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22-Spectro_6_web - 11/29,12/1 12/6 12/8 Spectroscopy...

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11/29,12/1 Spectroscopy 12/6 EXAMINATION # 3 12/8 Atomic Spectroscopy 12/16 FINAL EXAMINATION, 8-11 AM, in 100 NL
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LASERS, Harris 447- (Raman p. 411) LASER: L ight A mplification by S timulated E mission of R adiation, a powerful light source at useful wavelengths, A unique source of light! Properties of the laser emitted light: spatially narrow extremely intense coherent (the same frequency, phase and the direction of propagation as the stimulating photons ) highly monochromatic may be made tunable
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INVERTED POPULATION! (1) Lasing material (gas, solid, liquid, plasma) : subjected to light (heat, discharge) generated from an auxiliary source, nearly all atoms became excited, and most of them drop into a metastable excited energy level. (2) Spontaneous emission photons are emitted (to begin with). When the number of particles in one excited state exceeds the number of particles in some lower-energy state, population inversion is achieved and the amount of stimulated emission - due to light that passes through - is larger than the amount of absorption . Hence, the light is amplified. (3) When an optical amplifier is placed inside a resonant optical cavity, one obtains a laser.
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Principle of a stimulated emission in the 2-level laser.
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Schematic diagram of a laser including a “pumping” device Energy input : the lasing material is energized ( pumped ) by radiation from an external power source, like: (i) an external current; (ii) an electrical discharge; (iii) another laser. Before pumping, most of the atoms remain in the ground electronic state: a normal population (from which photon absorption may occur).
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On return to the ground state, the atoms radiate light, which binge parallel to the laser axis. The light is reflected back and forth as it keeps on reflecting between the mirrors. Laser functions as an oscillator ! (Between the two mirrors). The key property of a laser, or a lasing action : a few photons of proper energy trigger the formation of a cascade of photons of the same energy.
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Principle of a stimulated emission in the 2-level laser.
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Types of Lasers   (A  laser can operate in either continuous or pulsed mode ):
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