First ionization potentials of Fm, Md, No, and Lr; Verification of filling-up of 5f electrons and confirmation of the actinide series

Sato, Tetsuya   ; Asai, Masato  ; Borschevsky, A.*; Beerwerth, R.*; Kaneya, Yusuke*; Makii, Hiroyuki   ; Mitsukai, Akina*; Nagame, Yuichiro ; Osa, Akihiko  ; Toyoshima, Atsushi; Tsukada, Kazuaki  ; Sakama, Minoru*; Takeda, Shinsaku*; Oe, Kazuhiro*; Sato, Daisuke*; Shigekawa, Yudai*; Ichikawa, Shinichi*; Dllmann, Ch. E.*; Grund, J.*; Renisch, D.*; Kratz, J. V.*; Schdel, M.*; Eliav, E.*; Kaldor, U.*; Fritzsche, S.*; Stora, T.*

The first ionization potential (IP) yields information on valence electronic structure of an atom. IP values of heavy actinides beyond einsteinium (Es, Z = 99), however, have not been determined experimentally so far due to the difficulty in obtaining these elements on scales of more than one atom at a time. Recently, we successfully measured IP of lawrencium (Lr, Z = 103) using a surface ionization method. The result suggests that Lr has a loosely-bound electron in the outermost orbital. In contrast to Lr, nobelium (No, Z = 102) is expected to have the highest IP among the actinide elements owing to its full-filled 5f and the 7s orbitals. In the present study, we have successfully determined IP values of No as well as fermium (Fm, Z = 100) and mendelevium (Md, Z = 101) using the surface ionization method. The obtained results indicate that the IP value of heavy actinoids would increase monotonically with filling electrons up in the 5f orbital like heavy lanthanoids.