Pas de Deux for Atomic Electrons

An atom with many electrons is like a complex ballet, displaying an often chaotic interplay of orbits. Owing to the effects of quantum mechanics, the lowest energy orbits (or ground states) of many electron atoms are stable. Excited states of the atoms do experience interactions, however, in which one of the electrons is liberated while the other falls to a lower state. The connection between these autoionization processes and the classical orbits is generally obscured by these same quantum effects that stabilize the ground states, but in an experiment reported by Pisharody and Jones on page 813 of this issue, the connection is greatly clarified(1). In classical mechanics, a multi-electron atom is very similar to a planetary system [for a nice description of the classical three-body problem, see (2)]. There are some differences, particularly the fact that the gravitational forces coupling planets with each other and with the Sun are always attractive while the electrons all are attracted to the nucleus, but repel each other. The classical orbits of the two systems have many similarities including the presence of chaos, and collisions between planets or electrons which can throw them free of their respective systems. Of course, we observe in natur