Magneto-optical trapping of barium

Laser cooling and trapping of the heavy alkaline-earth-metal element barium has been achieved based on the strong 6s² ¹S₀-6s6p ¹P₁ transition. The excited state decays to a large fraction into metastable D states. Two schemes were implemented where three additional laser-driven transitions provide closed cooling cycles. This results in a fraction of 50(20)% of the trapped atoms in one of the metastable states. A total efficiency of 0.4(1)×10^-2 for slowing a thermal atomic beam and capturing atoms into a magneto-optical trap was obtained. Trap lifetimes of more than 1.5 s were observed. They are shortened at high laser intensities by three photon photoionization losses. The developed scheme can be transferred to other systems with large leakage from the cooling transition, such as Ra which is interesting for sensitive fundamental symmetry research.