Raman Spectroscopy and Thin Films

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Raman Spectroscopy and Thin Films

• Alexander Couzis
• ChE5535

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Principle of Raman Spectroscopy
Incident Light
Scattered Light
Sample
Raleigh Scatter (same wavelength as incident
light)
Raman Scatter (new wavelength)
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Raman Scattering Classical Description
Consider a diatomic molecule If light with an
electrical field EE0coswt is incident on this
molecule, the molecule will oscillate. The
induced dipole moment m, is then given by
a, is the polarizability of the molecule, and it
is a function of the separation between the
atoms. If x is the displacement from the
equilibrium separation between the atoms we can
expand the polarizability about x0, the
equilibrium separation
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Raman Scattering Classical Description
The molecule also vibrates at its resonant
(natural) vibration frequency w0, so that x
acoswt and substituting we get
Since the dipole radiates light at its
oscillating frequency, the molecule will emit
light at three frequencies, w, w-w0, and
ww0. w-w0 Stokes frequency ww0 Anti-Stokes
frequency
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Principle of Raman
The polarizability depends on how tightly the
electrons are bound to the nuclei. In the
symmetric stretch the strength of electron
binding is different between the minimum and
maximum internuclear distances. Therefore the
polarizability changes during the vibration and
this vibrational mode scatters Raman light (the
vibration is Raman active). In the asymmetric
stretch the electrons are more easily polarized
in the bond that expands but are less easily
polarized in the bond that compresses. There is
no overall change in polarizability and the
asymmetric stretch is Raman inactive.
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Experimental Set-Up
Filters
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Integrated Optics Raman Spectroscopy
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IO-Raman on Thin Films

• Rabe, J. P., J. D. Swalen, et al. (1978).
  Order-Disorder transitions in Langmuir-Blodgett
  Films. III. Polarized Raman Studies of Cadmium
  Arachidate Using Integrated Optical Techniques.
  J. Chem. Phys. 86(3) 1601-1607.
• Rabolt, J. F., R. Santo, et al. (1979). Raman
  Spectroscopy of Thin Polymer Films Using
  Integrated Optical Techniques. Applied
  Spectroscopy 33(6) 549-551.
• Rabolt, J. F., R. Santo, et al. (1980). Raman
  Measurements on Thin Polymer Films and Organic
  Monolayers. Applied Spectroscopy 34(5) 517-521.
• Rabolt, J. F., N. E. Schlotter, et al. (1981).
  Spectroscopic Studies of Thin Film Polymer
  Laminates Using Raman Spectroscopy and Intergated
  Optics. J. Phys. Chem 85 4141-4144.
• Rabolt, J. F., N. E. Schlotter, et al. (1983).
  Comparative Raman Studies of Molecular
  Interactions at a Dye/Polymer and a Dye/Glass
  Interface. Journal of Polymer Science Polymer
  Physics Edition 21 1-9.
• Schlotter, N. E. and J. F. Rabolt (1984).
  Measurements of the Optical Anisotropy of
  Trapped Molecules in Oriented Polymer Films by
  Waveguide Raman Spectroscopy (WRS). Applied
  Spectroscopy 38(2) 208-211.
• Schlotter, N. E. and J. F. Rabolt (1984). Raman
  Spectroscopy in Polymeric Thin Film Optical
  Waveguides. 1. Polarized Measurements and
  Orientational Effects in Two-Dimensional Films.
  J. Phys. Chem. 88 2062-2067.

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Internal Reflection Raman
Iwamoto, R., M. Miya, et al. (1981). J. Chem.
Phys. 74(9) 4780-4790.
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Surface-enhanced Raman scattering
Light incident on a molecule can lose energy to a
vibrational mode and be scattered at a lower
frequency. The Raman signal normally has a small
cross section but systems comprising rough silver
surfaces with adsorbed molecules show huge
enhancements to the cross section. The effect
only works for rough surfaces, and only for
metals whose conductivity is very high. The
effect is believed to be due to surface
resonances enhancing the local intensity of the
light, but detailed quantitative calculations
have only recently become meaningful with the
availability of a theory for photonic materials,
and the experimental possibility of making well
characterised ordered arrays of metallic
nanospheres.
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Raman at the Air-Liquid Interface
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DDPA
Stearic Acid
52 mN/m
35mN/m
25 mN/m
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Head Group Structure
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