First Ionization Potentials of Fm

We report the first ionization potentials (IP1) of the heavy actinides, fermium (Fm, atomic number Z = 100), mendelevium (Md, Z = 101), nobelium (No, Z = 102), and lawrencium (Lr, Z = 103), determined using a method based on a surface ionization process coupled to an online mass separation technique in an atom-at-a-time regime. The measured IP1 values agree well with those predicted by state-of-the-art relativistic calculations performed alongside the present measurements. Similar to the well-established behavior for the lanthanides, the IP1 values of the heavy actinides up to No increase with filling up the 5f orbital, while that of Lr is the lowest among the actinides. These results clearly demonstrate that the 5f orbital is fully filled at No with the [Rn]5f7s configuration and that Lr has a weakly bound electron outside the No core. In analogy to the lanthanide series, the present results unequivocally verify that the actinide series ends with Lr. E the periodic table and classifying newly discovered heavy elements are among the most fundamental and exciting aspects of the chemical sciences. This leads to architect the periodic table and revise its structure in the heavy element region. The most recent revision of the structure of the periodic table took place in the 1940s when Glenn T. Seaborg introduced the ground-breaking actinide concept, placing a new actinide series below the lanthanides. In this new series, the 5f electron shell is filled in a manner similar to the filling of the 4f electron shell in lanthanides. The actinide concept did not only allow for the immediate discoveries of the elements 95, americium, and 96, curium, but was also instrumental for the discovery of heavier ones. Chemical properties of weighable amounts of nuclear-reactorproduced actinides up to Fm have been extensively studied. However, much less is known about the heavier actinides due to stringent limitation on experimental procedures with increasing atomic number as these heavy elements are available in decreasing quantities of only one atom at a time. The first ionization potential (IP1) of an atom is one of the most fundamental chemical and physical quantities of every element. The first measurements of IP1 of actinides were performed by a surface ionization technique. Then laser spectroscopy and resonance ionization mass spectroscopy of macroscopically available actinides up to einsteinium have been conducted to measure accurate IP1 values. 8−11 Received: September 7, 2018 Published: October 25, 2018 Communication pubs.acs.org/JACS Cite This: J. Am. Chem. Soc. 2018, 140, 14609−14613 © 2018 American Chemical Society 14609 DOI: 10.1021/jacs.8b09068 J. Am. Chem. Soc. 2018, 140, 14609−14613 This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. D ow nl oa de d vi a U N IV G R O N IN G E N o n M ar ch 1 , 2 01 9 at 1 4: 28 :4 5 (U T C ). Se e ht tp s: //p ub s. ac s. or g/ sh ar in gg ui de lin es f or o pt io ns o n ho w to le gi tim at el y sh ar e pu bl is he d ar tic le s. Recently, we reported the successful measurement of IP1 of Lr in an atom-at-a-time scale experiment using a method based on surface ionization coupled to mass separation and α-particle detection techniques. The result suggested that Lr has the lowest IP1 value of all actinide elements, although those of other heavy actinides, Fm, Md, and No, have not yet been determined experimentally. According to the systematic variation of the IP1 values of heavy actinides, an increasing trend is anticipated up to No due to filling electrons up in the 5f orbital. Nobelium is expected to have the highest IP1 among the actinides due to the closed-shell structure of [Rn]5f7s. Very recently laser resonance ionization spectroscopy of No, using No (half-life, T1/2 = 51.2 s) in one-atomat-a-time quantities, was performed and the IP1 has been measured to be 6.62621 ± 0.00005 eV, supporting the scenario of closed 5f and 7s atomic shells in No. However, to unequivocally confirm the filling of the 5f electron shell in the heavy actinides, it is indispensable to experimentally determine the successive IP1 values from Fm to Lr. In the present study, we have applied the earlier developed surface-ionization method to determine the IP1 values of Fm, Md, and No. In addition, IP1 of Lr has been also measured to improve the accuracy of the previously reported IP1. 12 Surface ionization process takes place on a solid surface kept at a high temperature and can be described by the Saha−Langmuir (SL) equation. The ionization efficiency (Ieff) depends on the work function of the ionizing material, φ (eV), the temperature of the material surface, T (K), and IP1 of the element. The detailed experimental setup and the analytical method used in this work have been described in our previous papers. Short-lived isotopes Fm (T1/2 = 2.6 min), 251 Md (T1/2 = 4.27 min), No (T1/2 = 24.5 s), and Lr (T1/2 = 27 s) were produced in nuclear fusion reactions (Supplement Table 1). The produced atoms, recoiling from the target, were transported via a Teflon capillary to a surface ion-source installed at the JAEA-ISOL (Isotope Separator Online) by the He/CdI2 gas-jet transport system. 21 Transported products were injected into the ionization cavity of the ion-source. Metallic tantalum (Ta) was selected as the cavity material in this work. The products were surface-ionized on the hot surface of the Ta cavity kept at a temperature between 2550 and 3000 K. Produced ions are extracted and mass separated in the ISOL. The number of collected ions after the massseparation was determined by α spectrometry. The Ieff value was calculated from a ratio of the number of massseparated ions to that of directly collected atoms transported by the gas-jet system. The α spectra after surface ionization and following massseparation are shown in Supplement Figures 1−4. The measured Ieff values for Fm, 251 Md, No, and Lr are listed in Table 1 with the related surface temperature. On the basis of the S-L equation, Ieff in a small cavity configuration can be expressed as