Phosphorescence
Phosphorescence is a process in which energy absorbed by a substance is released relatively slowly in the form of light. This is in some cases the mechanism used for "glow-in-the-dark" materials which are "charged" by exposure to light. Unlike the relatively fast reactions in a common fluorescent tube, phosphorescent materials used for these materials absorb the energy and "store" it for a longer time as the processes required to re-emit the light occur less often.
Phosphorescence is a process where the photon is emitted, not from a singlet-excited state, but from a forbidden triplet state. The time scale of emission is generally in the picoseconds to nanosecond range, while phosphorescence typically lasts for fluorescence microseconds, milliseconds, or even longer minutes or hours. Researchers typically use a pulsed source such as a flash lamp or LED to measure phosphorescence spectra and decays on these longer time scales. Phosphorescence measurements use a longer-lived pulsed source, such as a xenon flash lamp. The timing of the flashing lamp can be used to measure spectra at different phosphorescence lifetimes.
PRINCIPLE :- When a molecule in the ground state (singlet) absorbs UV or visible radiations of the proper frequency, one electron passes into the vacant orbital present at a higher energy level. If the spin of the promoted electron does not undergo any change and the net spin is still zero. This is called the excited singlet state, while if the promoted electron has a spin parallel to its original partner, then it is called the excited triplet state. When incident light strikes a phosphorescent substance, the molecules absorb energy hv and are excited from the singlet ground state (So) to the singlet excited state (S1). Through intersystem crossing, a singlet excited electron transits to the triplet state by vibrational relaxation. Direct transition from the ground singlet electronic state to the triplet electronic state is impossible because this leads to a change in multiplicity and thus has a low probability of occurring which is a forbidden transition. Because the transition from triplet (T1) to singlet (ground state) is also forbidden as the electron spin in the ground state is parallel to the spin in the triplet excited state, the excited molecules stay in the triplet state (T1) for a longer time. This time could be about a few seconds, minutes, or even hours. The electron, then eventually come back down, accompanied by the emission of a photon. This phenomenon is called “phosphorescence“, and can be illustrated in the Jablonski diagram
