Raman Spectroscopy

1. Raman Spectroscopy 1923 – Inelastic light scattering is predicted by A. Smekel 1928 – Landsberg and Mandelstam see unexpected frequency shifts in scattering from quartz 1928 – C.V. Raman and K.S. Krishnan see “feeble fluorescence” from neat solvents First Raman Spectra: Filtered Hg arc lamp spectrum: C6H6 Scattering http://www.springerlink.com/content/u4d7aexmjm8pa1fv/fulltext.pdf

2. Raman Spectroscopy 1923 – Inelastic light scattering is predicted by A. Smekel 1928 – Landsberg and Mandelstam see unexpected frequency shifts in scattering from quartz 1928 – C.V. Raman and K.S. Krishnan see “feeble fluorescence” from neat solvents 1930 – C.V. Raman wins Nobel Prize in Physics 1961 – Invention of laser makes Raman experiments reasonable 1977 – Surface-enhanced Raman scattering (SERS) is discovered 1997 – Single molecule SERS is possible

3. Rayleigh Scattering • Elastic ( does not change) • Random direction of emission • Little energy loss Eugene Hecht, Optics, Addison-Wesley, Reading, MA, 1998.

4. virtual state Scattered Excitation v” = 1 v” = 0 Infrared (absorption) Raman (scattering) Raman Spectroscopy 1 in 107 photons is scattered inelastically Rotational Raman Vibrational Raman Electronic Raman

5. Classical Theory of Raman Effect mind = aE polarizability Colthup et al., Introduction to Infrared and Raman Spectroscopy, 3rd ed., Academic Press, Boston: 1990

6. Photon-Molecule Interactions When light interacts with a vibrating diatomic molecule, the induced dipole moment has 3 components: Rayleigh scatter Anti-Stokes Raman scatter Stokes Raman scatter Kellner et al., Analytical Chemistry

8. Must also have a change in polarizability Raman Scattering Selection rule: Dv = ±1 Overtones: Dv = ±2, ±3, … Classical Description does not suggest any difference between Stokes and Anti-Stokes intensities www.andor.com

9. Are you getting the concept? Calculate the ratio of Anti-Stokes to Stokes scattering intensity when T = 300 K and the vibrational frequency is 1440 cm-1. h = 6.63 x 10-34 Js k = 1.38 x 10-23 J/K

10. Presentation of Raman Spectra lex = 1064 nm = 9399 cm-1 Breathing mode: 9399 – 992 = 8407 cm-1 Stretching mode: 9399 – 3063 = 6336 cm-1

11. Mutual Exclusion Principle • For molecules with a center of symmetry, no IR active transitions are Raman active and vice versa • Symmetric molecules IR-active vibrations are not Raman-active. Raman-active vibrations are not IR-active. O = C = O O = C = O Raman active Raman inactive IR inactive IR active

12. Raman vs IR Spectra Ingle and Crouch, Spectrochemical Analysis

13. Raman vs Infrared Spectra McCreery, R. L., Raman Spectroscopy for Chemical Analysis, 3rd ed., Wiley, New York: 2000

14. Raman vs Infrared Spectra McCreery, R. L., Raman Spectroscopy for Chemical Analysis, 3rd ed., Wiley, New York: 2000

15. Raman Intensities Radiant power of Raman scattering: s(nex) – Raman scattering cross-section (cm2) nex – excitation frequency E0 – incident beam irradiance ni – number density in state i exponential – Boltzmann factor for state i s(nex) - target area presented by a molecule for scattering

16. Raman Scattering Cross-Section s(nex) - target area presented by a molecule for scattering Table adapted from Aroca, Surface Enhanced Vibrational Spectroscopy, 2006

17. Raman Scattering Cross-Section CHCl3: C-Cl stretch at 666 cm-1 Table adapted from Aroca, Surface Enhanced Vibrational Spectroscopy, 2006

18. Advantages of Raman over IR • Water can be used as solvent. • Very suitable for biological samples in native state (because water can be used as solvent). • Although Raman spectra result from molecular vibrations at IR frequencies, spectrum is obtained using visible light or NIR radiation. • =>Glass and quartz lenses, cells, and optical fibers can be used. Standard detectors can be used. • Few intense overtones and combination bands => few spectral overlaps. • Totally symmetric vibrations are observable. • Raman intensities a to concentration and laser power.

19. Advantages of IR over Raman • Simpler and cheaper instrumentation. • Less instrument dependent than Raman spectra because IR spectra are based on measurement of intensity ratio. • Lower detection limit than (normal) Raman. • Background fluorescence can overwhelm Raman. • More suitable for vibrations of bonds with very low polarizability (e.g. C–F).

20. Raman and Fraud Lewis, I. R.; Edwards, H. G. M., Handbook of Raman Spectroscopy: From the Research Laboratory to the Process Line, Marcel Dekker, New York: 2001.0

21. Ivory or Plastic? Lewis, I. R.; Edwards, H. G. M., Handbook of Raman Spectroscopy: From the Research Laboratory to the Process Line, Marcel Dekker, New York: 2001.

22. The Vinland Map: Genuine or Forged? Brown, K. L.; Clark, J. H. R., Anal. Chem. 2002, 74,3658.

23. The Vinland Map: Forged! Brown, K. L.; Clark, J. H. R., Anal. Chem. 2002, 74,3658.

24. Resonance Raman Raman signal intensities can be enhanced by resonance by factor of up to 105 => Detection limits 10-6 to 10-8 M. Typically requires tunable laser as light source. Kellner et al., Analytical Chemistry

25. Resonance Raman Spectra Ingle and Crouch, Spectrochemical Analysis

26. Resonance Raman Spectra lex = 441.6 nm lex = 514.5 nm http://www.photobiology.com/v1/udaltsov/udaltsov.htm

27. Raman Instrumentation Tunable Laser System Versatile Detection System

28. Dispersive and FT-Raman Spectrometry McCreery, R. L., Raman Spectroscopy for Chemical Analysis, 3rd ed., Wiley, New York: 2000

29. Spectra from Background Subtraction McCreery, R. L., Raman Spectroscopy for Chemical Analysis, 3rd ed., Wiley, New York: 2000

30. Fluorescence Background in Raman Scattering McCreery, R. L., Raman Spectroscopy for Chemical Analysis, 3rd ed., Wiley, New York: 2000

31. Rotating Raman Cells Rubinson, K. A., Rubinson, J. F., Contemporary Instrumental Analysis, Prentice Hall, New Jersey: 2000

32. Raman Spectroscopy: PMT vs CCD McCreery, R. L., Raman Spectroscopy for Chemical Analysis, 3rd ed., Wiley, New York: 2000