Direct determination of the magnetic quadrupole contribution to the Lyman-α₁ transition in a hydrogenlike ion.

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}{sub 1} (2p{sub 3/2}{yields}1s{sub 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}{sub 1} radiation due to the E1-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{sub 3/2} state.