Experimental determination of the helium 2 3P1-1 1S0 transition rate

We present the first experimental determination of the 2 (3)P(1)-1 (1)S(0) transition rate in helium and compare this measurement with theoretical quantum-electrodynamic predictions. The experiment exploits the very long (similar to 1 minute) confinement times obtained for atoms magneto-optically trapped in an apparatus used to create a Bose-Einstein condensate of metastable (2 (3)S(1)) helium. The 2 (3)P(1)-1 (1)S(0) transition rate is measured directly from the decay rate of the cold atomic cloud following 1083 nm laser excitation from the 2 (3)S(1) to the 2 (3)P(1) state, and from accurate knowledge of the 2 (3)P(1) 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.