Determination of the ionization energy of NpO2 and comparative ionization energies of actinide oxides

The ionization energy of NpO2 was determined by the electron-transfer bracketing technique employing Fourier transform ion cyclotron resonance mass spectrometry. Under thermoneutral conditions the NpO2+ ion was found to abstract an electron from N,N,N',N'-tetramethyl-l,4-phenylenediamine (IE = 6.20 - 0.05 eV) but not from N,Ndimethyl-l,4-phenylenediamine (IE = 6.46 - 0.02 eV). Accordingly it is concluded that IE[NpO2] = 6.33 - 0.18 eV. This result contrasts with a previously reported value for IE[NpO2] of ~5.0 eV. The new value for the ionization energy of NpO2 from the present study is compared with those of UO2, PuO2 and AmO2. Literature values for the ionization energies of actinide monoxides and dioxides are tabulated, and systematic trends are considered. The recently reported second ionization energies for UO2, NpO2, PuO2 and AmO2 are also assessed. Comparisons of the various actinide oxide ionization energies are made with ionization energies of actinide atoms and with values for other relevant molecules.