Ab initio study of the positronation of the CaO and SrO molecules including calculation of annihilation rates

Abstract

Ab initio multireference single- and double-excitation configuration interaction calculations have been performed to compute potential curves for ground and excited states of the CaO and SrO molecules and their positronic complexes, e(+)CaO, and e(+)SrO. The adiabatic dissociation limit for the (2)Σ(+) lowest states of the latter systems consists of the positive metal ion ground state (M(+)) and the OPs complex (e(+)O(-)), although the lowest energy limit is thought to be e(+)M + O. Good agreement is found between the calculated and experimental spectroscopic constants for the neutral diatomics wherever available. The positron affinity of the closed-shell X (1)Σ(+) ground states of both systems is found to lie in the 0.16-0.19 eV range, less than half the corresponding values for the lighter members of the alkaline earth monoxide series, BeO and MgO. Annihilation rates (ARs) have been calculated for all four positronated systems for the first time. The variation with bond distance is generally similar to what has been found earlier for the alkali monoxide series of positronic complexes, falling off gradually from the OPs AR value at their respective dissociation limits. The e(+)SrO system shows some exceptional behavior, however, with its AR value reaching a minimum at a relatively large bond distance and then rising to more than twice the OPs value close to its equilibrium distance.

Calculation of positron binding energies using the generalized any particle propagator theory

“…85 Similar geometrical changes arising from positron binding have been reported for alkaline monoxides, with PBEs above 0.3 eV. 86 Positron binding to alkaline-earth oxides has also been addressed, 87,88 although changes in bond lengths and vibrational frequencies were found to be much smaller (around 1%) than those of alkali hydrides and alkaline oxides. 3 PBEs of diatomic molecules, XY, were calculated at two different configurations: the equilibrium geometries of neutral (XY) and positronic ([XY;e + ]) molecules.…”

“…3,85,86 However, previous calculations for alkaline-earth oxides reported the opposite trend, presumably as a result of basis sets deficiencies. 87,88,91 We observe in Figure 5(b) that PBEs also increase with molecular polarizabilities. The only exception, that occurs for alkaline earth oxides, is caused by the similar polarizabilities of MgO and BeO.…”