Only certain atoms and ions have optical transitions amenable to laser cooling, since it is extremely difficult to generate the amounts of laser power needed at wavelengths much shorter than 300 nm. Furthermore, the more hyperfine structure an atom has, the more ways there are for it to emit a photon from the upper state and not return to its original state, putting it in a dark state and removing it from the cooling process. It is possible to use other lasers to optically pump those atoms back into the excited state and try again, but the more complex the hyperfine structure is, the more (narrow-band, frequency locked) lasers are required. Since frequency-locked lasers are both complex and expensive, atoms which need more than one extra repump laser are rarely cooled; the common rubidium magneto-optical trap, for example, requires one repump laser. This is also the reason why molecules are in general difficult to laser cool: in addition to hyperfine structure, molecules also have rovibronic couplings and so can also decay into excited rotational or vibrational states. However, laser cooling of molecules has been demonstrated, first with SrF molecules, and subsequently with other diatomics such as CaF and YO.