Nanoparticle Optics Lab Part II

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Nanoparticle Optics LabPart II

• Light Scattering

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Theory

• A collimated light source is the most basic tool
  for nanoparticle work. Often called a Tyndall
  beam. Named after the 19th century scientist
  John Tyndall who studied light scattering in
  detail.
• HOTS Higher Order Tyndall Spectra

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Theory Scattering Angle

• How is the angle measured?
• Zero is the forward direction, the direction of
  the undeviated rays
• 180 is backward, rays scattered directly back
  into the source.
• Note that in the diagram to the right the
  scattering angles are 129 (180 51) and
  139 (180 42), respectively.

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Theory Scattering Plane

• the scattering plane is defined by the two rays
  involved, the source-particle ray and the
  particle-observer ray
• The scattering plane is determined by
  observation, it is not fixed in space.
• For example, if the observer moves, the
  scattering plane will move with the observer
• The scattering plane is useful to define the
  direction of polarization of light (parallel and
  perpendicular)

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Theory Rayleigh Scattering (electric dipole)
vertical sourcepolarization
horizontal source polarization
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Theory Rayleigh Scattering (electric dipole)

• unpolarized source
• Note that 90 scatteringis polarized
  perpendicular to the scattering plane.

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Theory MieAbsorption and Scattering by a
Sphere(exact solution)

• Gustav Mie (1908) motivation The colors of
  colloidal gold.
• Multipole expansion (EM modes of a sphere)
• electric dipole
• magnetic dipole, electric quadrupole
• magnetic quadrupole, electric octupole
• etc.
• If d lt ?/20 then only the first term (dipole) is
  needed. In this limiting case, Mies theory
  reduces to Rayleighs theory
• small particle limit Mie ? Rayleigh

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Objective

• Learn about the scattering plane and the
  polarization of Rayleigh scattering.
• Learn about Mie scattering and the angular
  dependence of scattering.
• Observe HOTS and angular scattering for
  monodisperse sols.

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Procedure Rayleigh Scattering

• Shine Tyndall beam through colloidal silica
  without polarizer.
• Observe beam from top and side of jar.
• Use polarized lens to check polarity of light
  scattering from silica in jar.

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Procedure Rayleigh Scattering

• Place polarizer between Tyndall beam and jar.
• Observe light intensity from side of jar.
• Note difference in scattering intensity between
  parallel and perpendicular polarized source.

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Procedure HOTS

• Replace jar of colloidal silica with colloidal
  sulfur.
• With source polarized perpendicular, observe
  different colors of HOTS spectra.
• Rank particle size in the two jars by counting
  the number of times a certain color repeats when
  moving 180 degrees around the jar.
• Larger particles cause more repetitions.
• Use one eye and look for an easy color to see
  such as red.

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Procedure Scattering Angle

• Using the procedure for colloidal sulfur, rank
  three polystyrene samples in order of size.
• Put one polystyrene sample in the path of the
  543.5 nm HeNe laser.
• Prop one side of the sample container on a slide
  to point the back surface reflection of the
  container away from the laser.
• Line up laser beam emitted from sample container
  with iris.
• Use crossed polarizers to adjust laser beam
  intensity.

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Procedure Scattering Angle

• Find points of minimum scattering intensity.
• Use one eye to line up sight in the middle of the
  bottle at angle of minimum intensity.
• Record angles for each bottle.

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Results Rayleigh Scattering

• With unpolarized source, light scattered at 90
  degrees from the source was polarized
  perpendicular.
• With source polarized perpendicular, light
  scattered at 90 degrees was polarized
  perpendicular. Moving 180 degrees around the
  bottle produced changes in intensity with a
  minimum at 90 degrees.

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Results HOTS

• Observed different number of color repetitions
  for colloidal sulfur.
• For polystyrene observed one, five, and three
  repetitions for bottles D, E, and F respectively.

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Results Scattering Angle

• Saw different numbers of scattering intensity
  minimums for bottles D, E, and F.
• Observed one, five, and two minimums for bottles
  D, E, and F respectively. This led us to believe
  the largest particles were in bottle E, and the
  smallest were in D.
• Different observers recorded slightly different
  angles of minimums.

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Analysis

• Table lists averages of measured angles of
  minimum intensity from three observers.
• Angles were compared to Mie Plot data to estimate
  diameter of polystyrene.

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Analysis

• Graph shows best fit to observed data.
• Minimums above 160 degrees and below 20 degrees
  were not taken into account.

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Questions

• Size estimated for particles is 350 nm, 1160 nm,
  and 750 nm for bottles D, E, and F respectively.
• Polydispersed sol will cause light from different
  wavelengths to overlap in HOTS. Colors will be
  less distinct.
• A way to improve this experiment would be to use
  a light detector to measure the scattered
  intensity at different angles. Human error would
  be reduced.