Transcript Light Scattering - The University of Oklahoma Department

Light Scattering
Rayleigh Scattering &
Mie Scattering
Theory
• Characteristics of Polystyrene and Sulphur Nanoparticles.
-- Non absorbing
-- Refractive index is a weak function of wavelength.
• How does light interact with objects?
-- Reflection (light deviated from its original path)
-- Refraction
-- Diffraction
-- Absorption (light absorbed and converted to heat)
Theory Cont…
• A collimated light source is the most basic tool for nanoparticle work.
Often called a Tyndall beam.
• The Tyndall effect is the scattered path of light observed in the
suspension. Examples: Milk, smoke, Lake Thunderbird.
• Scattering Plane
-- The scattering plane is defined by the two rays involved, the sourceparticle 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.
Theory Cont….
• Theory of Rayleigh
-- Particles are treated as electric dipole.
• Results:
-- I 1/ λ4 (only true if the refractive index is a weak function of λ, i.e. not a
metal.)
-- I r6
-- scattered light at 90° is linearly polarized perpendicular to the scattering
plane.
Verticle Source
Polarization
Horizontal Source
Polarization
Theory Cont…
• Mie Scattering
-- Absorption and Scattering by a Sphere.
-- Multipole expansion (EM modes of a sphere)
-- electric dipole.
-- magnetic dipole, electric quadrupole.
-- magnetic quadrupole, electric octupole.
• If d < λ/20 then only the first term (dipole) is needed. In this limiting case,
Mie’s theory reduces to Rayleigh’s theory.
• Efficiency factors: Qsca, Qabs, Qext
– Plot Qext vs λ for the extinction spectra
– Qsca and Qabs vs λ show their contribution to Qext.
• Intensity for perpendicular and parallel polarized light
– Plot I vs θ for the angular intensity dependence for each polarization.
Objective
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Learn about Scattering plane
The Polarization of Rayleigh Scattering
Mie Scattering
Angular Dependence
Procedure
Rayleigh scattering
•
Using a light source and polarization lens
we observed the way light rays are
polarized through rayleigh scattering in
different solutions: Silica SOL, Sulfur SOL
and Fine Sulfur particles, by shining the
light source through the solutions
•
Shine light source though solutions in a
dark room
•
Place polarized lens in path of light source
to observe polarization effects of scattered
light in the scattering plane and outside of
scattering plane, I.E. view from top (90
degrees) and other angles of observation
Higher Order Tyndall Spectra (HOTS)
• Using the same procedure to observe the
Rayleigh scattering effects, observe the
different colors associated with the scattered
light and observe angle dependency
• Note the number of orders in each sample,
one order is one color change from red to
green
Scattering Angle
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Using the Helium Neon laser apparatus
(wavelength = 543.5 mm) the laser beam
was shone through our samples of
polystyrene latex
The samples were placed at an angle on
the observation stage to avoid multiple
reflection of the laser beam in the same
area of the solution
Note the minimum intensity zones in the
scattered light by observing in the
horizontal plane and recording the angle
these minima occur. This was done by
observing from about 35 degrees to 145
degrees from the laser beam in the
scattering plane. See picture
Laser
Sample
Observation
stage with
angle
measurement
site
Optical Microscopy
• Using an optical microscope in dark field
observation mode, observe the nanoparticles
in each of the three samples
• Note their movement and size (each sample
resembles a night sky filled with stars, the
particles can be seen but not studied in
detail)
Dark-Field
Optical Microscopy
•A central obstruction
blocks the central cone.
•The sample is only
illuminated by the
marginal rays.
•These marginal rays must
be at angles too large for
the objective lens to
collect.
•Only light scattered by the
object is collected by the
lens.
OU NanoLab/NSF NUE/Bumm & Johnson
www.microscopy.fsu.edu
Results: Rayleigh Scattering
• Silica SOL
– With the vertical polarization lens in place we noticed there is no angular
dependence in the scattering plane
– By using a second polarization lens at a 90 degree angle we verified that
the light is polarized perpendicular to the scattering plane
• Sulphur SOL
– Using the same techniques we noted the HOTS followed slightly different
result patterns, there was significantly more forward scattering which
caused the scattered light to blend together and appear simply white
toward larger scattering angles in the scattering plane
• Polystyrene latex
– By observing each of the three samples and noting their order using the
HOTS phenomenon we ranked the sample by particle size from smallest
to largest as follows: Sample C, Sample B, Sample A (This initial ranking
seemed to be correct according to our scattering angle experiment data)
Observation
Sample
A
Gaurav
50
74
98
Trevor
49
73
100
124
Ye
85
98
105
B
54
96
54
96
50
98
C
141
141
141
Results: Scattering Angle
• Sample C
– No minimum were found in this sample although the intensity
significantly decreased as the viewing angle decreased, this result is
consistant with the Mie plot data
– Estimated particle size <240 nm
• Sample B
– Minima recorded at scattering angles of 54 and 96 degrees
– Estimated particle size  600nm
• Sample A
– Minima recorded at 50, 74 and 98 degrees
– Estimated particle size  1060 nm
Mie plot results
Sample C: Particle size < 240nm
Sample A: Particle size = 1060nm
~ 50
~74
~98
Intensity decreases as angle increases
Sample B: Particle size = 600nm
~54
~ 96
Experiment discussion
• Experimentally determined particle size
– Sample A: Particle size  1060 nm
– Sample B: Particle size  600 nm
– Sample C: Particle size  < 240 nm
• Error and procedure improvement suggestions
– Lab results were recorded by human observation of three different lab
technicians, humans always make mistakes
• These results could be improved my taking more measurements and averaging
results
• Mono-dispersed vs. Poly-dispersed HOTS
– A more highly dispersed sample would appear more “milky” under
observation, that is to say the light spectrum would be blended together and
appear more like white light instead a showing distinct wavelengths