FIU CHM 4130 - Chapter 15 Xiao_Molecular Luminescence Spectrometry_2018 (51 pages)

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Chapter 15 Xiao_Molecular Luminescence Spectrometry_2018



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Molecular Luminescence Spectroscopy Chapter 15 Fluorescence Phosphorescence and Chemiluminescence What happens to the absorbed EM energy determines whether you have Absorbance molecule returns to the ground or lower energy state via a non radia ve transi on such as vibra on collision with other molecules etc These give o the energy absorbed rather than the emission of light Fluorescence Some energy is lost through various processes e g non radia ve transi ons and then light is given o Phosphorescence The molecule transi ons from an excited triplet state to an lower energy singlet state and gives o light Fluorescence Phosphorescence Chemiluminescence Absorption 10 14 1 to 10 15 s 10 8 For UV Vis need to observe Po and P difference which limits detection 10 9s M M heat Theory of Fluorescence and Phosphorescence 10 5 to 10 8 s fluorescence 10 4 to 10 s phosphorescence Excitation of e by absorbance of h Re emission of hv as e goes to ground state Use h 2 for qualitative and quantitative analysis Method Mass detec on limit mole Concentra on detec on limit molar Advantages UV Vis 10 13 to 10 16 10 5 to 10 8 Universal Fluorescence 10 15 to 10 17 10 7 to 10 9 Sensi ve Fluorescence Pauli Exclusion Principle 2 Fluorescence ground state to single state and back Phosphorescence ground state to triplet state and back Fluorescence Phosphorescence 10 5 to 10 8 s 10 4 to 10 s Spins paired No net magnetic field Spins unpaired net magnetic field Rates of Absorption and Emission Absorp on Emission Rate seconds Comments Photo Absorp on 10 14 to 10 15 Fast Fluorescence 10 5 to 10 10 Fast singlet to singlet transi on Phosphorescence 10 4 to 10 Slow triplet to singlet transi on S1 S0 Deactivation Processes a vibrational relaxation solvent collisions emission excitation Stokes shift vibrational relaxation is efficient and goes to lowest vibrational level of electronic state within 10 12s or less significantly shorter life time than electronically excited state fluorescence occurs from



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