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 5 S 1 / 2 F = 3 → → 5 P 3 / 2 F = 4 transition. On excitation, however, the detuning necessary for cooling gives a small, but non-zero overlap with the 5 P 3 / 2 F = 3 state. If an atom is excited to this state, which occurs roughly every thousand cycles, the atom is then free to decay either the F = 3 , light coupled upper hyperfine state, or the F = 2 "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 5 S 1 / 2 F = 2 → → 5 P 3 / 2 F = 3 transition is used to recycle the population back into the optical loop so that cooling can continue.