The suitability of barium monofluoride for laser cooling from ab initio study

ABSTRACT

The feasibility of laser cooling the ¹³⁸Ba¹⁹F molecule is performed using ab initio quantum chemistry. Three low-lying doublet electronic states X ²Σ⁺, A' ²Δ and A ²Π are determined by the multireference configuration-interaction (MRCI) method, where the spin–orbit coupling (SOC) effect is also taken into account in the electronic structure calculations. The computed spectroscopic constants and permanent dipole moments agree well with the available experimental data. The Franck–Condon factors of the A ²П → X ²Σ⁺ transition show highly diagonal dominance (f₀₀ = 0.981, f₁₁ = 0.940, f₂₂ = 0.896) and the A ²П state has a radiative lifetime of τ = 37.8 ns, allowing for rapid laser cooling. Our calculation indicates that the laser-cooling scheme require only three lasers at 822 nm, 855 nm and 856 nm proceeded on the A ²П (ν′) ← X ²Σ⁺ (ν′′) transitions. The appeared intervening state A' ²Δ between the X ²Σ⁺ and A ²П states is the main challenge for laser cooling this molecule. In fact, the calculated vibrational branching loss ratio to the intermediate A' ²Δ state is almost negligible at a level of η < 4.5 × 10^?9. Thus, BaF is a promising laser-cooling candidate with a relatively simple laser-cooling scheme.