Ch. 7 Notes - Sources of Radiation o Radiant power of a...

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Unformatted text preview: Sources of Radiation o Radiant power of a source varies exponentially with its power supply Thus, power supplies must be regulated o 2 types Continuum sources Emit radiation that changes in intensity only slowly as a function of wavelength Have widespread use in absorption and fluorescence spectroscopy UV region: deuterium lamp , or a gas-filled arc lamp for very high intensity Visible region: tungsten filament lamp Infrared region: inert solids heated to 1500-2000 K, which gives maximum radiant output o These sources called glo-bars b/c they are heated until they glow o Ex. tungsten lamp, Nernst glower , Line sources Emit a limited number of lines , or bands of radiation Find wide use in atomic absorption spectroscopy, atomic and molecular fluorescence spectroscopy, and Raman spectroscopy Uses mercury and sodium vapor lamps, or hollow-cathode lamps o Lasers (light amplification by stimulated emission of radiation) Highly useful b/c of high intensity, narrow bandwidth, and coherent nature of output Components a lasing material (solid crystal, solution, or gas) is activated, or pumped, by radiation from an external source so that some photons of the right energy will trigger a cascade of photons of the same energy o lasing material is one w/ long-lived excited state, ex. fluorescence functions as an oscillator where radiation passes back and forth through lasing medium output mirror only reflects 95%, allowing output Mechanism A molecular system will have vibrational levels Pumping o Several of the higher electronic and vibrational energy levels of the active species are populated o Lifetime of an excited vibrational state is brief, so electrons will quickly relax to the lowest excited vibrational level Spontaneous emission o Random release of energy as excited electrons decay to ground state o Radiation produced differs in direction and phase (incoherent) Stimulated emission o Excited laser species struck by photons of same energies as photons produced by spontaneous emission o Collisions like this cause the excited species to relax and to simultaneously emit a photon of exactly the same energy o the released photons are exactly in the same direction and in phase, thus coherent with, the original stimulating photon Absorption o Ground state laser species struck by photons of same energies as in spontaneous emission o Causes excitation to higher energy state To have light amplification, the number of photons produced by stimulated emission must exceed the number absorbed by absorption o This occurs only if more electrons are in excited state than in ground state (population inversion) o Population inversions created by pumping Multiple Level Systems o Lasing does not work with only a single ground state and a single excited state o 3-level system Fast nonradiative transition between 2 excited levels, and emission from lower excited level to...
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This note was uploaded on 10/24/2009 for the course CHEM 447 taught by Professor Staff during the Winter '08 term at University of Michigan.

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Ch. 7 Notes - Sources of Radiation o Radiant power of a...

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