Download - Scattering of Light: Raman Spectroscopy
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Scattering of Light: Raman Spectroscopy
Deanna O’DonnellInformal P-Chem Review
June 4th, 2009
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A review of light• Electromagnetic wave
– Oscillating electric and magnetic fields
• Classical Interactions of light and matter– Absorption– Reflection– Refraction– Scattering
• Scattering– Elastic (Rayleigh scattering)
– Inelastic (Raman scattering)
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Cross section ()
• Measure of the likelihood a molecule will absorb a photon
• Beer’s LawA = OD = c l
• Conversion (cm2)= 2303 (M-1cm-1)
Na
• units of cm2
• Typical values ~10-15 cm2
• Raman values ~10-30 cm2
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History
• Sir. C.V. Raman discovered light scattering in 1928
• Awarded Nobel Prize in physics in 1930
• Experiment composed of light source (sunlight), a sample, and detector (eye)
• His nephew, Dr. S. Chandrasekhar, of the University of Chicago won the Nobel prize in physics in 1983
Sir. C.V. Raman
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Raman Basics• Raman spectroscopy studies the frequency change of
light due to the interaction with matter
• The energy of a vibrational mode (m) depends on molecular structure and environment. • Atomic mass, Bond order, Molecular substituents,
Molecular geometry and Hydrogen bonding all contribute
• Raman signal is 10-6 time weaker than incident light (o)
• Photons are not absorbed• To observe Raman scattering
the molecule must be polarizable
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Selection Rules
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More Raman Basics• Raman shifts can be expressed as o ± m
Stokes and Anti-stokes produce same spectrum, differing in intensity. Intensity is governed by the Maxwell-Boltzmann Distribution law.
• Raman shifts are measured in wavenumbers (cm2)
600 400 200 0 -200 -400 -600
460 31
221
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- 21
7-
312
- 46
0
Rayleigh Scattering
Anti-Stokes Scattering
Stokes Scattering
Raman Shift cm-1
Stokes and Anti-stokes Raman Spectrum of CCl4
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Raman Basics• Raman shifts can be expressed as o ± m
Stokes and Anti-stokes produce same spectrum, differing in intensity. Intensity is governed by the Maxwell-Boltzmann Distribution law.
• Raman shifts are measured in wavenumbers (cm-1)
600 400 200 0 -200 -400 -600
460 31
221
7
- 21
7-
312
- 46
0
Rayleigh Scattering
Anti-Stokes Scattering
Stokes Scattering
Raman Shift cm-1
Stokes and Anti-stokes Raman Spectrum of CCl4
E1
E0
StokesScattering
Anti-StokesScattering
RayleighScattering
- m + m
virtualstates
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More Raman Basics
Normal Raman≤100
Resonance Raman≥103
En
erg
y
o = 500nmo = 334nm
S1250 300 350 4000.000
0.005
0.010
0.015
0.020
OD
wavelength nm33
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Absorption of BenzoateOH-adduct(s)
SoSo
Spectra simplified, only totally symmetric modes enhanced – why?
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Signal Enhancement• Common method to enhance the Raman scattering is
• Resonance Raman
Resonance Raman• Occurs when o em
• Enhancement is on the order of 103 to 108
i = ij Ej
i = induced electric dipole ij = polarizability E = electric field of the iiiiiiiiiielectromagnetic radiation
Imn = Io(o-mn)4|(ij)mn|2
(ij)mn (em-o)-1
http://www.personal.dundee.ac.uk/~tjdines/Raman/RR3.HTM
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How do you enhance the signal?
• Two commonly used methods to enhance the Raman scattering are• Resonance Raman• Surface Enhanced Raman
Resonance Raman• Occurs when o em
• Enhancement is on the order of 103 to 108
280 300 320 340 360 380 400 420 440 460 480 500 520
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
0.040
0.045
0.050
2mM Benzoic Acid, 1M t-BuOHpH 13.2, N
2
e- adduct
440 nm
320 nm
Ab
sorp
tio
n
Wavelength nm
i = ij Ej
= induced electric dipole ij = polarizability E = electric field of the iiiiiiiiiielectromagnetic radiation
Imn = Io(o-mn)4|(ij)mn|2
(ij)mn (em-o)-1
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• Discovery• Experimentally discovered by Fleischmann et al. (1974)• Later explained by Van Duyne and Creighton (1977)
• Produces 105 to 106 enhancement• Metal surfaces utilized include
• Ag, Au, Cu, Li, Na, K, In, Pt, Rh• SERS is possible due to Electromagnetic and Chemical
enhancement• Other factors contribute to further enhancement
• NaCl, “hot spots”, concentration, orientation
Surface Enhanced Raman Scattering (SERS)
i = ij Ej
= induced electric dipole
ij = polarizability
E = electric field of the electromagnetic radiation
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Good References
Vibrational Spectroscopy
Wilson, E.B.; Decius, J.C.; Cross, P.C.; Molecular Vibrations, ISBN:0-486-63941-X
Harris, D.C.; Bertolucci, M.D.; Symmetry and Spectroscopy, ISBN: 0-486-66144-X
Raman Spectroscopy
Ferraro, J.R.; Nakamoto, K.; Brown, C.W.; Introductory Raman Spectroscopy, ISBN: 978-0-12-254105-6
Radiation Chemistry Rates
(use index, search engine not reliable)