lecture19 clh class - 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:

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

Filtered Hg arc lamp spectrum:

C6H6 Scattering

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

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

Rayleigh Scattering

•Elastic (λ does not change)

•Random direction of emission

•Little energy loss 4 2 2

04 2

8 ( ') (1 cos )( )scEE

dθπ α θ

λ+

=

Raman Spectroscopy 1 in 107 photons is scattered inelastically

Infrared (absorption)

Raman (scattering)

v” = 0

v” = 1

virtual state

Exci

tatio

n

Scat

tere

d

Rotational Raman Vibrational Raman Electronic Raman

Classical Theory of Raman Effect

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

µind = αE

polarizability

Kellner et al., Analytical Chemistry

max 0

max max 0

max max 0

( ) cos 21 cos 2 ( )21 cos 2 ( )2

equilz zz

zzvib

zzvib

t E td r E tdr

d r E tdr

µ α πνα π ν ν

α π ν ν

= +

∆ + +

∆ −

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

www.andor.com

max 0

max max 0

max max 0

( ) cos 21 cos 2 ( )21 cos 2 ( )2

equilz zz

zzvib

zzvib

t E td r E tdr

d r E tdr

µ α πνα π ν ν

α π ν ν

= +

∆ + +

∆ −

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

vibhkTN e

N

ν−

=

Calculate the ratio of 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

Presentation of Raman Spectra

λex = 1064 nm = 9399 cm-1

Breathing mode: 9399 – 992 = 8407 cm-1

Stretching mode: 9399 – 3063 = 6336 cm-1

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

Ingle and Crouch, Spectrochemical Analysis

Raman vs Infrared Spectra

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

Raman Intensities

σ(νex) – Raman scattering cross-section (cm2) νex – excitation frequency E0 – incident beam irradiance ni – number density in state i exponential – Boltzmann factor for state i

40 α ( )

iEkT

R ex ex iE n eσ ν ν−

Φ

Radiant power of Raman scattering:

σ(νex) - target area presented by a molecule for scattering

Raman Scattering Cross-Section

σ(νex) - target area presented by a molecule for scattering

scattered flux/unit solid angleindident flux/unit solid angle

dd

ddd

σ

σσ

=Ω

= ΩΩ∫

Process Cross-Section of σ (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

Raman Scattering Cross-Section

λex (nm) σ ( 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

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 α to concentration and laser power.

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

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?

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?

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

The Vinland Map: Forged!

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

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

Ingle and Crouch, Spectrochemical Analysis

Resonance Raman Spectra

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

λex = 441.6 nm

λex = 514.5 nm

Raman Instrumentation

Tunable Laser System Versatile Detection System

Dispersive and FT-Raman

Spectrometry

McCreery, R. L., Raman Spectroscopy for Chemical

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

Spectra from Background Subtraction


Rotating Raman Cells

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

Raman Spectroscopy: PMT vs CCD


Fluorescence Background in Raman Scattering


Confocal Microscopy Optics


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 Depth


lecture19 clh class - raman spectroscopy

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