On the stability and lifetime of GaO2+ in the gas phase

The electronic structure, stability, and lifetime of GaO2+ have been investigated using high-level ab initio calculations. The potential energy curves have been calculated at the CCSD(T)/aug-cc-pV5Z and at the MS-CASPT2/ANO-RCC levels of theory. Lifetimes were evaluated using the Exterior Complex Scaling (ECS) method and B-spline basis functions. Our calculations show that GaO2+ is a metastable species in the gas phase, since the diatomic dication, in its ground state, lies 97.1 kcal/mol above the Ga+ (S-1) + O+ (S-4) dissociation limit. However, the energy barrier that has to be overcome to reach this limit is 3 kcal/mol high so that five vibrational resonances can be accommodated between the bottom of the well and the top of the barrier. The evaluated lifetimes vary from hundreds of femtoseconds to approximately 1 s, so at least two of them have long enough lifetimes (1 s and 91 mu s) to be detected using mass spectrometry techniques, in agreement with the experimental evidence. In the experiment (Fiser et al. in Eur J Mass Spectrom 15:315-324, 2009), GaO2+ was observed for an ion flight time of about similar to 12 mu s through a magnetic-sector mass spectrometer and unambiguously identified by its isotopic abundance. Our results also show that isotopic effects on the resonances' energies and on their lifetimes, when Ga-70 is replaced by Ga-69 or Ga-71, are very small (similar to 0.1 and similar to 1%, respectively), reflecting the large mass of the system.