to remove the most weakly bound electron—of lawrencium (Z = 103). Such a measurement usually requires a sample of 10¹⁰–10¹² atoms, but Nagame and colleagues did it with just a few thousand using the approach sketched in the figure. They produced ²⁵⁶Lr atoms, one every few seconds, by bombarding a californium-249 target with a boron-11 beam and then guided them into a 2700-K tantalum cavity, where some of the atoms were ionized on the hot metal surface. The newly formed Lr⁺ ions were extracted electrostatically and their masses verified with a dipole magnet. The ion yield depends on Lr’s ionization potential, Ta’s work function, the temperature, and the experimental geometry. Accounting for all those factors, the researchers determined the ionization potential to be 4.96 ± 0.08 eV. Though that uncertainty is still relatively high, the measurement will help guide future theoretical and experimental work on Lr and other superheavy elements. (T. K. Sato et al., Nature 520, 209, 2015 .)