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

Abstract

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 A${}^2{\Pi }_{1/2}$, H${}^2{\Delta }_{3/2}$ and E${}^2{\Pi }_{1/2}$ states are calculated (136 ns, 5.8 μs and 46 ns respectively) for the first time, while the theoretical value for B${}^2{\Sigma }_{1/2}^{+}$ 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 A${}^2\Pi$ - X${}^2{\Sigma }^{+}$ transition is superior to B${}^2{\Sigma }^{+}$ - X${}^2{\Sigma }^{+}$, where multiple tiny decay channels degrade its efficiency. Further possible improvements to the cooling method are proposed.