Electron attachment to volatile uranyl molecules

Negative ion production by endoergic charge transfer from fast alkali atoms to a crossed molecular beam of uranyl complexes has been studied for three systems, UO₂L₂, UO₂L₂·THF, and UO₂L₂·TMP, where L denotes hexafluoroacetylacetonate, THF tetrahydrofuran, and TMP trimethyl phosphate. Near the lowest threshold, only negative ions of the parent molecules appear. The corresponding nominal electron affinities are 1.9 ± 0.3, 1.6 ± 0.2, and 1.5 ± 0.3 eV, respectively. At higher collision energies, smaller negative ions appear, all containing the L^- anion; the variation with collision energy and source temperature indicates these come from dissociative electron attachment to the parent molecules rather than attachment to products of thermal unimolecular decomposition. Qualitative electronic structure arguments suggest the attaching electron first enters a uranium 5f orbital and then migrates to the lowest vacant π* orbital of the L ligand.