Verification of Filling-Up of 5f Electrons and Confirmation of the Actinide Series
by Determination of the First Ionization Potentials of Fm, Md, No, and Lr
T. K. Sato1), M. Asai1), A. Borschevsky3), R. Beerwerth4,5), Y. Kaneya1,2), H. Makii1),A. Mitsukai1,2),
Y. Nagame1), A. Osa6), A. Toyoshima1), K. Tsukada1), M. Sakama7), S. Takeda7), K. Ooe8), D. Sato8),
Y. Shigekawa9), S. Ichikawa10), C. E. Düllmann11,12,13), J. Grund11,12), D. Renisch11,12), J. V. Kratz11),
M. Schädel11), E. Eliav14), U. Kaldor14), S. Fritzsche4,5), and T. Stora15)
1): R.G. for Heavy Element Nuclear Science, JAEA 2): Ibaraki Univ. 3): Univ. of Groningen4): Friedrich-Schiller-Universität 5): Helmholtz-Institut Jena,
6): Section for Research Reactor and Accelerator, JAEA 7): Tokushima Univ. 8) Niigata Univ.
9): Osaka Univ. 10): RIKEN, 11): Johannes Gutenberg-Universität Mainz
12): Helmholtz-Institut Mainz 13): GSI 14): Tel Aviv Univ., 15): ISOLDE, CERN

The first ionization potential (IP1) of an atom is one of the mostfundamental chemical and physical quantities of every element,
which gives direct information about the binding energy of anelectron in the outermost electronic orbital of an atom. Accurate
IP1 values of heavy elements provide crucial tests for ourunderstanding of their electronic structure affected by strong
relativistic effects caused by their large electric charge of heavy
nuclei. IP1 values of heavy elements with atomic number Z ≥ 100,
however, could not be determined experimentally, because theirproduction rates and half-lives drastically decrease as the atomic
number increases. The study of these elements therefore requires
new techniques on an atom-at-a-time scale.
Recently, the successful measurements of IP1 of lawrencium(Lr) and nobelium (No) in an atom-at-a-time scale have beenreported [1, 2]. Those were measured by using a method based on
surface ionization coupled to mass separation and α-particle
detection techniques, and a laser resonance ionization
spectroscopy, respectively. However, a systematic IP1
measurement from fermium (Fm) to Lr has not been performed.
In this work, we have applied the surface-ionization method [1]to determine the IP1 values of Fm, mendelevium (Md), and No.
In addition, IP1 of Lr has been also measured to improve theaccuracy of the previously reported IP1. Surface ionization
process takes place on a solid surface kept at a high temperatureand can be described by the Saha−Langmuir equation. Theionization efficiency (Ieff) depends on the work function of theionizing material, ϕ (eV), the temperature of the material surface,T (K), and IP1 of the element. Short-lived isotopes 249Fm (T1/2 =2.6 min), 251Md (T1/2 = 4.27 min), 257No (T1/2 = 24.5 s), and 256Lr(T1/2 = 27 s) were produced in nuclear fusion reactions of actinide
targets with heavy ion beams. The produced atoms recoiling fromthe target were transported to a surface ion-source installed at theJAEA-ISOL (Isotope Separator On-Line) by the He/CdI2 gas-jet
transport system. Transported products were surface ionized on
the surface of the Ta cavity kept at temperatures between 2550
and 3000 K in the ion source. Produced ions were extracted andmass separated in the ISOL. The number of collected ions after
the mass separation was determined by α spectrometry tocalculate Ieff values. Measured Ieff values for each isotope gave
IP1 values of the elements. The obtained IP1 values of Fm, Md,
No and Lr were determined to be 6.52 ± 0.13, 6.59 ± 0.13,
0.06+ 0.05
6.62+
− 0.07 , and 4.96− 0.04 eV, respectively.
The variation of the IP1 values of the heavy actinides withatomic number in comparison with those of the heavy lanthanides

is shown in Figure 1. The measured IP1 values agree well withthose predicted by state-of-the-art relativistic calculations
performed alongside the present measurements [3]. As expectedfrom the prediction, the IP1 values increase up to No via Fm andMd with atomic number in analogy to the heavy lanthanides. We
take this as an indication that the 5f orbital is fully filled at Nowith the [Rn]5f147s2 configuration. The lowest IP1 value of Lr isconfirmed; the ground-state electronic configuration of the Lr
atom has closed 5f14 and 7s2 shells with an additional weakly
bound electron in the valence orbital. This IP1 structure is similar
to that found in heavy lanthanides, confirming unambiguouslythat the actinide series end with Lr.
References[1] T. K. Sato et al., Nature 520, 209-211(2015).
[2] P. Chhetri et al., Phys. Rev. Lett. 120, 263003 (2018).
[3] T. K. Sato et al., J. Am. Chem. Soc. 140, 14609-14613(2018).
[4] A.Kramida et al., NIST Atomic Spectra Database,https://physics.nist.gov/asd

Fig.1 The first ionization potentials of heavy lanthanides (blue) andactinides (red) including the present results for Fm, Md, No and Lr.
A closed symbol indicates the data obtained in this work. Opensymbols are taken from literature values [4]