Revisiting the Optical Spectrum of the Plutonyl Ion (PuO₂)²⁺ in 1 M HClO₄

The analysis of the solution absorption spectrum of the plutonyl ion in an aqueous environment was given by Eisenstein and Pryce (E&P) in 1968. In 2011 a new spectrum was published of the (PuO₂)²⁺ ion in 1 M HClO₄. We have been provided with the original data of this spectrum and have found in the data a previously unreported low-lying transition at 7385 cm^-1 which we have assigned as a magnetic dipole transition. We have fit most of the near-infrared and optical transitions with Gaussian fits and tabulated a new energy level list up to 22,000 cm^-1 which mostly agrees with the data of E&P. We assumed a crystal field of D_∞h (only axial symmetry) and utilized the intensity calculations published for the isoelectronic (NpO₂)¹⁺ ion using a complete basis set for the 5f² problem including the Coulombic, spin-orbit as well as the crystal field Hamiltonian. Our results differ substantially from those of E&P. Subsequently, we used a truncated Hamiltonian to try to establish the effects of assuming the σ antibonding orbitals are at such high energies that we can ignore their contributions to the lower lying φ and δ orbitals.