Mean Lifetimes of the2pand3pLevels in He ii

The foil-excitation technique has been used to measure the mean lifetimes of the $2p$ and $3p$ levels in He ii. The measured lifetimes agree with the theoretical values, the measured accuracy being ±3% for the $2p$ level and ±10% for the $3p$ level. The mean lives were determined from the decay of the two emission lines $1s-2p$ at 304 Å and $1s-3p$ at 256 Å as a function of distance from the intersection of the foil and beam. Cascading into the $2p$ and $3p$ levels was observed, and corrections for cascading were necessary to determine the lifetimes. The observed cascading was found to be in reasonable agreement with the theoretically predicted cascading from adjacent excited levels of He ii.