On the formation of positive and negative ions in gaseous UF6

Total cross sections for the formation of positive ions by electron impact on UF6 have been determined from threshold to 1 keV. A trapped electron apparatus is used to determine cross sections for attachment of electrons to UF6 from 1 to 20 eV. Maxima are observed at 2.15±0.05 eV and ∼7 eV. The cross section at 2.15 eV is 2.7±1 Å2. Our previous studies showed that UF−5 also peaks at this energy. The negative ion products resulting from collisions between orthogonal crossed beams of alkali atoms (Na, K, Cs) with UF6 are also investigated and the ions observed are UF−6, UF−5, and F− in order of increasing endothermicity for the reactions. UF−6 ions are observed to occur at approximately zero energy for all alkali reactions. From these observations, the electron affinity of UF6 is estimated to be greater than or equal to 5.1 eV. UF−6 ions are also detected when only a thermal alkali beam enters the reaction region of the mass spectrometer. Although the possibility of a surface reaction is not entirely ruled out, evidence is presented for a two-body collisional ionization reaction between thermal alkali atoms and UF6. The threshold for the production of F− gives a bond dissociation energy, D (UF5–F) =3.0±0.2 eV, in good agreement with the unpublished value of Hildenbrand. Using this bond dissociation energy, together with the threshold for forming UF−5, we calculate the electron affinity of UF5 to be 4±0.4 eV. This value is derived assuming that the final products are A++F+UF5− (A=alkali atom).