Coulomb four-body problem

The dynamics of a quantum system with four charged particles can be a tough nut to crack, and competing theoretical models often differ qualitatively in their predictions. Helium provides a good four-body system to study when an electron collides with the atom to knock out both native electrons and leave the doubly charged bare nucleus behind. Theorists have disagreed about the directions the three escaping electrons would take when the incoming projectile is near the threshold energy for such an electron-impact double ionization process. Alexander Dorn, Joachim Ullrich, and colleagues at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, have now measured the momenta of the three electrons in that very energy regime and have found that the electrons tend to emerge in an equilateral triangle shape, separated by angles of 120 degrees, as predicted by some theories. Interestingly, one of the successful theories predicts that the escape pattern depends on the initial bound-state configuration and that the electron paths for triply photoionized lithium would take a T-shape, with two electrons emerging back-to-back. (X. Ren, A. Dorn, J. Ullrich, Phys. Rev. Lett., in press; A. Emmanouilidou, P. Wang, J. M. Rost, Phys. Rev. Lett. 100, 063002, 2008.) — Stephen G. Benka