theory study of laser cooling of barium monohalides.

The feasibility of laser cooling barium monohalides BaX (X = F, Cl, Br, I) is investigated using methods with the inclusion of spin-orbit coupling (SOC) effects. Calculated spectroscopic constants for BaF, BaCl, BaBr and BaI are in very good agreement with the available experimental measurements. The results demonstrate that the calculated electronic structure is accurate and can be used to establish the optical scheme of laser cooling. The highly diagonal Franck-Condon factors (FCFs) (BaF: = 0.980, = 0.939, = 0.894; BaCl: = 0.998, = 0.995, = 0.992) between the XΣ and AΠ states are determined, which are found to be in good agreement with previous theoretical results. The radiative lifetimes (BaF: 39.13-41.20 ns; BaCl: 117.99-110.23 ns) of the AΠ-XΣ transition for the first five vibrational levels show that the AΠ is a rather short lifetime state. The current study indicates that BaF and BaCl are two good choices of molecules for laser cooling. Therefore, BaI and BaBr are not promising laser-cooling candidates because the FCFs of the AΠ-XΣ transition are off-diagonal. We further propose the three-laser cooling schemes based on the AΠ-XΣ transition for BaF and BaCl.