Chemistry of the elements at the end of the actinide series using their low-energy ion-beams

Studies of the chemical properties of the elements at the uppermost end of the Periodic Table are extremely challenging both experimentally and theoretically. One of the most important and interesting subjects is to clarify the basic chemical properties of these elements as well as to elucidate the influence of relativistic effects on their electronic configuration. Isotopes of these elements produced at accelerators, however, are short-lived, and the number of produced atoms is so small; any chemistry to be performed must be done on an atom-at-a-time basis that imposes stringent limits on experimental procedures. Here we describe our recent achievements in the effective production of low-energy ion-beams of the elements at the end of the actinide series, fermium (Fm, atomic number Z= 100), mendelevium (Md, Z= 101), nobelium (No, Z= 102), and lawrencium (Lr, Z= 103), using a surface ionization ion-source installed in the ISOL (Isotope Separator On-Line) at the Tandem accelerator facility of JAEA (Japan Atomic Energy Agency). Then the successful measurements of the first ionization potentials (IP1) of these elements with the ISOL setup are reviewed. The measured IP1values increased up to No via Fm and Md, while that of Lr was the lowest among the actinides. Based on the variation of the IP1values of the heavy actinides with the atomic number in comparison with those of the heavy lanthanides, the results clearly demonstrated that the 5forbitals are fully filled at No, and the actinide series ends with Lr. Furthermore, the IP1value of Lr provoked controversy over its position in the Periodic Table, so a short introduction to this issue is presented. The feasibility of the extension of chemical studies to still heavier elements with their ion-beams generated by ISOL is briefly discussed.