PAIR EXCITATIONS IN SOLIDS

The pair-excitation probability in the weak-field regime is calculated using a fully quantized interaction Hamiltonian, for both the rotatin g- and fixed-atom cases. The peak of the absorption occurs at the sum of the final-state energies of the two atoms and the transition probab ility is shown to be proportional to the inverse square of the laser d etuning and to the inverse cube of the nearest-neighbor distance. In a ddition, retardation effects result in a system size dependence for th e transition rate not previously predicted. The significance of retard ation effects is shown to depend on the excited-state coherence lifeti me, the distance of closest approach, and the wavelength of the transi tion.