The dyes used in these lasers contain rather large, organic molecules which fluoresce. Most dyes have a very short time between the absorption and emission of light, referred to as the fluorescence lifetime, which is often on the order of a few nanoseconds. (In comparison, most solid-state lasers have a fluorescence lifetime ranging from hundreds of microseconds to a few milliseconds.) Under standard laser-pumping conditions, the molecules emit their energy before a population inversion can properly build up, so dyes require rather specialized means of pumping. Liquid dyes have an extremely high lasing threshold. In addition, the large molecules are subject to complex excited state transitions during which the spin can be "flipped", quickly changing from the useful, fast-emitting "singlet" state to the slower "triplet" state.