Experimental Determination of the Helium 2 ³P₁–1 ¹S₀ Transition Rate

We present the first experimental determination of the 2 ³P₁–1 ¹S₀ transition rate in helium and compare this measurement with theoretical quantum-electrodynamic predictions. The experiment exploits the very long (∼1  minute) confinement times obtained for atoms magneto-optically trapped in an apparatus used to create a Bose-Einstein condensate of metastable (2 ³S₁) helium. The 2 ³P₁–1 ¹S₀ transition rate is measured directly from the decay rate of the cold atomic cloud following 1083 nm laser excitation from the 2 ³S₁ to the 2 ³P₁ state, and from accurate knowledge of the 2 ³P₁ population. The value obtained is 177±8  s^-1, which agrees very well with theoretical predictions, and has an accuracy that compares favorably with measurements for the same transition in heliumlike ions higher in the isoelectronic sequence.