Quantitative theoretical analysis of lifetimes and decay rates relevant in laser cooling BaH

Tiny radiative losses below the 0.1% level can prove ruinous to the effective laser cooling of a molecule. In this paper the laser cooling of a hydride is studied with rovibronic detail using ab initio quantum chemistry in order to document the decays to all possible electronic states (not just the vibrational branching within a single electronic transition) and to identify the most populated final quantum states. The effect of spin-orbit and associated couplings on the properties of the lowest excited states of BaH are analysed in detail. The lifetimes of the A21=2, H23=2 and E21=2 states are calculated (136 ns, 5.8 s and 46 ns respectively)for the first time, while the theoretical value for B2+1=2 is in good agreement with experiments. Using a simple rate model the numbers of absorption-emission cycles possible for both one- and two-colour cooling on the competing electronic transitions are determined, and it is clearly demonstrated that the A2 – X2+ transition is superior to B2+ – X2+, where multiple tiny decay channels degrade its efficiency. Further possible improvements to the cooling method are proposed.