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

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.