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  • Fluorescence and Phosphorescence

    Types of luminescence

    Photoluminescence:
    Light is produced by exposing the substance to UV radiation.
    Photoluminescence involves Fluorescence and Phosphorescence phenomenon

    Bioluminescence:
    Emission of light by a living organism and biological process.

    Chemiluminescence:
    Emission of light by chemical reaction. called Chemiluminescence.

    Radioluminescence:
    Emission of light by interaction of radioactive particles with the compounds called Radioluminescence.

    Mechanism of fluorescence and phosphorescence
    Fluorescence involves singlet-singlet transition while phosphorescence involves triplet-singlet transition.
    Out of two electron in “ground singlet state” –> one of electron is excited to higher energy level. –> if excitation not associated with change in spin of electron still spins are in opposite direction electron present in “excited singlet state” and if excitation associated with change in spin and both the electron spin in same direction –> electron present in “excited triplet state”.
    Fluorescence and Phosphorescence  are the deactivation processes
    excited singlet state –> ground singlet state –> no change in spin à emits absorb energy –> fluorescence
    excited triplet state –> ground singlet state –> change in spin –> emits absorb energy –> phosphorescence

    Fluorescence

    Phosphorescence

    No change in spin Change in spin
    excited singlet state à ground singlet state excited triplet state à ground singlet state
    singlet-singlet transition triplet-singlet transition
    Life time of excited state is small (10-6 to 10-9 sec) Life time of excited state is long (10-4 to second or minutes)
    It stops immediately as soon as the source of excitation is removed It seen for longer period even after removing source of  excitation

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