Optical cycling in charged complexes with Ra-N bonds

Abstract: The extension of laser cooling and trapping techniques to polyatomic molecular ions would have advanced scientific applications such as search of physics outside of the Standard Model, ultracold chemistry etc. We apply the Fock space relativistic coupled cluster method to study low-lying electronic states of molecular ions with Ra--N bonds, namely RaNCH$^+$, RaNH$^+_3$ and RaNCCH$^+_3$. Prospects of laser cooling of these species are estimated, and the peculiarities of unpaired-electron distributions are analyzed from the point of view of the molecular electronic structure. RaNH$^+_3$ and RaNCCH$^+_3$ are the first symmetric top molecular ions expected to be suitable for direct laser cooling.