Raman

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Raman
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Scattering

• Tyndall scattering if small particles are
  present
• During Rayleigh scattering (interaction of light
  with relatively small molecules) incident light
  is scattered in all directions
• Some of the incident energy can be
  converted into rotational or vibrational energy-
  so wavelength of scattered light is longer

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In a fluorescence experiment, the scattered light
will be collected along with the fluorescence

• Thus we may see peaks in our fluorescence
  spectrum that do not arise by emission.
• Especially true with low levels of fluorescence

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• The Raman effect arises when a photon is incident
  on a molecule and interacts with the electric
  dipole and causes perturbation
• In quantum mechanics the scattering is described
  as an excitation to a virtual state lower in
  energy than a real electronic transition with
  nearly coincident de-excitation and a change in
  vibrational energy.

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• At room temperature the thermal population of
  vibrational excited states is low
• Therefore the initial state is usually the ground
  state and the scattered photon will have lower
  energy (longer wavelength) than the exciting
  photon.
• This Stokes shift is what is usually observed in
  Raman spectroscopy.

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• The selection rule for a Raman-active vibration
  is that there be a change in polarizability
  during the vibration
• If a molecule has a centre of symmetry,
  vibrations which are Raman-active will be silent
  in the infrared, and vice versa.

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Polarization effects

• Raman scatter from totally symmetric vibrations
  will be strongly polarized parallel to the plane
  of polarization of the incident light.
• The scattered intensity from non-totally
  symmetric vibrations is ¾ as strong in the plane
  perpendicular to the plane of polarization of the
  incident light as in the plane parallel
• to it.

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• Typical strong Raman scatterers are moieties with
  distributed electron clouds, such as
  carbon-carbon double bonds.
• The pi-electron cloud of the double bond is
  easily distorted in an external electric field.
• Bending or stretching the bond changes the
  distribution of electron density substantially,
  and causes a large change in induced dipole
  moment.

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Instrumentation
Laser UV, vis, NIR
Sample Cell
Monochromator
Detector
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Fluorescence Interferes

• Just as a Raman peak can show up in a
  fluorescence spectrum, fluorescence can show up
  in and often swamps out a Raman spectrum
• Change laser wavelength to longer wavelength
• Raman shifts are independent of the wavelength of
  excitation

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In summary

• Raman spectroscopy gives information about
  vibrations
• It uses UV, visible or NIR laser light rather
  than IR
• The information is often complementary to that
  obtaine by IR especially for molecules with a
  centre of symmetry
• Water is less of a problem
• Can use quartz cells and fibre
  
• optics

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Positions of the Raman bands of various solvents
when excited at selected wavelengths
Excitation wavelength Excitation wavelength Excitation wavelength Excitation wavelength
Solvents 313 366 405 436
water 350 418 469 511
acetonitrile 340 406 457 504
cyclohexane 344 409 458 499
chloroform 346 411 461 502
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Raman peak maxima of water at various wavelengths
of excitation
Excitation/nm Raman emission/nm
200 212
250 272
300 337
350 397
400 463
450 530
500 602
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Distinguishing Raman from Fluorescence peaks

• Raman the scattering peak shifts as the
  excitation wavelength shifts
• The amount of energy abstracted is always
  constant (a vibrational energy)
• Fluorescence changing the excitation wavelength
  does not affect the emission wavelength

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• Fluorescence can be avoided by using
  longer-wavelength lasers in the near-infrared
  (NIR) region.
• The weaker Raman signal resulting from NIR
  excitation requires specialized components
  optimized for maximum throughput and
  signal-to-noise ratio in the NIR region.