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A quantum electrodynamic calculation of the interaction of an excited-stateatom with a ground-state atom is performed. For an excited reference state anda lower-lying virtual state, the contribution to the interaction energynaturally splits into a pole term, and a Wick-rotated term. The pole term isshown to dominate in the long-range limit, altering the functional form of theinteraction from the retarded 1/R^7 Casimir-Polder form to a long-rangetail-provided by the Wick-rotated term-proportional to cos[2 (E_m-E_n) R/(hbarc)]/R^2, where E_m < E_n is the energy of a virtual state, lower than thereference state energy E_n, and R is the interatomic separation. Generalexpressions are obtained which can be applied to atomic reference states ofarbitrary angular symmetry. Careful treatment of the pole terms in the Feynmanprescription for the atomic polarizability is found to be crucial in obtainingcorrect results.

Long-Range Tails in van der Waals Interactions of Excited-State and Ground-State Atoms