When a beam of spin-polarized metastable ${\mathrm{He}}^{+}2s_{\frac{1}{2}}$ ions is quenched by an electric field $\overrightarrow{\mathrm{E}}$, the emitted radiation intensity contains an asymmetry term proportional to $\left(\hat{k}·\hat{E}\right)(\overrightarrow{\mathrm{P}}·\hat{k}\times \hat{E})$ where $\overrightarrow{\mathrm{P}}$ is the spin-polarization vector and $\hat{k}$ is the direction of observation. The resulting asymmetry is nearly proportional to the level width of the $2p_{\frac{1}{2}}$ state in ${\mathrm{He}}^{+}$. The experiment provides the first observation of the asymmetry. The measured asymmetry is (0.007 69±0.000 10) in agreement with the theoretical value 0.007 618 and corresponds to a lifetime ${\tau }_{2p}=(0.988\mathrm{}\pm 0.013)\times {10}^{-10\mathrm{}}$ sec.

• Received 8 September 1981

DOI:https://doi.org/10.1103/PhysRevA.25.862