Observation of stable HO4+ and DO4+ ions from ion–molecule reactions in helium nanodroplets

Abstract

Ion–molecule reactions between clusters of H₂/D₂ and O₂ in liquid helium nanodroplets were initiated by electron-induced ionization (at 70 eV). Reaction products were detected by mass spectrometry and can be explained by a primary reaction channel involving proton transfer from H₃⁺ or H₃⁺(H₂)_n clusters and their deuterated equivalents. Very little HO₂⁺ is seen from the reaction of H₃⁺ with O₂, which is attributed to an efficient secondary reaction between HO₂⁺ and H₂. On the other hand HO₄⁺ is the most abundant product from the reaction of H₃⁺ with oxygen dimer, (O₂)₂. The experimental data suggest that HO₄⁺ is a particularly stable ion and this is consistent with recent theoretical studies of this ion.