The transfer of excitation in a dimer is treated as a resonance, using Feshbach's nuclear reaction formalism. The radiation field-matter interaction is treated simultaneously with the intermonomeric potential. This gives a unified point of view and allows one to see what the interference effects are. The F\orster vibrational-relaxation mechanism and the Frenkel exciton are studied for the dimers, and explicit expressions for the decay probability are obtained.