First measurement of ionization potential casts light on ‘last’ actinide

Description

 Publication in the CERN Courier concerning our article in Nature on the measurement of the first ionization potential of lawrencium (Nature 520, 209 (2015), cover page feature).

 

The quest for new heavy chemical elements is the subject of intense research, as the synthesis and identification of these new elements fill up empty boxes in the familiar Periodic Table. The measurement of their properties for a proper classification in the table has proved challenging, because the isotopes of these elements are short-lived and new methods must be devised to cope with synthesis rates that yield only one atom at a time. Now, an international team led by researchers from the Japanese Atomic Energy Agency (JAEA) in Tokai has developed an elegant experimental strategy to measure the first ionization potential of the heaviest actinide, lawrencium (atomic number, Z = 103).

Using a new surface ion source (figure 1) and a mass-separated beam, the team’s measurement of 4.96±0.08 eV – published recently in Nature (Sato et al. 2015) – agrees perfectly with state-of-the-art quantum chemical calculations that include relativistic effects, which play an increasingly important role in this region of the Periodic Table. The result confirms the extremely low binding energy of the outermost valence electron in this element, therefore confirming its position as the last element in the actinide series. This is in line with the concept of heavier homologues of the lanthanide rare earths, which was introduced by Glenn Seaborg in the 1940s.

In the investigations at JAEA the researchers have exploited the isotope-separation online (ISOL) technique, which has been used for nuclear-physics studies at CERN’s ISOLDE facility since the 1960s (CERN Courier December 2004 p16). The technique has now been adapted to perform ionization studies with the one-atom-at-a-time rates that are accessible for studies of lawrencium. A new surface-ion source was developed and calibrated with a series of lanthanide isotopes of known ionization potentials. The ionization probability of the mass-separated lawrencium could then be exploited to determine its ionization potential using the calibration master curve.

Fig. 2.

The special position of lawrencium in the Periodic Table has placed the element at the focus of questions on the influence of relativistic effects, and the determination of properties to confirm its position as the last actinide. The two aspects most frequently addressed have concerned its ground-state electronic configuration and the value of its first ionization potential.

Relativistic effects strongly affect the electron configurations of the heaviest elements. In the actinides, the relativistic expansion of the 5f orbital contributes to the actinide contraction – the regular decrease in the ionic radii with increasing Z. Together with direct relativistic effects on the 7s and 7p_1/2 orbitals, this influences the binding energies of valence electrons and the energetic ordering of the electron configurations. However, it is difficult to measure the energy levels of the heaviest actinides with Z > 100 by a spectroscopic method because these elements are not available in a weighable amount.

The ground-state electronic configuration of lawrencium (Lr) is expected to be [Rn]5f¹⁴7s²7p_1/2. This is different from that of its homologue in the lanthanide series, lutetium, which is [Xe]4f¹⁴6s²5d. The reason for this change is the stabilization by strong relativistic effects of the 7p_1/2 orbital of Lr below the 6d orbital. Lr, therefore, is anticipated to be the first element with a 7p_1/2 orbital in its electronic ground state. As the measurement of the ionization potential directly reflects the binding energy of a valence electron under the influence of relativistic effects, its experimental determination provides direct information on the energetics of the electronic orbitals of Lr, including relativistic effects, and a test for modern theories. However, this measurement cannot answer questions about the electronic configuration itself. Nevertheless, as figure 2 shows, the experimental result is in excellent agreement with a new theoretical calculation that includes these effects and favours the [Rn]5f¹⁴7s²7p_1/2 ground-state configuration.