We theoretically characterize interactions, energetics, and chemical reaction paths in ionic twobody and three-body systems of alkali-metal and alkaline-earth-metal atoms in the context of modern experiments with cold hybrid ion-atom mixtures. Using ab initio techniques of quantum chemistry such as the coupled-cluster method, we calculate ground-state electronic properties of all diatomic AB + and most of triatomic A2B + molecular ions consisting of Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, and Yb atoms. Different geometries and wave-function symmetries of the ground state are found for different classes of molecular ions. We analyze intermolecular interactions in the investigated systems including additive two-body and nonadditive three-body ones. As an example we provide twodimensional interaction potential energy surfaces for KRb + +K and Rb + +Sr2 mixtures. We identify possible channels of chemical reactions based on the energetics of the reactants. The present results may be useful for investigating controlled chemical reactions and other applications of molecular ions formed in cold hybrid ion-atom systems.

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Molecular ions formed in cold hybrid ion–atom experiments may find interesting applications ranging from precision measurements to controlled chemical reactions. Here, we investigate the electronic structure of the Sr 2 + molecular ion, which may be produced by photoassociation of laser-cooled Sr+ ions immersed in an ultracold gas of Sr atoms or by ionization of ultracold Sr2 molecules. Using ab initio electronic structure methods, such as the coupled cluster and configuration interaction ones with small-core relativistic energy-consistent pseudopotentials and large Gaussian basis sets, we calculate potential energy curves for the ground and 41 excited electronic states, and electric dipole transition moments between them. We show that alkaline-earth molecular ions, despite their apparently simple structure with only three valence electrons, are challenging for state-of-the-art quantum chemistry methods due to their multireference nature and high density of states. Finally, we calculate and analyze Franck–Condon factors governing the photoionization of ground-state Sr2 molecules into Σ u + 2 and Σ g + 2 states of Sr 2 + molecular ions. The present results may be useful for studying and guiding the formation and spectroscopy of cold Sr 2 + molecular ions.

Molecular ions formed in cold hybrid ion-atom experiments may find interesting applications ranging from precision measurements to controlled chemical reactions. Here, we investigate electronic structure of the Sr + 2 molecular ion, which may be produced by photoassociation of laser-cooled Sr + ions immersed into an ultracold gas of Sr atoms or by ionization of ultracold Sr2 molecules. Using ab initio electronic structure methods, such as the coupled cluster and configuration interaction ones with small-core relativistic energy-consistent pseudopotentials and large Gaussian basis sets, we calculate potential energy curves for the ground and 41 excited electronic states, and electric dipole transition moments between them. We show that alkaline-earth molecular ions despite of their apparently simple structure with three valence electrons only are challenging for state-of-the-art quantum chemistry methods due to their multireference nature and high density of states. Finally, we calculate and analyze Franck-Condon factors governing the photoionization of ground-state Sr2 molecules into 2 Σ + u and 2 Σ + g states of Sr + 2 molecular ions. The present results may be useful for studying and guiding formation and spectroscopy of cold Sr + 2 molecular ions.