Oxygen-containing gas-phase diatomic trications and tetracations: ReO ^z+, NbO^z+ and HfO^z+ (z = 3, 4)

Three oxygen-containing gas-phase diatomic trications ReO³⁺, NbO³⁺ and HfO³⁺ as well as the diatomic tetracation NbO⁴⁺ have been observed by mass spectrometry at non-integer m/z values. These unusual triply charged molecular ion species, together with the corresponding diatomic dications ReO²⁺, NbO²⁺ and HfO ²⁺, were produced by energetic, high-current oxygen ( ¹⁶O^-) ion beam sputtering of rhenium, niobium and hafnium metal samples, respectively, whose surfaces were dynamically oxidized by oxygen primary ion incorporation. In addition, NbO^z+ (z ≤ 4) were generated by intense femtosecond laser excitation and photofragmentation (Coulomb explosion) of Nb_xO_y clusters and were detected through Time-of-Flight Mass Spectrometry (TOF). Our experimental results confirm previous reports on the detection of NbO⁴⁺, NbO³⁺, NbO²⁺, HfO³⁺ and HfO²⁺ with Atom Probe mass spectrometry, whereas ReO³⁺ and ReO²⁺ apparently had not been observed before. In addition, these multiply charged molecular ions have been studied theoretically for the first time. Ab initio calculations of their electronic structures show that the diatomic trications ReO³⁺, NbO³⁺ and HfO³⁺ are long-lived metastable gas-phase species, with bond lengths of 1.61 Å, 1.62 Å and 1.86 Å, respectively. They present large potential barriers with respect to dissociation of more than 2.7 eV. The corresponding diatomic dications are thermochemically stable molecules with very large dissociation energies (>3.5 eV). Our calculations predict the diatomic tetracation ReO⁴⁺ to be a metastable ion species in the gas phase. We compute a potential barrier toward fragmentation of 0.6 eV; its formation requires a quadruple adiabatic ionization energy of 85.7 eV. Even though our calculations show that NbO⁴⁺ is a weakly bound (dissociation barrier ∼0.1 eV) metastable molecule, it is here identified via linear time-of-flight mass spectrometry.