Calcium-containing diatomic dications in the gas phase

Abstract

Sputtering (ion surface bombardment) of various calcium-containing powder samples with an energetic (17 keV), high-current ¹⁶O⁻ beam has produced the diatomic dications of CaSi²⁺, CaP²⁺, CaF²⁺, CaH²⁺, CaCl²⁺, CaBr²⁺ and CaI²⁺. These molecular gas-phase species have been identified in positive ion mass spectra at half-integer m/z values; their ion flight times through a magnetic-sector mass spectrometer were roughly 10⁻⁵ s. Most of them appear to be novel molecular ions; the stability of the latter four (CaH²⁺, CaCl²⁺, CaBr²⁺ and CaI²⁺) had been demonstrated in previous theoretical studies, whereas only CaF²⁺ and CaBr²⁺ had been observed before. Here we combine the results of our experimental search with a detailed theoretical study of the remaining three systems CaSi²⁺, CaP²⁺ and CaF²⁺. All electronic states correlating with the first dissociation channel are characterized using high level ab initio electronic structure calculations. In their ground states, we find CaSi²⁺ to be a long-lived metastable molecule, whereas CaF²⁺ and CaP²⁺ are thermodynamically stable, with respective equilibrium internuclear distances of 6.253, 4.740, and 5.731 a₀. CaSi²⁺ has a well depth of 7116 (0.88) cm⁻¹ (eV) and a dissociation asymptote 7956 (0.99) cm⁻¹ (eV) below the ground state minimum. The dissociation energy of CaF²⁺ is estimated to be 3404 (0.42) cm⁻¹ (eV), whereas for CaP²⁺ we found 2547 (0.32) cm⁻¹ (eV), and a barrier height of 8118 (1.01) cm⁻¹ (eV). Their adiabatic double ionisation energies are 22.87, 16.91, and 17.32 eV, respectively, for the F, Si, and P containing dications.