Resonant interaction between identical atoms including recoil

The emission spectrum is calculated for a system of two identical atoms, having relative coordinate r. Initially one atom is in an excited state and the other in its ground state. Both atom and field variables are fully quantized, so that the calculation automatically includes effects related to both retardation and atomic recoil on the absorption or emission of radiation. It is shown that for k(0)r much greater than 1 (where k(0) = 2 pi/lambda and lambda is the wavelength associated with the resonant excited- to ground-state transition), the emission spectrum can consist of a triplet. The frequency separation between the components of the triplet can be interpreted in terms of the recoil the atoms undergo individually on emitting or absorbing radiation. For k(0)r much less than 1, the atoms emit as a composite system and the recoil (while not resolvable) is that associated with a ''molecule'' of mass 2m, where m is the mass of one of the atoms.