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

TL;DR

This paper provides a detailed quantum chemistry analysis of BaH molecule's electronic states, lifetimes, and decay channels, informing more efficient laser cooling strategies by identifying optimal transitions and decay pathways.

Contribution

It offers the first theoretical calculations of BaH excited state lifetimes and decay rates, including detailed rovibronic decay pathways, enhancing understanding of molecular laser cooling limitations.

Findings

Calculated lifetimes of excited states (136 ns, 5.8 μs, 46 ns) for the first time.

Identified A$^2{\Pi}$ - X$^2{\Sigma}^+$ transition as more efficient for cooling.

Demonstrated tiny decay channels significantly impact cooling cycle efficiency.

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 {\mu}s 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.