Ab initio electronic structure of the Sr$_2^+$ molecular ion

TL;DR

This paper provides detailed ab initio calculations of the electronic structure of the Sr$_2^+$ molecular ion, including potential energy curves and transition moments, aiding experimental efforts in cold ion-atom chemistry.

Contribution

It presents comprehensive electronic structure data for Sr$_2^+$, including excited states and Franck-Condon factors, using advanced quantum chemistry methods.

Findings

Calculated potential energy curves for 42 electronic states.

Identified challenges of multireference character in alkaline-earth ions.

Analyzed Franck-Condon factors for photoionization processes.

Abstract

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 Sr$_2$ molecules. Using \textit{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 Sr$_2$ molecules into $^2\Sigma^+_u$ and $^2\Sigma^+_g$ 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.