Many atoms that do not contain closed optical loops can still be laser cooled, however, by using repump lasers which re-excite the population back into the optical loop after it has decayed to a state outside of the cooling cycle. The magneto-optical trapping of rubidium 85, for example, involves cycling on the closed formula_55 transition. On excitation, however, the detuning necessary for cooling gives a small, but non-zero overlap with the formula_56 state. If an atom is excited to this state, which occurs roughly every thousand cycles, the atom is then free to decay either the formula_57, light coupled upper hyperfine state, or the formula_58 "dark" lower hyperfine state. If it falls back to the dark state, the atom stops cycling between ground and excited state, and the cooling and trapping of this atom stops. A repump laser which is resonant with the formula_59 transition is used to recycle the population back into the optical loop so that cooling can continue.