The oscillator strength for the resonance (1S1-2P1) transition in atomic helium has been determined from a measurement of the lifetime of the 2P1 state. This lifetime was obtained using a modified zero-field level crossing (Hanle) method. The scattering sample was a thermal velocity beam of 2S1 metastable atoms. Resonant 2- (2S1-2P1) radiation is absorbed by the 2S1 atoms, which are excited to the 2P1 state. This state then primarily decays to the ground 1S1 state with the emission of resonance (584) radiation. The lifetime of the 2P1 state was determined from measurements of the magnetic-field dependence of the angular distribution of the 584 intensity. This new technique eliminates the problem of constructing a vacuum ultraviolet resonance lamp. The measured lifetime (2P1) is (5.63 0.22) 10-10 sec. The resulting absorption oscillator strength for the resonance transition is 0.273 0.011. This oscillator strength agrees with the theoretical value. 1969 The American Physical Society.
