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Fluorescence Spectrum [Анг.]

A molecule that initially absorbs ultraviolet light to get to the excited state and then emit UV or visible light to return to the ground state is said to have undergone photoluminescence. The emission of light is either fluorescence or phosphorescence depending on how the electron returns to the ground state. It is fluorescence if it is a singlet excited state and phosphorescence if it is a triplet excited state. Fluorescence normally has a longer wavelength than the radiation used for the excitation. This is because some of the internal energy is lost before the fluorescent emission occurs. Many molecules do not have any photoluminescence even though they can absorb ultraviolet light. The return to the ground state in these molecules occurs through the internal conversion of the excitation energy into vibrational energy. The result is heat rather than photoluminescence. Generally a molecule that phosphoresces or fluoresces contain at least one aromatic ring. Photoluminescent analysis is more sensitive and selective than absorption spectrophotometry.
The Guide To Acing Physical Pharmacy

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