OPTICAL
ABSORPTION :
Dr. Satyanarayan Dhal
Lecture-2
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What is Optical Absorption?
1. Definition : This is the phenomena when any material object takes up the light energy (photon) and alters electromagnetic energy into heat energy of the absorber.
2. Linear absorption : When the absorption of waves does not depend on their intensity.
3. Nonlinear absorption : Sometimes the medium's transparency changes by a factor that varies as a function of wave intensity, and nonlinear absorption occurs.
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How to measure radiation absorption ??
1. The absorbance of an object : How much of the incident light is absorbed by it (instead of being reflected or refracted).
2. What we will know by knowing absorbance ? 3. Ans : Identification of a substance via
absorption spectroscopy
1. Attenuation : Reduction of the intensity of light waves as they propagate through a medium.
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How to measure radiation absorption ??
1. In absorption spectroscopy, a sample is irradiated from one side, and the light that exits from the sample in every direction is measured.
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Beer–Lambert law
1. relates the attenuation of light to the properties of the material through which the light is travelling.
2. Applied to chemical analysis measurements
3. used in understanding attenuation in physical optics, for photons, neutrons, or rarefied gases.
• is the optical path length
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1. M olecules containing bonding and non-bonding electrons absorb energy in the form of UV or visible light.
2. They excite these electrons to higher anti-bonding molecular orbitals.
3. The energy gap between the HOM O and the LUM O is lower, then the wavelength of light absorbed will be longer.
4. 4 possible types of transitions : σ–σ* > n–σ* > π–π* > n–π*
• is the optical path length
• In this region of the electromagnetic spectrum,
• atoms and molecules undergo electronic transitions.
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UV/Vis spectrophotometer
1. It measures the intensity of light after passing through a sample.
2. It compares it to the intensity of light before it passes through the sample.
3. Transmittance : is the ratio expressed as a percentage (%T).
4. Absorbance (A) = -(log(%T/100%)
5. The UV–visible spectrophotometer can also be designed to determine reflectance; where the spectrophotometer determines the intensity of light reflected from a sample , and compares it to the intensity of light reflected from a reference material.
• is the optical path length
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How UV/Vis spectrophotometer works ?
• is the optical path length
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Band Gap is a very important property for PV Solar Cells !
• is the optical path length
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Efficiency of PV Cell (Shockley–Queisser limit)
• is the optical path length
1. Maximum theoretical efficiency of a PV using a single p-n junction to gather power from the cell.
2. The limit is around 33.7% for a single p-n junction PV cell.
3. The corresponding band gap of 1.34 eV.
4. Why silicon ?
5. Si has a less favorable band gap of 1.1 eV, resulting in a maximum
efficiency of about 32%.
How we can calculate band gap from UV-Vis Spectroscopy ?
Reference : Kumar, H., & Rani, R. (2013). Structural and optical characterization of ZnO nanoparticles synthesized by microemulsion route. International Letters of Chemistry, Physics and Astronomy, 14, 26-36.
Tauc plot shows the quantity hν (the energy of the light) on the abscissa and the
quantity (αhν)1/r on the ordinate, where α is the absorption coefficient of the material.
Band Gap of ZnO NP = 3.7 eV
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Other Few Examples
• Ultraviolet–visible spectroscopy,
• Infrared spectroscopy,
• X-ray absorption spectroscopy.
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