We report the observation of an interference between the electric dipole ($E1$) and the magnetic quadrupole ($M2$) amplitudes for the linear polarization of the Ly-${\alpha }_1$ ($2p_{3/2}\to 1s_{1/2}$) radiation of hydrogenlike uranium. This multipole mixing arises from the coupling of the ion to different multipole components of the radiation field. Our observation indicates a significant depolarization of the Ly-${\alpha }_1$ radiation due to the $E1\mathrm{\text{−}}M2$ amplitude mixing. It proves that a combined measurement of the linear polarization and of the angular distribution enables a very precise determination of the ratio of the $E1$ and the $M2$ transition amplitudes and the corresponding transition rates without any assumptions concerning the population mechanism for the $2p_{3/2}$ state.

• Received 21 July 2010

DOI:https://doi.org/10.1103/PhysRevLett.105.243002