We investigate the angular distribution of photoionization fragments at low photon energies (12–40 eV) in an open shell atom, by synchrotron radiation recoil ion momentum spectroscopy in a laser cooled and trapped sample. For cesium atoms, for which relativistic effects play an important role and the ion recoil is relatively small, we could determine large and rapid changes of the asymmetry parameter $\beta$ from two, observed for $s$ electrons outside resonances and far from the Cooper minimum. They can be explained by relativistic effects and interchannel coupling arising from final state configuration mixing.

• Received 10 December 2003

DOI:https://doi.org/10.1103/PhysRevLett.93.183001