Molecular negative surface ionization of UF₆

Investigations have been carried out to elucidate the process of formation of UF₆⁻ molecular ions on heated surfaces. On a carbon coated Pt surface the absolute efficiency of conversion of UF₆ to UF₆⁻ was found to be 99⁺¹₋₅% over a wide temperature range in accordance with anticipations based solely on measured thermionic work functions and the molecular electron affinity of UF₆. The heat of adsorption of UF₆⁻ determined from residence time measurements is 33.6±1.1 kcal/mol on this surface. On clean Pt and other pure metal surfaces the ionization efficiency is orders of magnitude lower than predictions based on the Saha–Langmuir equation. The reasons for this behavior are discussed in terms of alternative dissociation processes on the surface.