The ab initio study of laser cooling of BBr and BCl

We investigate the feasibility of laser cooling BBr and BCl using ab initio quantum chemistry. The multi-reference configuration interaction method (MRCI) is used to calculate the ground state X1Σ+ and the low-lying excited state A1Π, where Davidson modification with the Douglas–Kroll scalar relativistic correction is also taken into account. The calculated spectroscopic constants are in good agreement with available experimental values. The potential energy curves, permanent dipole moments (PDMs), transition dipole moments (TDMs) followed by Franck–Condon factors and radiative times for the transitions from the A1Π state to the ground state X1Σ+ are obtained as well. The determined Franck–Condon factors are highly diagonally distributed and the evaluated radiative lifetimes are of the order of nanoseconds. Furthermore, the a3Π → X1Σ+ transitions of BBr and BCl are also strongly diagonal and the X1Σ+ → A1Π transitions perhaps can be followed by the X1Σ+ → a3Π transitions to attain a lower Doppler temperature. Long-range behavior of BBr and BCl has also been studied, and a double well is found in the A1Π state of BBr. The shallow long-range well might open up even more channels for laser cooling of BBr. The results demonstrate the possibility of laser cooling BBr and BCl, and provide a promising theoretical reference for further research on BBr and BCl.