Existence of triply charged actinide-hydride molecules

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Molecules consisting of actinide elements (Th, U) and hydrogen were detected as diatomic trications using low-energy accelerator mass spectrometry (AMS). In many AMS setups the molecules are destroyed by sending a negative ion beam through a foil or a gas medium (the so-called stripper) and by selecting ions in charge state 3+ or higher afterwards. During measurements of samples containing the abundant isotopes ${}^{232}$Th, ${}^{235}$U, and ${}^{238}$U, a stripper thickness dependent counting rate on the next higher mass, i.e., 233, 236, and 239, was observed when selecting the 3+ charge state. The dependence of the signal intensity on the thickness of the stripper gas indicates a partial survival of triply charged molecules of mass 233, 236, and 239, respectively. In a dedicated experiment, the existence of the molecule ${}^{232}$ThH${}^{3+}$ could be verified by directly detecting the breakup fragment ${}^{232}$Th${}^{3+}$. Quantum chemical ab initio calculations at the scalar-relativistic complete active space self-consistent field (CASSCF) and multireference configuration interaction (MRCI) level confirm the existence of both ThH${}^{3+}$ and UH${}^{3+}$ by revealing local minima on the potential curves.