Chapter 27 - Chapter 27 Molecular Fluorescence Spectroscopy...

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Unformatted text preview: Chapter 27 Molecular Fluorescence Spectroscopy Fluorescence is a photoluminescence process in which atoms or molecules are excited by absorption of electromagnetic radiation. The excited species then relax to the ground state, giving up their excess energy as photons. One of the most attractive features of molecular fluorescence is its inherent sensitivity, which is often one to three orders of magnitude better than absorption spectroscopy. continued Another advantage is the large linear concentration range of fluorescence methods, which is significantly greater than those encountered in absorption spectroscopy. Fluorescence methods are, however, much less widely applicable than absorption methods because of the relatively limited number of chemical systems that show appreciable fluorescence. Principles of Molecular Fluorescence Molecular fluorescence is measured by exciting the sample at the absorption wavelength, also called excitation wavelength , and measuring the emission at a longer wavelength called the emission or fluorescence wavelength . Usually, fluorescence emission is measured at right angles to the incident beam so as to avoid measuring the incident radiation. The short-lived emission that occurs is called fluorescence , whereas luminescence that is much longer lasting is called phosphorescence . Excitation Spectra and Fluorescence Spectra Because the energy differences between vibrational states is about the same for both ground and excited states, the absorption, or excitation spectrum , and the fluorescence spectrum for a compound often appear as approximate mirror images of one another with overlap occurring near the origin transition (0 vibrational level of E 1 to 0 vibrational level of E ). There are many exceptions to this mirror-image rule, particularly when the excited and ground states have different molecular geometries or when different fluorescence bands originate from different parts of the molecule. Concentration and Fluorescence Intensity...
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This note was uploaded on 05/09/2010 for the course CHEM 3245 taught by Professor Anwara.bhuiyan during the Spring '10 term at Arkansas Tech.

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Chapter 27 - Chapter 27 Molecular Fluorescence Spectroscopy...

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