Lawrence Berkeley National Laboratory

Experiments were performed at the Lawrence Berkeley National Laboratory 88-Inch Cyclotron facility to investigate the electron-transfer reduction reaction of dipositive Lr (Z = 103) with O₂ gas. Ions of ²⁵⁵Lr were produced in the fusion-evaporation reaction ²⁰⁹Bi(⁴⁸Ca,2n) ²⁵⁵Lr and were studied with a novel gas-phase ion chemistry technique. The produced ²⁵⁵Lr²⁺ ions were trapped and O₂ gas was introduced, such that the charge-exchange reaction to reduce ²⁵⁵Lr²⁺ to ²⁵⁵Lr¹⁺ was observed and the reaction rate constant was determined to be k = 1.5(7) × 10^-10 cm³/mol/s. The observation that this reaction proceeds establishes the lower limit on the second ionization potential of Lr to be 13.3(3) eV. This gives further support that the actinide series terminates with Lr. Additionally, this result can be used to better interpret the situation concerning the placement of Lu and Lr on the periodic table within the current framework of the actinide hypothesis. The success of this experimental approach now identifies unique opportunities for future gas-phase reaction studies on actinide and super heavy elements.