Magneto-optical trap for metastable helium at 389 nm

We have constructed a magneto-optical trap (MOT) for metastable triplet helium atoms utilizing the 2³S₁→3³P₂ line at 389 nm as the trapping and cooling transition. The far-red-detuned MOT (detuning Δ=-41MHz) typically contains few times 10⁷ atoms at a relatively high (~10⁹cm^-3) density, which is a consequence of the large momentum transfer per photon at 389 nm and a small two-body loss rate coefficient (2×10^-10cm³/s<β<1.0×10^-9cm³/s). The two-body loss rate is more than five times smaller than in a MOT on the commonly used 2³S₁→2³P₂ line at 1083 nm. Furthermore, laser cooling at 389 nm results in temperatures somewhat lower than those achieved using 1083 nm. The 389-nm MOT exhibits small losses due to two-photon ionization, which have been investigated as well.