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  1. 1. And God said, let there be light: and there was light.
  2. 2. Milestones of Photochemistry/physics/biology <ul><li>Primitive Earth Age: Small gas molecules + Sun light Nucleic acids and Proteins </li></ul><ul><li>1805: Young’s double slit experiment (Wave nature of light) </li></ul><ul><li>1864: Electromagnetic Wave theory of light (Maxwell) </li></ul><ul><li>1886: First organic photochemical reaction by Giacomo Luigi Ciamician </li></ul><ul><li>1887: Photoelectric effect (Heinrich Hertz) </li></ul><ul><li>Early 1900’s: Grotthus- Draper law </li></ul><ul><li>Early 1990s: Beer-Lambert law </li></ul><ul><li>1900: Black body radiation: Quantum nature of light (Max Planck) </li></ul><ul><li>1905: Quantum theory of radiation (Einstein). </li></ul><ul><li>1909: Wave-particle duality (Einstein) </li></ul><ul><li>1912: Stark-Einstein law </li></ul><ul><li>1916: Concept of spontaneous absorption and emission (Einstein) </li></ul><ul><li>1917: Concept of stimulated emission: Concept of MASER and LASER (Einstein) </li></ul><ul><li>1918: Nobel prize: Max Planck: Energy quanta </li></ul><ul><li>1921: Nobel Prize: Einstein: Photoelectric effect </li></ul><ul><li>1926: Nomenclature of the light quantum as photon (Gilbert Lewis) </li></ul><ul><li>1927: Nobel Prize: Arthur Compton: Compton effect: Photons carry momentum </li></ul><ul><li>1953: Discovery of MASER: Charles H. Townes </li></ul><ul><li>1960: Discovery of LASER: Theodore H. Maiman </li></ul><ul><li>1967: Nobel prize: Manfred Eigen: Flash Photolysis to study ms-ns processes </li></ul><ul><li>1971: Nobel prize: Gerhard Herzberg: Electronic structure of molecules </li></ul><ul><li>1992: Nobel prize: Rudolf A. Marcus: Theory of electron transfer reactions </li></ul><ul><li>1997: Nobel prize: Steven Chau: Laser cooling and trapping of atoms </li></ul><ul><li>1999: Nobel prize: Ahmed H. Zewail: Femtosecond laser spectroscopy </li></ul><ul><li>2000: Development of attosecond laser. </li></ul><ul><li>2008: Nobel prize: Osamu Shimomura: Green fluorescent protein </li></ul><ul><li>2009: Nobel prize: Charles K. Kao: Optical fibre cable </li></ul>
  3. 3. Giacomo Luigi Ciamician: Father of Modern Molecular Photochemistry His first photochemistry experiment was published in 1886 and was titled “On the conversion of quinone into quinol by light”. Born August 25, 1857(1857-08-25) Trieste, Austria Died January 2, 1922 (aged 64) Bologna, Italy Education University of Vienna Employer University of Bologna
  4. 4. Fundamental theories of Photochemistry <ul><li>Grotthus-Draper law: Only that light which is absorbed by a system can cause chemical change. </li></ul><ul><li>Stark-Einstein law: One quantum of light is absorbed per molecule of absorbing and reacting substance that disappears. </li></ul><ul><li>Beer-Lambert law: </li></ul>
  5. 5. Fundamental theories of Photochemistry
  6. 6. Fundamental theories of Photochemistry <ul><li>Selection rules: </li></ul><ul><ul><li>Radiation transitions (Absorption, fluorescence) </li></ul></ul><ul><ul><li>Allowed transitions: g u, e.g.: S 0 S 1 and T 1 S 0 </li></ul></ul><ul><ul><li>Forbidden transitions: g g, u u, e.g.: S 1 T 1 </li></ul></ul><ul><ul><li>2. Radiationless transitions (vibration relaxation, Internal conversion, Intersystem crossing) </li></ul></ul><ul><ul><li>Allowed transitions: g g, u u, e.g.: S 1 T 1 </li></ul></ul><ul><ul><li>Forbidden transitions: g u, e.g.: S 0 S 1 and T 1 S 0 </li></ul></ul>
  7. 7. Instrumentation
  8. 10. Femtosecond Transient Absorption Spectroscopy
  9. 11. Femtosecond Transient Absorption Spectroscopy
  10. 12. Femtosecond Transient Absorption Spectroscopy
  11. 14. Femtosecond fluorescence upconversion setup
  12. 15. Femtosecond Spectroscopy lab
  13. 16. Absorption of Light by Ionic Compounds <ul><li>Electrons can jump between “bands” </li></ul><ul><li>Incident light with energy ≥ than the “band gap” energy can be used to excite the electrons </li></ul>
  14. 17. So What Does this Mean for Solar Cells? <ul><li>In dye-sensitized solar cells… </li></ul><ul><ul><li>Talk about highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) </li></ul></ul><ul><li>In single-crystal silicon solar cells… </li></ul><ul><ul><li>Talk about “conduction band” (excited states) and “valence band” (ground states) </li></ul></ul>
  15. 18. A Closer Look at Solar Cells <ul><li>How do traditional, silicon-based solar cells and newer, dye-sensitized solar cells work? </li></ul><ul><li>What are the advantages and disadvantages of each type of cell? </li></ul>Silicon-based solar cell Dye-sensitized solar cell
  16. 19. How a Silicon-Based Solar Cell Works <ul><li>A positive “hole” is left in the electron’s place </li></ul><ul><li>This separation of electrons and holes creates a voltage and a current </li></ul><ul><li>Light with energy greater than the band gap energy of Si is absorbed </li></ul><ul><li>Energy is given to an electron in the crystal lattice </li></ul><ul><li>The energy excites the electron; it is free to move </li></ul>Click image to launch animation (requires web access)
  17. 20. How a Dye-Sensitized Cell Works Click image to launch animation (requires web access) <ul><li>Light with high enough energy excites electrons in dye molecules </li></ul><ul><li>Excited electrons infused into semiconducting TiO 2 , transported out of cell </li></ul><ul><li>Positive “holes” left in dye molecules </li></ul><ul><li>Separation of excited electrons and “holes” creates a voltage </li></ul>
  18. 22. P=1.367kWm -2 - the solar constant – solar radiation power outside the Earth’s atmosphere Taken from: S. M. SZE; Physics of Semiconductor Devices; Second Edition; John Wiley & Sons;New York; 1981 SOLAR SPECTRAL IRRADIANCE