The oscillator strength for the resonance ($1{}_{}{}^1{}_{}{}^{}S-2{}_{}{}^1{}_{}{}^{}P$) transition in atomic helium has been determined from a measurement of the lifetime of the $2{}_{}{}^1{}_{}{}^{}P$ state. This lifetime was obtained using a modified zero-field level crossing (Hanle) method. The scattering sample was a thermal velocity beam of $2{}_{}{}^1{}_{}{}^{}S$ metastable atoms. Resonant 2-μ ($2{}_{}{}^1{}_{}{}^{}S-2{}_{}{}^1{}_{}{}^{}P$) radiation is absorbed by the $2{}_{}{}^1{}_{}{}^{}S$ atoms, which are excited to the $2{}_{}{}^1{}_{}{}^{}P$ state. This state then primarily decays to the ground $1{}_{}{}^1{}_{}{}^{}S$ state with the emission of resonance (584 Å) radiation. The lifetime of the $2{}_{}{}^1{}_{}{}^{}P$ state was determined from measurements of the magnetic-field dependence of the angular distribution of the 584 Å intensity. This new technique eliminates the problem of constructing a vacuum ultraviolet resonance lamp. The measured lifetime $\tau \left(2\mathrm{}{}_{}{}^1{}_{}{}^{}P\right)$ is (5.63 ± 0.22) × ${10}^{-10\mathrm{}}$ sec. The resulting absorption oscillator strength for the resonance transition is 0.273 ± 0.011. This oscillator strength agrees with the theoretical value.

• Received 17 January 1969

DOI:https://doi.org/10.1103/PhysRev.183.81