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Measurement of the first ionization potential of lawrencium, element 103

Abstract

The chemical properties of an element are primarily governed by the configuration of electrons in the valence shell. Relativistic effects influence the electronic structure of heavy elements in the sixth row of the periodic table, and these effects increase dramatically in the seventh row—including the actinides—even affecting ground-state configurations1,2. Atomic s and p1/2 orbitals are stabilized by relativistic effects, whereas p3/2, d and f orbitals are destabilized, so that ground-state configurations of heavy elements may differ from those of lighter elements in the same group. The first ionization potential (IP1) is a measure of the energy required to remove one valence electron from a neutral atom, and is an atomic property that reflects the outermost electronic configuration. Precise and accurate experimental determination of IP1 gives information on the binding energy of valence electrons, and also, therefore, on the degree of relativistic stabilization. However, such measurements are hampered by the difficulty in obtaining the heaviest elements on scales of more than one atom at a time3,4,5. Here we report that the experimentally obtained IP1 of the heaviest actinide, lawrencium (Lr, atomic number 103), is electronvolts. The IP1 of Lr was measured with 256Lr (half-life 27 seconds) using an efficient surface ion-source and a radioisotope detection system coupled to a mass separator. The measured IP1 is in excellent agreement with the value of 4.963(15) electronvolts predicted here by state-of-the-art relativistic calculations. The present work provides a reliable benchmark for theoretical calculations and also opens the way for IP1 measurements of superheavy elements (that is, transactinides) on an atom-at-a-time scale.

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Figure 1: Schematic experimental set-up used to measure the IP1 of Lr on an atom-at-a-time scale.
Figure 2: The ionization efficiency (Ieff) of various short-lived isotopes as a function of the effective IP1 (IP1*) at 2,700 K.

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Acknowledgements

We thank the JAEA tandem accelerator crew for supplying intense and stable beams for the experiments. The 249Cf was made available by H. Nitsche (Univ. California, Berkeley); it was produced in the form of 249Bk through the former Transplutonium Element Production Program at Oak Ridge National Laboratory (ORNL) under the auspices of the Director, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division of the US Department of Energy. Financial support by the Helmholtz-Institut Mainz is acknowledged. This work has been partly supported by the Grant-in-Aid for Scientific Research (C) no. 26390119 of the Ministry of Education, Science, Sports and Culture (MEXT).

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T.K.S., M.A., Y.N. and M.S. prepared the main part of the manuscript, A.B., E. E. and U.K. contributed to the theory part, and T.S. to the experimental part. C.E.D. and J.V.K. commented on the manuscript. T.K.S, M.A., T.S., N.S., K.T. and S.I. developed the surface ion-source in the ISOL setup at the JAEA tandem accelerator facility. T.K.S. and M.A. were responsible for data acquisition and analysis. T.S. commented on ion-source optimizations and the data analysis procedure. K.T. prepared the 249Cf target. K.E., J.R., P.T.-P., C.E.D. and N.T. separated and provided the 249Cf for the target. The on-line experiments were performed by T.K.S., M.A., N.S., Y.K., K.T., S. I., S.M., Y.N., K.O., A.O., D.R., M.S. and A.T., while theoretical calculations were carried out by A.B., E.E. and U.K. All authors discussed the results and commented on the manuscript.

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Correspondence to Y. Nagame.

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The authors declare no competing financial interests.

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Sato, T., Asai, M., Borschevsky, A. et al. Measurement of the first ionization potential of lawrencium, element 103. Nature 520, 209–211 (2015). https://doi.org/10.1038/nature14342

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