Strong Alignment Observed for the Time-Reversed Photoionization Process Studied in Relativistic Collisions with Bare Uranium Ions

In high-energy atomic collisions between bare high- $Z$ projectiles and low- $Z$ target atoms, an electron may be captured radiatively into an excited projectile state which subsequently decays by x-ray emission. This process is the inverse of two-photon–one-electron ionization, in which the first photon resonantly excites an electron from the hydrogenic ${1s}_{1/2}$ ground state and a second photon ionizes the excited electron. We present an experimental and theoretical study of the angular distribution of the Ly- ${\alpha }_1$ ( ${2p}_{3/2}\to {1s}_{1/2}$) x rays following radiative electron capture. From the observed anisotropic emission pattern a significant alignment of the intermediate ${2p}_{3/2}$ state is deduced.