Reactions of Atomic Thorium and Uranium Cations with SF6 Studied by Guided Ion Beam Tandem Mass Spectrometry.

Abstract

The fundamental chemistry of the thorium and uranium fluorides continues to be an area of interest because of the use of thorium and uranium fluoride compounds in nuclear fuel systems. Here, we study the reaction of thorium cations with sulfur hexafluoride for the first time and revisit the reaction of uranium cations with sulfur hexafluoride. By using guided ion beam tandem mass spectrometry, we explore the reaction pathways that become accessible well above thermal energies (E ∼ 0.04 eV). Overall, we find that both Th+ and U+ react very efficiently with SF6, approaching the collision limit at both thermal and elevated energies. The primary products observed at low energies include Th1-3+, UF1-4+, and SF1-4+, all of which are formed in barrierless, exothermic processes. SF5+ was also observed, although the pressure dependence of this channel reveals that SF5+ forms exothermically through secondary reactions, which the energy dependences suggest result from reactions between ThF2+ and UF3+ with SF6. At higher energies, both AnF3+ products are observed to decay to AnF+ + F2, and both SF4+ and SF2+ exhibit cross sections with endothermic features. For both systems, the rise in SF4+ can be attributed to a secondary collision between AnF+ with SF6 on the basis of the pressure dependence of the SF4+ channel at higher energies, and the rise in SF2+ appears to result from the decomposition of SF3+ to SF2+ + F.