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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

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raman spectroscopy
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

raman spectroscopy2
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

slide3

Rayleigh Scattering

  • Elastic ( does not change)
  • Random direction of emission
  • Little energy loss

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

raman spectroscopy4

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

classical theory of raman effect
Classical Theory of Raman Effect

mind = aE

polarizability

Colthup et al., Introduction to Infrared and Raman Spectroscopy, 3rd ed., Academic Press, Boston: 1990

photon molecule interactions
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

raman scattering

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

are you getting the concept
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

presentation of raman spectra
Presentation of Raman Spectra

lex = 1064 nm = 9399 cm-1

Breathing mode:

9399 – 992 = 8407 cm-1

Stretching mode:

9399 – 3063 = 6336 cm-1

mutual exclusion principle
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

raman vs ir spectra
Raman vs IR Spectra

Ingle and Crouch, Spectrochemical Analysis

raman vs infrared spectra
Raman vs Infrared Spectra

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

raman vs infrared spectra14
Raman vs Infrared Spectra

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

raman intensities
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

raman scattering cross section
Raman Scattering Cross-Section

s(nex) - target area presented by a molecule for scattering

Table adapted from Aroca, Surface Enhanced

Vibrational Spectroscopy, 2006

raman scattering cross section17
Raman Scattering Cross-Section

CHCl3:

C-Cl stretch at 666 cm-1

Table adapted from Aroca, Surface Enhanced

Vibrational Spectroscopy, 2006

advantages of raman over ir
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.
advantages of ir over raman
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).
raman and fraud
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

ivory or plastic
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.

the vinland map genuine or forged
The Vinland Map: Genuine or Forged?

Brown, K. L.; Clark, J. H. R., Anal. Chem. 2002, 74,3658.

the vinland map forged
The Vinland Map: Forged!

Brown, K. L.; Clark, J. H. R., Anal. Chem. 2002, 74,3658.

resonance raman
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

resonance raman spectra
Resonance Raman Spectra

Ingle and Crouch, Spectrochemical Analysis

resonance raman spectra26
Resonance Raman Spectra

lex = 441.6 nm

lex = 514.5 nm

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

raman instrumentation
Raman Instrumentation

Tunable Laser System

Versatile Detection System

dispersive and ft raman spectrometry
Dispersive and FT-Raman Spectrometry

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

spectra from background subtraction
Spectra from Background Subtraction

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

rotating raman cells
Rotating Raman Cells

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

raman spectroscopy pmt vs ccd
Raman Spectroscopy: PMT vs CCD

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

fluorescence background in raman scattering
Fluorescence Background in Raman Scattering

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

confocal microscopy optics
Confocal Microscopy Optics

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

confocal aperture and field depth
Confocal Aperture and Field Depth

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

confocal aperture and field depth35
Confocal Aperture and Field Depth

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