Ab initio electronic structure of the Sr2 + molecular ion

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 2+ u and 2+ g 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.