Measurement of the first ionization potential of lawrencium, element 103

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 configurations^{1, 2}. Atomic s and p_1/2 orbitals are stabilized by relativistic effects, whereas p_3/2, 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 time^{3, 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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