415-Lecture 6_Raman-LH

415-Lecture 6_Raman-LH - Analytical Chemistry II Analytical...

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Analytical Chemistry II Analytical Chemistry II Raman Spectroscopy Lecture 9, Chapter 18
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Analytical Chemistry II Analytical Chemistry II Another spectroscopic technique which probes the vibrational structure of molecules. C.V. Raman discovered in 1928; received Nobel Prize in 1931. Resurgence in recent years due to the development of new detectors with improved sensitivity. Raman Spectroscopy
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Analytical Chemistry II Analytical Chemistry II Energy Diagram Excited electronic state Ground electronic state 0 1 2 3 0 1 2 3 Virtual electronic states Excitation Rayleigh Scattering Raman Scattering Anti-Stokes Stokes 1 in 10 7 photons
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Analytical Chemistry II Analytical Chemistry II Raman Scattering • Radiation induces oscillations in electric charges of particles; • Does not require dipole moment to exist; but requires “momentary” induced dipole moment; • induced dipoles radiate secondary waves; • excitation frequency is “modulated” by the vibrational frequency of the bond; • particle dimensions are in the same order as wavelength or smaller; larger particles - reflect how easy the electrons deform upon the impinging light: Polarizability -- α
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Analytical Chemistry II Analytical Chemistry II Mathematical Equation of Raman Effect The oscillating electric field of the excitation light. The induced dipole moment from this oscillating field. The molecular polarizability changes with bond length.
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415-Lecture 6_Raman-LH - Analytical Chemistry II Analytical...

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