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 P-4(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 tau(5/2)=(359.0 +/- 0.7)mu s.