Thermodynamically stable diatomic dications: The cases of SrO²⁺ and SrH²

A high level theoretical investigation of the low-lying electronic states of the diatomic dications SrO²⁺ and SrH²⁺ is presented for the first time along with experimental results of their mass spectra where they were detected. A global and reliable picture of the potential energy curves of the electronic states and the associated spectroscopic parameters provide quantitative results attesting to the thermodynamic stability of both species. Inclusion of spin-orbit interactions does not significantly change the energetic characterization. For SrO²⁺, the ground (X ³Σ^-) and first excited (A ³Π, T_e = 3971 cm^-1) states are bound (D_e) by 15.94 kcal mol^-1 and 4.71 kcal mol^-1, respectively. Transition probabilities (A_v'v″) have been evaluated and radiative lifetimes estimated for the vibrational states of A ³Π (v'), and transition probabilities are expected to be diagonally dominant and fall in the far-IR region of the spectrum. For the singlet states a ¹Δ, b ¹Π, c ¹Σ⁺, and d ¹Σ⁺, transition probabilities have also been calculated for all symmetry allowed transitions and the radiative lifetimes evaluated for selected vibrational states of the upper levels. The transitions associated with the band systems d ¹Σ⁺-b ¹Π and d ¹Σ⁺-c ¹Σ⁺, although falling in the yellow region of the spectrum, with overlapping bands, are expected to show quite distinct intensities since the transition moment associated with d ¹Σ⁺-c ¹Σ⁺ is much larger. For singlet transitions, the prediction of relative intensities using the Franck-Condon approximation fails in most of the cases. For SrH²⁺, only the ground state is bound (D_e = 6.54 kcal mol^-1); with an equilibrium distance of 5.117 a₀, the associated spectroscopic parameters (ω_e, ω_ex_e, B_e) turned out to be (518.9, 32.77, 2.3227) in cm^-1. For both species, dipole moment functions illustrate the variation of the molecular polarity with the internuclear distance.