Physical Review A
When a beam of spin-polarized metastable He+(2s12) ions is quenched by an electric field E', the emitted radiation intensity contains an asymmetry term proportional to (kE)(P'k×E), where P' is the spin-polarization vector and k is the direction of observation. The resulting asymmetry is nearly proportional to the level width of the 2p12 state in He+. The measured asymmetry 0.007 602 70.000 020 3 corresponds to a lifetime 2p=(0.99920.0026)×10-10 sec, in fair agreement with the theoretical value 2p=0.9972×10-10 sec. © 1983 The American Physical Society.
Drake, Gordon W. F.; Patel, J.; and Van Wijngaarden, A.. (1983). Lifetime of the 2p state in He II. Physical Review A, 28 (6), 3340-3348.