Alignment caused by photoionization and in radiative electron capture into excited states of hydrogenic high-Z ions

In high-energy atomic collisions between bare high-Z projectiles and low-Z target atoms, an electron may be captured radiatively into the ground state or, alternatively, into an excited projectile state, which subsequently decays by x-ray emission. These processes are the inverse of a single-step or a two-step 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. In this paper, we present a theoretical analysis with a particular emphasis on a detailed multipole decomposition of the photon wave function. This treatment is suitable for connecting angular correlations with alignment studies of the excited state. The anisotropy factors for angular correlations beween the beam axis and the decay x rays following radiative electron capture (REC) into the 2p(3/2), 3P(3/2), 3d(3/2), and 3d(5/2) levels of bare Xe54+, Au79+, Pb82+, and U92+ projectiles with energies ranging from 10 MeV/u to 10 GeV/u are explicitly presented and the connection with the alignment is given. It is predicted that REC occurs predominantly into states with the minimal magnetic quantum number +/-1/2 indicating a strong alignment perpendicular to the beam axis.