a forbidden energy state

Uncle Wiki tells us: Phosphorescence is a specific type of photoluminescence related to fluorescence. Unlike fluorescence, a phosphorescent material does not immediately re-emit the radiation it absorbs. The slower time scales of the re-emission are associated with “forbidden” energy state transitions in quantum mechanics. As these transitions occur less often in certain materials, absorbed radiation may be re-emitted at a lower intensity for up to several hours. As seen (by flash, so you don’t see their forbidden green fluorescent state) at Penders… aka Ghost Fungus.