  We have measured the alignment of the L-shell magnetic-substates following
the K-shell excitation of hydrogen- and helium-like uranium in relativistic
collisions with a low-Z gaseous target. Within this experiment the population
distribution for the L-shell magnetic sublevels has been obtained via an
angular differential study of the decay photons associated with the subsequent
de-excitation process. The results show a very distinctive behavior for the H-
and He-like heavy systems. In particular for $K \rightarrow L$ excitation of
He-like uranium, a considerable alignment of the L-shell levels was observed. A
comparison of our experimental findings with recent rigorous relativistic
predictions provides a good qualitative and a reasonable quantitative
agreement, emphasizing the importance of the magnetic-interaction and many-body
effects in the strong-field domain of high-Z ions.
