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Raman spectroscopy


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Raman Spectroscopy(Principle and applications)

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Raman spectroscopy

  1. 1. Chemistry of Raman Spectroscopy • Monochromatic light applied to sample • Incident light is scattered – Rayleigh (elastic) and Raman (inelastic) • Rayleigh scatter is filtered out • The returned scattered light is a different wavelength • This difference corresponds to an energy shift which provides a unique chemical fingerprint
  2. 2. Careful observation, however, reveals that: A very small fraction of the radiation is transmitted at all angles from the original path and that the intensity of this scattered radiation increases with particle size. Types of Scattering: • Rayleigh scattering Scattering by molecules or aggregates of molecules with dimensions significantly smaller than the wavelength of the radiation. Its intensity is proportional to: - The inverse fourth power of the wavelength (. - The dimensions of the scattering particles. • The square of the polarizability of the particles. • An everyday manifestation of Rayleigh scattering is the blue color of the sky, which results from greater scattering of the shorter wavelengths of the visible spectrum.
  3. 3. The blue color of the sky is caused by the scattering of sunlight off the molecules of the atmosphere. This scattering, called, is more effective at short wavelengths (the blue end of the visible spectrum). Therefore the light scattered down to the earth at a large angle with respect to the direction of the sun's light is predominantly in the blue end of the spectrum.
  4. 4. Raman Scattering • The Raman scattering effect differs from ordinary scattering in that part of the scattered radiation suffers quantized frequency changes. • These changes are the result of vibrational energy level transitions that occur in the molecules as a consequence of the polarization process.
  5. 5. • Basic Physical Realization – Illuminate a specimen with laser light (e.g. 532nm) – Scattered (no absorbed) Light in two forms • Elastic (Rayleigh) → scattered = incident • InElastic (Raman) → scattered  incident – Light Experiences a “Raman Shift” in Wave Length.
  6. 6. Raman vs. Rayleigh • Raman Intensity is About 0.1 ppm of the Incoming Laser Intensity
  7. 7. • Inelastic Light Scattering Mechanisms • Raman Shift Can be: – To Longer WaveLengths (Stokes Scattering) • Loses Energy – Predominant Raman Shift – To Shorter WaveLengths (AntiStokes Scattering) • Gains Energy – Subordinate Raman Shift Raman Stokes Scattering Raman AntiStokes Scattering
  8. 8. 14-6 The Doppler Effect The Doppler effect is the change in pitch of a sound when the source and observer are moving with respect to each other. When an observer moves toward a source, the wave speed appears to be higher, and the frequency appears to be higher as well.
  9. 9. Application of Doppler Effect Nexrad: Next Generation Weather Radar
  10. 10. Ultrasonic Scanner
  11. 11. The cavitron ultrasonic surgical aspirator (CUSA) Neurosurgeons use a cavitron ultrasonic surgical aspirator (CUSA) to “cut out” brain tumors without adversely affecting the surrounding healthy tissue.
  12. 12. Bohr Model of the Atom Electrons in Atoms nucleus (+) electron (-) Courtesy Christy Johannesson
  13. 13. Atomic Spectrum How color tells us about atoms
  14. 14. Prism • White light is made up of all the colors of the visible spectrum. • Passing it through a prism separates it. Author: Thomas V. Green Jr.
  15. 15. If the light is not white • By heating a gas or with electricity we can get it to give off colors. • Passing this light through a prism does something different. Author: Thomas V. Green Jr.
  16. 16. Atomic Spectrum • Each element gives off its own characteristic colors. • Can be used to identify the atom. • How we know what stars are made of. Author: Thomas V. Green Jr.
  17. 17. • These are called line spectra • unique to each element. • These are emission spectra • The light is emitted given off. Author: Thomas V. Green Jr.
  18. 18. Line-Emission Spectrum ground state excited state ENERGY IN PHOTON OUT Courtesy Christy Johannesson 656 nm486 nm410 nm 434 nm Wavelength (nm) PrismSlits
  19. 19. Bohr Model • electrons exist only in orbits with specific amounts of energy called energy levels • Therefore… • electrons can only gain or lose certain amounts of energy • only certain photons are produced Courtesy Christy Johannesson
  20. 20. Bohr Model 1 2 3 4 5 6 • Energy of photon depends on the difference in energy levels • Bohr’s calculated energies matched the IR, visible, and UV lines for the H atom Courtesy Christy Johannesson nucleus
  21. 21. Other Elements • Each element has a unique bright-line emission spectrum. i.e. “Atomic Fingerprint” Helium Bohr’s calculations only worked for hydrogen!  Courtesy Christy Johannesson
  22. 22. Bohr’s Experiment Kelter, Carr, Scott, Chemistry A Wolrd of Choices 1999, page 76 Animation by Raymond Chang – All rights reserved.
  23. 23. Copyright © 2007 Pearson Benjamin Cummings. All rights reserved. (a) Electronic absorption transition (b) H2 emission spectrum (top), H2 absorption spectrum (bottom)
  24. 24. continuous spectrum absorption spectrum emission spectrum hot source gas absorption spectrum emission spectrum
  25. 25. Hydrogen Spectral Lines Lyman series (ultraviolet) Balmer series (visible) Paschen series (infrared) Frequency (hertz) 1016 1015 1014 7 6 5 4 3 2 1n =
  26. 26. Copyright © 2007 Pearson Benjamin Cummings. All rights reserved. (ultraviolet) (visible) (infrared) HYDROGEN SPECTRAL LINES
  27. 27. Hydrogen Spectral Lines A B C D E F Lyman series (UV) A B C D E Balmer (Visible) A B C D Paschen (IR) E1 E2 E3 E4 E5 E6 Energy Bohr’s model of the atom accounted mathematically for the energy of each of the transitions shown. IR region UV region 656 nm 486 nm 434 nm 410 nm Davis, Metcalfe, Williams, Castka, Modern Chemistry, 1999, page 97 ionization