Ab initio study on the electronic states and laser cooling of AlCl and AlBr

We investigate whether AlCl and AlBr are promising candidates for laser cooling. We report new ab initio calculations on the ground state X¹Σ⁺ and two low-lying states (A¹Π and a³Π) of AlCl and AlBr. The calculated spectroscopic constants show good agreement with available theoretical and experimental results. We also obtain the permanent dipole moments (PDMs) curve at multi-reference configuration interaction (MRCI) level of theory. The transition properties of A¹Π and a³Π states are predicted, including the transition dipole moments (TDMs), Franck-Condon factors (FCFs), radiative times and radiative width. The calculated radiative lifetimes are of the order of a nanosecond, implying that they are sufficiently short for rapid laser cooling. Both AlCl and AlBr have highly diagonally distributed FCFs which are crucial requirement for molecular laser cooling. The results demonstrate the feasibility of laser cooling AlCl and AlBr, and we propose laser cooling schemes for AlCl and AlBr.