Rayleigh%20scattering

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Rayleigh scattering
Technique Synopsis Solid State Physics
Absorption spectrometry Scan ?in , measure intensity of transmission. Optical absorption, band gap, energy level spacing
Photoluminescence (PL) Fixed ?in (laser), scan ?out Optical recombination transitions.
Photoluminescence Excitation (PLE) Fix ?out , scan ?in (tunable laser or monochromator) Sensitive to transitions that pump optical emissions.
Raman scattering Laser in, scan ?out very close to ?in Stokes/anti-Stokes peaks provide information about phonon energies.
Raman scattering
transmission
excitation photons
photoluminescence

• At a glance
• Transmission untouched photons
• Photoluminescence includes fluorescence (emission
  within 10-5 s) and phosphorescence (emission
  after 10-5 s). Emission wavelength usually longer
  than excitation wavelength (Stokes shift)
• Raman scattering inelastic scattering, in
  semiconductors, it can be photon-phonon
  scattering
• Rayleigh scattering elastic scattering, no
  change in wavelength

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• PL of bulk semiconductor usually have peak at
  band gap, while absorption and PLE is broad and
  can determine density of state.

Molecular energy level

• In molecules absorption and PLE peaks are couple
  of S1 and S2 with vibrational energy, while PL
  peaks are couple of S0 and vibrational energy.

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Left The evolution of the UV-Vis and PL spectra
of the core/shell nanocrystals upon the growth of
the CdS shell in a typical reaction. Right
Asymmetric PL of core/shell nanocrystals with
five monolayers of CdS shell.

• CdSe quantum dot
• PL and PLE peaks in CdSe quantum dots can be used
  to compute energy spacing and relaxation
  characteristics for electrons and holes

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• Measurement Principle
• For single beam instrument, data is acquired
  twice, once with a reference cell, once with a
  sample cell in place. Signal ration is taken to
  give absorbance.
• A double-beam instrument adjust zero with the
  shutter closed when the shutter opens the
  absorbance is read directly from the difference
  amplifier.
• Application
• Characterize optical absorption
• Advantage
• Relatively simple instrument
• Disadvantage
• Limited sensitivity especially when the change in
  absorption is small compared to transmission.

(a) single-beam instrument, (b) double-beam
instrument

• Setup
• Both setups have a filter or monochromator for
  wavelength selection, a transducer and a readout
  device for data collection.
• Double-beam instrument splits the excitation
  source for faster acquisition and greater
  accuracy
• Our UV-VIS system is a single beam instrument
  with a monochromator

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PLPLE spectra for quinine solution
PL
PLE

• Measurement Principle
• PL excitation wavelength is fixed, emission
  intensity vs. wavelength is obtained by scanning
  a monochromator of spectrometer.
• PLE emission is detected at a fixed wavelength
  while excitation wavelength is scan (by a
  monochromator or tunable laser) to obtain
  emission intensity vs. excitation wavelength.
• Application
• Provide both optical absorption and emission
  information
• Advantage
• PLE is similar to absorption in some sense, with
  much better sensitivity. Detection limits can be
  three orders of magnitude smaller than those
  encountered in absorption spectroscopy.

Diagram of a PLPLE system

• Setup
• A combined PL and PLE system has 2
  monochromators for wavelength selection of
  excitation and emission. A single PL system can
  have a laser as an excitation source. A tunable
  laser can also be used instead of the excitation
  monochromator.
• A beam splitter and a reference detector is used
  to compensate for the variation in excitation
  intensity

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entrance slit, f/4
xyz stage
fiber optics, f/2.5
collimating mirror
f 127 mm f/2.4
focusing mirror
CCD 1024x256
f 63.5 mm f/1.2
SP-150 Spectrometer f 150 mm f/4 dual grating
turrets
cryostat
Excitation laser