Lifetime of the2S12state in hydrogen like fluorine and oxygen

Beam-foil measurements of the lifetimes of the $2S_{\frac{1}{2}}$ states in hydrogenlike fluorine and oxygen have been performed. The experimental results of 237 ± 17 nsec (fluorine) and 453 ± 43 nsec (oxygen) are in good agreement with the respective theoretical values of 228 and 464 nsec. Our results indicate that meaningful beam-foil lifetime measurements can be made on systems whose lifetimes are in the hundreds-of-nanosecond region and whose decay can be followed over only a fraction of a decay length.