Measurement of an E1-M1 interference effect in the electric field quenching of spin-polarized He+ 2s/sub 1/2/ ions

When a beam of spin-polarized metastable He+ 2s/sub 1/2/ ions is quenched by an electric field E, the emitted radiation intensity contains an asymmetry term which is proportional to kxE x P, where P is the spin-polarization vector and k is the direction of observation. The effect is due to interference between spontaneous magnetic-dipole (M1) and induced electric-dipole (E1) decay modes to the ground state. At Vertical BarEVertical Bar = 38.14 V/cm, the measured asymmetry is (0.323 +- 0.085) x 10-3 in agreement with the theoretical value 0.3443 x 10-3. The experiment provides the first measurement of the relativistic M1 matrix element for the 2s/sub 1/2/-1s/sub 1/2/ transition in a hydrogenic ion. The paper contains an extensive discussion of all possible asymmetry effects, including higher-order relativistic and electric-field perturbation corrections