The lifetime of the helium anion

We report the result of a correction-free measurement of the lifetime of the metastable atomic negative helium ion. For all previous experimental investigations of the lifetime of the most long lived level of the He^- ion, 1s2s2p ⁴P_5/2, it was necessary to correct for systematic effects of the experimental environment as magnetically induced mixing with more short-lived fine structure components and/or photo detachment by the thermal radiation. In the present experiment any such influence on the measured lifetime has been effectively eliminated by performing the measurement in a cryogenic electrostatic ion-beam trap. With this technique we have reached a higher accuracy than earlier obtained. Our result with one standard deviation error is τ_5/2 = (359.0±0.7)μs.