Download to read offline
Raman.ppt
- 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: http://www.springerlink.com/content/u4d7aexmjm8pa1fv/fulltext.pdf Filtered Hg arc lamp spectrum: C6H6 Scattering
- 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. Eugene Hecht, Optics,Addison-Wesley, Reading, MA, 1998. Rayleigh Scattering •Elastic ( does not change) •Random direction of emission •Little energy loss 4 2 2 0 4 2 8 ( ') (1 cos ) ( ) sc E E d
- 4. Raman Spectroscopy 1 in107 photons is scattered inelastically Infrared (absorption) Raman (scattering) v” = 0 v” = 1 virtual state Excitation Scattered Rotational Raman Vibrational Raman Electronic Raman
- 5. Classical Theory ofRaman Effect Colthup et al., Introduction to Infrared and Raman Spectroscopy, 3rd ed., Academic Press, Boston: 1990 mind = E polarizability
- 6. Kellner et al.,Analytical Chemistry max 0 max max 0 max max 0 ( ) cos2 1 cos2 ( ) 2 1 cos2 ( ) 2 equil z zz zz vib zz vib t E t d r E t dr d r E t dr m When light interacts with a vibrating diatomic molecule, the induced dipole moment has 3 components: Photon-Molecule Interactions Rayleigh scatter Anti-Stokes Raman scatter Stokes Raman scatter
- 8. www.andor.com max 0 max max0 max max 0 ( ) cos2 1 cos2 ( ) 2 1 cos2 ( ) 2 equil z zz zz vib zz vib t E t d r E t dr d r E t dr m Selection rule: v = ±1 Overtones: v = ±2, ±3, … Raman Scattering Must also have a change in polarizability Classical Description does not suggest any difference between Stokes and Anti-Stokes intensities 1 0 vib h kT N e N
- 9. Calculate the ratioof Anti-Stokes to Stokes scattering intensity when T = 300 K and the vibrational frequency is 1440 cm-1. Are you getting the concept? h = 6.63 x 10-34 Js k = 1.38 x 10-23 J/K
- 10. Presentation of RamanSpectra ex = 1064 nm = 9399 cm-1 Breathing mode: 9399 – 992 = 8407 cm-1 Stretching mode: 9399 – 3063 = 6336 cm-1
- 11. Mutual Exclusion Principle Formolecules 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 IRSpectra Ingle and Crouch, Spectrochemical Analysis
- 13. Raman vs InfraredSpectra McCreery, R. L., Raman Spectroscopy for Chemical Analysis, 3rd ed., Wiley, New York: 2000
- 14. Raman vs InfraredSpectra McCreery, R. L., Raman Spectroscopy for Chemical Analysis, 3rd ed., Wiley, New York: 2000
- 15. Raman Intensities s(ex) –Raman scattering cross-section (cm2) ex – excitation frequency E0 – incident beam irradiance ni – number density in state i exponential – Boltzmann factor for state i 4 0 α ( ) i E kT R ex ex i E n e s Radiant power of Raman scattering: s(ex) - target area presented by a molecule for scattering
- 16. Raman Scattering Cross-Section s(ex)- target area presented by a molecule for scattering scattered flux/unit solid angle indident flux/unit solid angle d d d d d s s s Process Cross-Section of s (cm2) absorption UV 10-18 absorption IR 10-21 emission Fluorescence 10-19 scattering Rayleigh 10-26 scattering Raman 10-29 scattering RR 10-24 scattering SERRS 10-15 scattering SERS 10-16 Table adapted from Aroca, Surface Enhanced Vibrational Spectroscopy, 2006
- 17. Raman Scattering Cross-Section ex(nm) s ( x 10-28 cm2) 532.0 0.66 435.7 1.66 368.9 3.76 355.0 4.36 319.9 7.56 282.4 13.06 Table adapted from Aroca, Surface Enhanced Vibrational Spectroscopy, 2006 CHCl3: C-Cl stretch at 666 cm-1
- 18. Advantages of Ramanover 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 to concentration and laser power.
- 19. Advantages of IRover 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 signalintensities 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 Ingleand Crouch, Spectrochemical Analysis
- 26. Resonance Raman Spectra http://www.photobiology.com/v1/udaltsov/udaltsov.htm ex= 441.6 nm ex = 514.5 nm
- 27. Raman Instrumentation Tunable LaserSystem 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 BackgroundSubtraction McCreery, R. L., Raman Spectroscopy for Chemical Analysis, 3rd ed., Wiley, New York: 2000
- 30. Rotating Raman Cells Rubinson,K. A., Rubinson, J. F., Contemporary Instrumental Analysis, Prentice Hall, New Jersey: 2000
- 31. Raman Spectroscopy: PMTvs CCD McCreery, R. L., Raman Spectroscopy for Chemical Analysis, 3rd ed., Wiley, New York: 2000
- 32. Fluorescence Background inRaman Scattering McCreery, R. L., Raman Spectroscopy for Chemical Analysis, 3rd ed., Wiley, New York: 2000
- 33. Confocal Microscopy Optics McCreery,R. L., Raman Spectroscopy for Chemical Analysis, 3rd ed., Wiley, New York: 2000
- 34. Confocal Aperture andField Depth McCreery, R. L., Raman Spectroscopy for Chemical Analysis, 3rd ed., Wiley, New York: 2000 and http://www.olympusfluoview.com/theory/confocalintro.html
- 35. Confocal Aperture andField Depth McCreery, R. L., Raman Spectroscopy for Chemical Analysis, 3rd ed., Wiley, New York: 2000