Raman SpectroscopyRaman Spectroscopy1923 – 1923 – Inelastic light scattering is predicted by A. SmekelInelastic light scattering is predicted by A. Smekel1928 – 1928 – Landsberg and Mandelstam see unexpected Landsberg and Mandelstam see unexpected frequency shifts in scattering from quartzfrequency shifts in scattering from quartz1928 – 1928 – C.V. Raman and K.S. Krishnan see “feeble C.V. Raman and K.S. Krishnan see “feeble fluorescence” from neat solvents fluorescence” from neat solvents

First Raman Spectra:First Raman Spectra:

http://www.springerlink.com/content/u4d7aexmjm8pa1fv/fulltext.pdfhttp://www.springerlink.com/content/u4d7aexmjm8pa1fv/fulltext.pdf

Filtered Hg arcFiltered Hg arclamp spectrum:lamp spectrum:

CC66HH66

ScatteringScattering

Raman SpectroscopyRaman Spectroscopy1923 – 1923 – Inelastic light scattering is predicted by A. SmekelInelastic light scattering is predicted by A. Smekel1928 – 1928 – Landsberg and Mandelstam see unexpected Landsberg and Mandelstam see unexpected frequency shifts in scattering from quartzfrequency shifts in scattering from quartz1928 – 1928 – C.V. Raman and K.S. Krishnan see “feeble C.V. Raman and K.S. Krishnan see “feeble fluorescence” from neat solvents fluorescence” from neat solvents 1930 –1930 – C.V. Raman wins Nobel Prize in Physics C.V. Raman wins Nobel Prize in Physics1961 –1961 – Invention of laser makes Raman experiments Invention of laser makes Raman experiments reasonablereasonable1977 –1977 – Surface-enhanced Raman scattering (SERS) is Surface-enhanced Raman scattering (SERS) is discovereddiscovered1997 –1997 – Single molecule SERS is possible Single molecule SERS is possible

Eugene Hecht, Eugene Hecht, OpticsOptics, Addison-Wesley, Reading, MA, 1998., Addison-Wesley, Reading, MA, 1998.

Rayleigh ScatteringRayleigh Scattering

•Elastic (Elastic ( does not change) does not change)

•Random direction of emissionRandom direction of emission

•Little energy lossLittle energy loss4 2 2

04 2

8 ( ') (1 cos )( )sc

EE

d

Raman SpectroscopyRaman Spectroscopy1 in 101 in 1077 photons is scattered inelastically photons is scattered inelastically

Infrared(absorption)

Raman(scattering)

v” = 0

v” = 1

virtualstate

Exc

itat

ion

Sca

tter

ed

Rotational RamanRotational RamanVibrational RamanVibrational RamanElectronic RamanElectronic Raman

Classical Theory of Raman EffectClassical Theory of Raman Effect

Colthup et al., Colthup et al., Introduction to Infrared and Raman Spectroscopy, 3rd ed.Introduction to Infrared and Raman Spectroscopy, 3rd ed. , Academic Press, Boston: , Academic Press, Boston: 19901990

indind = = EE

polarizabilitypolarizability

Kellner et al., Kellner et al., Analytical ChemistryAnalytical Chemistry

max 0

max max 0

max max 0

( ) cos 2

1cos 2 ( )

21

cos 2 ( )2

equilz zz

zzvib

zzvib

t E t

dr E t

drd

r E tdr

When light interacts with a vibrating diatomic molecule, the inducedWhen light interacts with a vibrating diatomic molecule, the induceddipole moment has 3 components:dipole moment has 3 components:

Photon-Molecule InteractionsPhoton-Molecule Interactions

Rayleigh scatterRayleigh scatter

Anti-Stokes Raman scatterAnti-Stokes Raman scatter

Stokes Raman scatterStokes Raman scatter

www.andor.comwww.andor.com

max 0

max max 0

max max 0

( ) cos 2

1cos 2 ( )

21

cos 2 ( )2

equilz zz

zzvib

zzvib

t E t

dr E t

drd

r E tdr

Selection rule: Selection rule: v = v = ±±11Overtones: Overtones: v = ±2, ±3, …v = ±2, ±3, …

Raman ScatteringRaman Scattering

Must also have a change in polarizabilityMust also have a change in polarizability

Classical Description does not suggest any difference Classical Description does not suggest any difference between Stokes and Anti-Stokes intensitiesbetween Stokes and Anti-Stokes intensities

1

0

vibh

kTN

eN

Calculate the ratio of Anti-Stokes to Stokes scatteringCalculate the ratio of Anti-Stokes to Stokes scatteringintensity when T = 300 K and the vibrational frequencyintensity when T = 300 K and the vibrational frequencyis 1440 cmis 1440 cm-1-1..

Are you getting the concept?Are you getting the concept?

h = 6.63 x 10h = 6.63 x 10-34-34 Js Jsk = 1.38 x 10k = 1.38 x 10-23-23 J/K J/K

Presentation of Raman SpectraPresentation of Raman Spectra

exex = 1064 nm = 9399 cm = 1064 nm = 9399 cm-1-1

Breathing mode:Breathing mode:9399 – 992 = 8407 cm9399 – 992 = 8407 cm-1-1

Stretching mode:Stretching mode:9399 – 3063 = 6336 cm9399 – 3063 = 6336 cm-1-1

Mutual Exclusion PrincipleMutual Exclusion Principle

For molecules with a center of symmetry, no IR active For molecules with a center of symmetry, no IR active transitions are Raman active and vice versatransitions are Raman active and vice versa

Symmetric moleculesSymmetric molecules

IR-active vibrations are not Raman-active.IR-active vibrations are not Raman-active.

Raman-active vibrations are not IR-active.Raman-active vibrations are not IR-active.

O = C = OO = C = O O = C = O O = C = O

Raman active Raman inactiveRaman active Raman inactive IR inactiveIR inactive IR active IR active

Raman vs IR SpectraRaman vs IR Spectra

Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis

Raman vs Infrared SpectraRaman vs Infrared Spectra

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

Raman vs Infrared SpectraRaman vs Infrared Spectra

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

Raman IntensitiesRaman Intensities

((exex) – Raman scattering cross-section (cm) – Raman scattering cross-section (cm22))

exex – excitation frequency – excitation frequency

EE00 – incident beam irradiance – incident beam irradiance

nnii – number density in state i – number density in state i

exponential – Boltzmann factor for state iexponential – Boltzmann factor for state i

40 α ( )

iE

kTR ex ex iE n e

Radiant power of Raman scattering:Radiant power of Raman scattering:

((exex) - target area presented by a molecule for ) - target area presented by a molecule for

scatteringscattering

Raman Scattering Cross-SectionRaman Scattering Cross-Section

((exex) - target area ) - target area

presented by a presented by a molecule for scatteringmolecule for scattering

scattered flux/unit solid angle

indident flux/unit solid angle

d

d

dd

d

Process Cross-Section of (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, Table adapted from Aroca, Surface EnhancedSurface EnhancedVibrational SpectroscopyVibrational Spectroscopy, 2006, 2006

Raman Scattering Cross-SectionRaman Scattering Cross-Section

ex (nm) ( 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 13.06Table adapted from Aroca, Table adapted from Aroca, Surface EnhancedSurface EnhancedVibrational SpectroscopyVibrational Spectroscopy, 2006, 2006

CHClCHCl33::

C-Cl stretch at 666 cmC-Cl stretch at 666 cm-1-1

Advantages of Raman over IRAdvantages of Raman over IR

• Water can be used as solvent. Water can be used as solvent.

•Very suitable for biological samples in native state Very suitable for biological samples in native state (because water can be used as solvent).(because water can be used as solvent).

• Although Raman spectra result from molecular Although Raman spectra result from molecular vibrations at IR frequencies, spectrum is obtained using vibrations at IR frequencies, spectrum is obtained using visible light or NIR radiation.visible light or NIR radiation.

=>Glass and quartz lenses, cells, and optical fibers =>Glass and quartz lenses, cells, and optical fibers can be used. Standard detectors can be used.can be used. Standard detectors can be used.

• Few intense overtones and combination bands => few Few intense overtones and combination bands => few spectral overlaps. spectral overlaps.

• Totally symmetric vibrations are observable.Totally symmetric vibrations are observable.

•Raman intensities Raman intensities to concentration and laser power. to concentration and laser power.

Advantages of IR over RamanAdvantages of IR over Raman

• Simpler and cheaper instrumentation.Simpler and cheaper instrumentation.

• Less instrument dependent than Raman spectra Less instrument dependent than Raman spectra because IR spectra are based on measurement of because IR spectra are based on measurement of intensity intensity ratioratio..

• Lower detection limit than (normal) Raman.Lower detection limit than (normal) Raman.

• Background fluorescence can overwhelm Raman.Background fluorescence can overwhelm Raman.

• More suitable for vibrations of bonds with very low More suitable for vibrations of bonds with very low polarizability (e.g. C–F).polarizability (e.g. C–F).

Raman and FraudRaman and Fraud

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

Ivory or Plastic?Ivory or Plastic?

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

The Vinland Map: Genuine or Forged?The Vinland Map: Genuine or Forged?

Brown, K. L.; Clark, J. H. R., Brown, K. L.; Clark, J. H. R., Anal. Chem. Anal. Chem. 20022002, 74,, 74,3658.3658.

The Vinland Map: Forged!The Vinland Map: Forged!

Brown, K. L.; Clark, J. H. R., Brown, K. L.; Clark, J. H. R., Anal. Chem. Anal. Chem. 20022002, 74,, 74,3658.3658.

Resonance RamanResonance Raman

Raman signal intensities can be enhanced by resonance Raman signal intensities can be enhanced by resonance by factor of up to 10by factor of up to 1055 => Detection limits 10 => Detection limits 10-6-6 to 10 to 10-8-8 M. M.

Typically requires tunable laser as light source.Typically requires tunable laser as light source.

Kellner et al., Kellner et al., Analytical ChemistryAnalytical Chemistry

Resonance Raman SpectraResonance Raman Spectra

Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis

Resonance Raman SpectraResonance Raman Spectra

http://www.photobiology.com/v1/udaltsov/udaltsov.htmhttp://www.photobiology.com/v1/udaltsov/udaltsov.htm

exex = 441.6 nm = 441.6 nm

exex = 514.5 nm = 514.5 nm

Raman InstrumentationRaman Instrumentation

Tunable Laser System Versatile Detection System

Dispersive and Dispersive and FT-Raman FT-Raman

SpectrometrySpectrometry

McCreery, R. L., McCreery, R. L., Raman Raman Spectroscopy for Chemical Spectroscopy for Chemical

Analysis, 3rd ed.Analysis, 3rd ed., Wiley, New , Wiley, New York: 2000York: 2000

Spectra from Background SubtractionSpectra from Background Subtraction

McCreery, R. L., McCreery, R. L., Raman Spectroscopy for Chemical Analysis, 3rd ed.Raman Spectroscopy for Chemical Analysis, 3rd ed., Wiley, New York: , Wiley, New York: 20002000

Rotating Raman CellsRotating Raman Cells

Rubinson, K. A., Rubinson, J. F., Rubinson, K. A., Rubinson, J. F., Contemporary Instrumental AnalysisContemporary Instrumental Analysis, Prentice Hall, New , Prentice Hall, New Jersey: 2000Jersey: 2000

Raman Spectroscopy: PMT vs CCDRaman Spectroscopy: PMT vs CCD

McCreery, R. L., McCreery, R. L., Raman Spectroscopy for Chemical Analysis, 3rd ed.Raman Spectroscopy for Chemical Analysis, 3rd ed., Wiley, New York: , Wiley, New York: 20002000

Fluorescence Background in Raman Fluorescence Background in Raman ScatteringScattering

McCreery, R. L., McCreery, R. L., Raman Spectroscopy for Chemical Analysis, 3rd ed.Raman Spectroscopy for Chemical Analysis, 3rd ed., Wiley, New York: , Wiley, New York: 20002000

Confocal Microscopy OpticsConfocal Microscopy Optics

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

Confocal Aperture and Field DepthConfocal Aperture and Field Depth

McCreery, R. L., McCreery, R. L., Raman Spectroscopy for Chemical Analysis, 3rd ed.Raman Spectroscopy for Chemical Analysis, 3rd ed., Wiley, New York: , Wiley, New York: 2000 a2000 and nd http://www.olympusfluoview.com/theory/confocalintro.html

Confocal Aperture and Field DepthConfocal Aperture and Field Depth

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