Measurements of the first ionization potentials of heavy actinides

Sato, Tetsuya   ; Asai, Masato  ; Kaneya, Yusuke*; Tsukada, Kazuaki  ; Toyoshima, Atsushi; Mitsukai, Akina*; Takeda, Shinsaku*; Vascon, A.*; Sakama, Minoru*; Sato, Daisuke*; Oe, Kazuhiro*; Miyashita, Sunao*; Shigekawa, Yudai*; Borschevsky, A.*; Makii, Hiroyuki   ; Osa, Akihiko  ; Ichikawa, Shinichi*; Nagame, Yuichiro ; Schdel, M.*; Eberhardt, K.*; Runke, J.*; Thrle-Pospiech, P.*; Renisch, D.*; Eliav, E.*; Kaldor, U.*; Kratz, J. V.*; Stora, T.*; Dllmann, Ch. E.*; Trautmann, N.*

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.