  A quantum electrodynamic calculation of the interaction of an excited-state
atom with a ground-state atom is performed. For an excited reference state and
a lower-lying virtual state, the contribution to the interaction energy
naturally splits into a pole term, and a Wick-rotated term. The pole term is
shown to dominate in the long-range limit, altering the functional form of the
interaction from the retarded 1/R^7 Casimir-Polder form to a long-range
tail-provided by the Wick-rotated term-proportional to cos[2 (E_m-E_n) R/(hbar
c)]/R^2, where E_m < E_n is the energy of a virtual state, lower than the
reference state energy E_n, and R is the interatomic separation. General
expressions are obtained which can be applied to atomic reference states of
arbitrary angular symmetry. Careful treatment of the pole terms in the Feynman
prescription for the atomic polarizability is found to be crucial in obtaining
correct results.
