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 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 and states of molecular ions. The present results may be useful for studying and guiding the formation and spectroscopy of cold molecular ions.

在冷杂化离子-原子实验中形成的分子离子可能会发现从精密测量到受控化学反应的有趣应用。在这里，我们研究了分子离子的电子结构，这可能是由浸入Sr原子超冷气体中的激光冷却Sr ⁺离子的光缔合或超冷Sr ₂分子的电离产生的。使用从头电子结构方法，例如具有小核相对论能量一致伪势和大高斯基集的耦合簇和配置相互作用方法，我们计算了基态和41个激发电子态的势能曲线，以及它们之间的电偶极跃迁矩。我们表明，碱土分子离子，尽管它们的表面结构简单，只有三个价电子，但由于其多参比性质和高态密度，因此对最新的量子化学方法具有挑战性。最后，我们计算并分析了控制基态Sr ₂分子光电离成正电子和反正电子的Franck-Condon因子。分子离子。目前的结果可能对研究和指导冷分子离子的形成和光谱学很有用。