Why can phosphorescence continue after the exciting radiation is removed?

Phosphorescence persists after the removal of exciting radiation because of the presence of a metastable energy level, often referred to as an electron trap, located between the ground level and the excited level.1

In this process, an electron is promoted to an excited state and subsequently falls into this metastable level. Transitions from this state back to the ground level are considered "forbidden" or highly improbable by the laws of quantum mechanics, meaning the electron remains trapped for a significant duration. Because a change in intrinsic electron spin—such as the transition from a triplet state back to a singlet state—is statistically unlikely, the emission of light is delayed.12

The trapped electron eventually returns to the ground state either through a slow, spontaneous forbidden transition or by gaining additional energy from external sources, such as thermal agitation or optical stimulation, which helps it overcome the energy barrier. Consequently, the duration of the afterglow is determined by the time the electron spends in these metastable states.1