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Raman

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This Stokes shift is what is usually observed in Raman spectroscopy. The selection rule for a Raman-active vibration is that there be a change in polarizability ... – PowerPoint PPT presentation

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Title: Raman


1
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.
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