sem-tem lecture19 clh class

7/29/2019 SEM-TEM Lecture19 Clh Class

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Raman Spectroscopy

1923Inelastic light scattering is predicted by A. Smekel

1928Landsberg and Mandelstam see unexpected

frequency shifts in scattering from quartz

1928C.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:

C6H6Scattering


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Raman Spectroscopy

1923Inelastic light scattering is predicted by A. Smekel

1928Landsberg and Mandelstam see unexpected

frequency shifts in scattering from quartz

1928C.V. Raman and K.S. Krishnan see feeble

fluorescence from neat solvents

1930 C.V. Raman wins Nobel Prize in Physics1961 Invention of laser makes Raman experiments

reasonable

1977 Surface-enhanced Raman scattering (SERS) is

discovered1997 Single molecule SERS is possible


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

Rayleigh Scattering

Elastic ( does not change)

Random direction of emission

Little energy loss

4 2 2

04 2

8 ( ') (1 cos )( )sc

EE d


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Raman Spectroscopy1 in 107 photons is scattered inelastically

Infrared(absorption)

Raman(scattering)

v = 0

v = 1

virtual

state

Excitatio

n

Sc

attered

Rotational Raman

Vibrational Raman

Electronic Raman


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Classical Theory of Raman Effect

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

mind = E

polarizability


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Kellner et al.,Analytical Chemistry

max 0

max max 0

max max 0

( ) cos 2

1cos 2 ( )

2

1cos 2 ( )

2

equil

z zz

zzvib

zzvib

t E t

dr E t

dr

dr E t

dr

m

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

Photon-Molecule Interactions

Rayleigh scatter

Anti-Stokes Raman scatter

Stokes Raman scatter


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8/35www.andor.com

max 0

max max 0

max max 0

( ) cos 2

1 cos 2 ( )2

1cos 2 ( )

2

equil

z zz

zzvib

zzvib

t E t

d r E tdr

dr 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

vibh

kTN eN


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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


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Presentation of Raman Spectra

ex = 1064 nm = 9399 cm-1

Breathing mode:

9399

992 = 8407 cm

-1

Stretching mode:

9399 3063 = 6336 cm-1


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Mutual Exclusion Principle

For molecules with a center of symmetry, no IR activetransitions 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


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Raman vs IR Spectra

Ingle and Crouch, Spectrochemical Analysis


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Raman vs Infrared Spectra

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


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Raman vs Infrared Spectra



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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 ( )iE

kTR ex ex iE n es

Radiant power of Raman scattering:

s(ex) - target area presented by a molecule for

scattering


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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

dd

dd

s

ss

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 EnhancedVibrational Spectroscopy, 2006


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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


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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 usingvisible 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.


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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. CF).


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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


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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.


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The Vinland Map: Genuine or Forged?

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


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The Vinland Map: Forged!

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


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Resonance Raman

Raman signal intensities can be enhanced by resonanceby 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


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Resonance Raman Spectra

Ingle and Crouch, Spectrochemical Analysis


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Resonance Raman Spectra

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

ex = 441.6 nm

ex = 514.5 nm


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Raman Instrumentation

Tunable Laser System Versatile Detection System


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Dispersive and

FT-Raman

Spectrometry

McCreery, R. L., Raman

Spectroscopy for Chemical

Analysis, 3rd ed., Wiley, New

York: 2000


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Spectra from Background Subtraction

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

2000


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Rotating Raman Cells

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

Jersey: 2000


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Raman Spectroscopy: PMT vs CCD


2000

Fl B k d i R


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Fluorescence Background in Raman

Scattering


2000


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Confocal Microscopy Optics



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Confocal Aperture and Field Depth


2000 and http://www.olympusfluoview.com/theory/confocalintro.html


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Confocal Aperture and Field Depth


sem-tem lecture19 clh class

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