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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

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

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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)