Lecture_22

Lecture_22 - Lecture 22-1 Lecture Beyond Bohr Model...

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Lecture 22 Lecture 22 -1 Beyond Bohr Model ¾ Unfortunately, the classical visualization of the orbiting electron turns out to be wrong – even though it still gives us a simple way to think of the atom. Quantum Mechanics is needed to truly understand and describe the atom. Wave-particle duality, Probabilistic formulation of quantum physics – Chap. 28
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Lecture 22 Lecture 22 -2 Electron Energy Levels in Solids ¾ When great many (order of Avogadro’s number) atoms come together to form a solid, the individual atom’s energy levels split up into dense groups of levels for the combined solid, called energy bands . These bands span essentially continuous range of energies.
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Lecture 22 Lecture 22 -3 Energy Bands for Solids ¾ Dark regions are filled (i.e., there are electrons occupying there). ¾ Electrons can only move to available (unoccupied) states. ¾ There are many unoccupied states nearby in a conductor but there is none in an insulator. ¾ Small number of electrons can make a transition in semiconductors. holes
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Lecture 22 Lecture 22 -4 Stimulated Emission of Light ¾ Emitted photon in phase with incident photon. Coherent amplification . ¾ Incident photon with hf = Δ E stimulates emission of photon of the same frequency. So more photons come out as have gone in. laser Cascading effect can occur!
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Lecture 22 Lecture 22 -5 Laser L ight a mplification by s timulated e mission of r adiation Coherent, narrow, and intense Monochromatic (can be tunable as in liquid dye lasers ) Can be continuous or pulsed Can be made using solid, liquid, gas, or even free electrons. Sustained population inversion is required.
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Lecture 22 Lecture 22 -6 Examples of Lasers He-Ne Laser (continuous) Little populated, thus population inversion easy.
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Lecture 22 Lecture 22 -7 Compton Scattering (Compton Effect) ¾ When X-ray strikes matter, EM radiation is found to scatter with longer wavelength than in the incident ray.
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Lecture_22 - Lecture 22-1 Lecture Beyond Bohr Model...

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