In which categories can light scattering techniques be divided?
In elastic and inelastic light scattering methods.
What is the difference between elastic and inelastic light scattering?
With elastic light scattering the ligth only changes direction and does not exchange energy.
Inelastic light scattering does exchange energy and therefore the wavelength of the light changes.
How is electromagnetic radiation absorbed and emitted by atoms?
Atoms can absorb energy by lifting the electons into an excited state. If the electron drops down, light is emitted.
Name inelastic scattering processes and explain them.
spontaneous emission:
Molecule sponatneously drops from high to low energy level and emitts radiation
absorption
molecule is lifted to a higher energy level by absorbing light
stimulated emission
A molecule in a high energy state gets triggered by a photon and drops down to the lower energy level.
What is a LASER?
Light Amplification by Stimulated Emission of Radiation:
monochromatic
coherent
strongly directed
often polarized
Explain the working principle of a Laser.
Four energy levels:
pump transition from E1 to E4
nonradiative transition from E4 to E3
stimulated transition from E3 to E2 create Laser beam
Explain the main components of a LASER.
Laser material with four energy levels
mirror M and semitransparent mirror P
Pump energy source
What type of LASERs do you know?
Laser material:
gas laser
metal-vapour laser
solid-state laser
dye laser
Pump Energy:
chemical laser
Continous / pulsed laser
semiconductor laser
What is LIF?
Laser-Induced Fluoresence
Explain the fundamental principle of LIF?
Absorption of the energy of the laser.
Spontaneous emission with small delay.
The specific energy needed must be known and deliverable.
How does the experimental setup for PLIF (Planar LIF) looks like?
Laser induces natural fluoresence of specific molecules. The natural fluoresence can be measured.
Name applications for LIF / PLIF.
Droplet dynamics:
Visualization of liquid mixing
temperature mapping of droplets impinging onto a heated wall with temperature sensitive fluorescein
Macro- and Micro-scale mixing
Explain the basic principle of Raman spectroscopy.
The molecule gets excited with less energy it takes to transfer into the next stable state.
The molecule gets lifted into an unstable virtual state and drops back to ground state.
Now the molecule can drop down to a different vibrational or rotational level
What does the term “Anti-Stokes” transition mean?
When is a signal “red-shifted”?
Depending on which vibrational level the Molecule drops down, the light increases or decreases its frequency.
Anti-Sokes-transition means a blue shift to higher frequencies.
Stokes-transition means a red shift to lower frequencies.
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