Metastable-ion lifetime studies utilizing a heavy-ion storage ring: Measurements on He-

The lifetime of the metastable autodetaching 1s2s2p 4P5/2 level of the He-4 ion has been measured by a time-of-flight technique to be 350±15 μs using a heavy-ion storage ring. Blackbody-induced photodetachment of this weakly bound level is shown to influence the decay rate by nearly 20%, an effect not discussed previously. The magnetic-field-induced mixing effects provide additional information about the lifetime of the close-lying 4P3/2 level, yielding τ(4P3/2)=12±2 μs. The observed lifetimes are consistent with previous experimental values, but the accuracy of the 4P5/2 lifetime is improved by a factor of 6, allowing a detailed test of theoretical predictions to be performed. The 4P5/2 lifetime is significantly shorter than most recent advanced theoretical results, but in agreement with a multiconfiguration Hartree-Fock approach accurately describing the initial bound 4P5/2 and the final continuum 2F5/2 states.