Presentation on theme: "Raman Spectroscopy 1923 – Inelastic light scattering is predicted by A. Smekel 1928 – Landsberg and Mandelstam see unexpected frequency shifts in scattering."— Presentation transcript:
1Raman Spectroscopy1923 – Inelastic light scattering is predicted by A. Smekel1928 – Landsberg and Mandelstam see unexpected frequency shifts in scattering from quartz1928 – C.V. Raman and K.S. Krishnan see “feeble fluorescence” from neat solventsFirst Raman Spectra:Filtered Hg arclamp spectrum:C6H6Scattering
2Raman Spectroscopy1923 – Inelastic light scattering is predicted by A. Smekel1928 – Landsberg and Mandelstam see unexpected frequency shifts in scattering from quartz1928 – C.V. Raman and K.S. Krishnan see “feeble fluorescence” from neat solvents1930 – C.V. Raman wins Nobel Prize in Physics1961 – Invention of laser makes Raman experiments reasonable1977 – Surface-enhanced Raman scattering (SERS) is discovered1997 – Single molecule SERS is possible
3Rayleigh Scattering Elastic ( does not change) Random direction of emissionLittle energy lossEugene Hecht, Optics, Addison-Wesley, Reading, MA, 1998.
41 in 107 photons is scattered inelastically Raman Spectroscopy1 in 107 photons is scattered inelasticallyInfrared(absorption)Raman(scattering)v” = 0v” = 1virtualstateExcitationScatteredRotational RamanVibrational RamanElectronic Raman
5Classical Theory of Raman Effect mind = aEpolarizabilityColthup et al., Introduction to Infrared and Raman Spectroscopy, 3rd ed., Academic Press, Boston: 1990
6Photon-Molecule Interactions When light interacts with a vibrating diatomic molecule, the induceddipole moment has 3 components:Rayleigh scatterAnti-Stokes Raman scatterStokes Raman scatterKellner et al., Analytical Chemistry
8Raman Scattering Selection rule: Dv = ±1 Overtones: Dv = ±2, ±3, … Must also have a change in polarizabilityClassical Description does not suggest any differencebetween Stokes and Anti-Stokes intensities
9Are you getting the concept? Calculate the ratio of Anti-Stokes to Stokes scatteringintensity when T = 300 K and the vibrational frequencyis 1440 cm-1.h = 6.63 x Jsk = 1.38 x J/K
11Mutual Exclusion Principle For molecules with a center of symmetry, no IR active transitions are Raman active and vice versaSymmetric moleculesIR-active vibrations are not Raman-active.Raman-active vibrations are not IR-active.O = C = O O = C = ORaman active Raman inactiveIR inactive IR active
12Ingle and Crouch, Spectrochemical Analysis Raman vs IR SpectraIngle and Crouch, Spectrochemical Analysis
13Raman vs Infrared Spectra McCreery, R. L., Raman Spectroscopy for Chemical Analysis, 3rd ed., Wiley, New York: 2000
14Raman vs Infrared Spectra McCreery, R. L., Raman Spectroscopy for Chemical Analysis, 3rd ed., Wiley, New York: 2000
15Raman Intensities Radiant power of Raman scattering: s(nex) – Raman scattering cross-section (cm2)nex – excitation frequencyE0 – incident beam irradianceni – number density in state iexponential – Boltzmann factor for state is(nex) - target area presented by a molecule for scattering
16Raman Scattering Cross-Section ProcessCross-Section ofs (cm2)absorptionUV10-18IR10-21emissionFluorescence10-19scatteringRayleigh10-26Raman10-29RR10-24SERRS10-15SERS10-16s(nex) - target area presented by a molecule for scatteringTable adapted from Aroca, Surface EnhancedVibrational Spectroscopy, 2006
17Raman Scattering Cross-Section lex (nm)s ( x cm2)532.00.66435.71.66368.93.76355.04.36319.97.56282.413.06CHCl3:C-Cl stretch at 666 cm-1Table adapted from Aroca, Surface EnhancedVibrational Spectroscopy, 2006
18Advantages 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.
19Advantages 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).
20Raman and FraudLewis, I. R.; Edwards, H. G. M., Handbook of Raman Spectroscopy: From the Research Laboratory to the Process Line, Marcel Dekker, New York:
21Ivory 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.
22The Vinland Map: Genuine or Forged? Brown, K. L.; Clark, J. H. R., Anal. Chem. 2002, 74,3658.
23The Vinland Map: Forged! Brown, K. L.; Clark, J. H. R., Anal. Chem. 2002, 74,3658.
24Kellner et al., Analytical Chemistry Resonance RamanRaman 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
25Resonance Raman Spectra Ingle and Crouch, Spectrochemical Analysis