Theoretical Insights into the Spectroscopic Properties and Prospects for the Direct Laser Cooling of the Radium Monohalides

Abstract

Alkaline earth metal monohalides are currently considered the most promising candidates for direct laser cooling due to the unique electronic structure of their lower states. Among the alkaline earth monohalides family, the radium compounds are of the highest interest because of the largest mass of the radium atom compared to other alkaline earth metals, which provides some advantages for applications of ultracold matter. In addition, the radium monohalides are still the least studied both theoretically and experimentally. Here the state-of-the-art ab initio studies of the lower states of radium monochloride RaCl and radium monoastatide RaAt diatomic radicals are presented for the first time. The potential energy curves of the ground and five low-lying excited states are calculated using the Fock–space relativistic coupled cluster method. Electronic term energies, equilibrium internuclear distances, transition and permanent dipole moments, sequences of vibrational energies, harmonic vibrational frequencies, Franck–Condon factors, and radiative lifetimes are predicted. Based on these results, we calculated cooling parameters for all radium monohalides and concluded that all of them, including the RaAt molecule, can be used for direct laser cooling. The probable vibrational laser cooling schemes were also proposed. This work continues the series of research of radium monohalides and includes the comparison of results with previously studied RaF, RaBr, and RaI molecules. It can provide an understanding of trends in spectroscopic and radiative properties of this alkaline earth metal monohalides series.