Page 57 - Practical book PC406-MNU-2025

MANSOURA NATIONAL UNIVERSIY
PHARM D- CLINICAL PHARMACY LEVEL II INSTRUNENTAL ANALYSIS (PC 406)

II- Spectrofluorimetry

Both fluorescence and phosphorescence are types of photoluminescence (luminescence)

Luminescence:

 It is the process of re-emission of previously absorbed light.

 When the molecules in the ground state absorb UV light, they are transfered to

the excited state, then, reemission of the previously absorbed light takes place

and the molecules return to the ground state where fluorescence or

phosphorescence takes place

Molecular Emission:

 After the absorption of UV Visible light, the excited molecular species are

extremely short-lived and deactivation occurs due to:

a. Internal collision ( internal conversion)

b. Cleavage of chemical bonds initiating photochemical reactions
c. Re-emission of light (luminescence)

d. Heat
e. Interaction between the solute and the solvent

 Re-emission of energy as luminescence occurs from molecules in which the
electron system is shielded from normal deactivation process so that complete

deactivation by collisions is discouraged.

 Molecules on excitation normally posses higher vibrational energy than they

had in the ground state.

 This extra vibrational energy is lost (Fig.1 ) by collision after which the

molecules return to the ground electronic state with the emission of light as

fluorescence.

 Deactivation as fluorescence is a rapid process occurring within 10 -10
-6
-9
seconds of the excitation.

 Figure 1 shows the energy transfer during the absorption, fluorescence and

phosphorescence of UV-Visible radiation.

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