Two electronic equilibrium configurations for two-electron atoms have been derived exactly from the classical equations of motion. In these configurations the atom rotates as a whole like a rigid body. No further equilibrium is shown to exist. Semiclassical quantization of the rotation predicts two series of resonances converging to the double ionization threshold. This atomic-rotation spectrum is substantially different from its molecular and nuclear analog due to long-range Coulomb interactions.

• Received 4 February 1986

DOI:https://doi.org/10.1103/PhysRevLett.57.66