For exothermic reactions, a PES is classified as attractive (or early-downhill) if R* AB > R* BC, so that the transition state is reached while the reactants are approaching each other. After the transition state, the A—B bond length continues to decrease, so that much of the liberated reaction energy is converted into vibrational energy of the A—B bond. An example is the harpoon reaction K + Br 2 → K—Br + Br, in which the initial long-range attraction of the reactants leads to an activated complex resembling K •••Br •••Br. The vibrationally excited populations of product molecules can be detected by infrared chemiluminescence.