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L14_Molec_spectr_lasers - Molecular Spectroscopy ­part...

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Unformatted text preview: Molecular Spectroscopy ­part IV: lasers & photochemistry Interac8on of photons with molecules and materials Electronic excita8ons S8mulated emission •  •  A laser is a device that emits light through a process of op8cal amplifica8on based on the s8mulated emission of photons. The term "laser" originated as an acronym for Light Amplifica.on by S.mulated Emission of Radia.on. The emiFed laser light is notable for its high degree of spa8al and temporal coherence, unaFainable using other technologies. Spa8al and temporal coherence: For any electromagne8c wave (em) wave, if at 8me t=0 and t0 the phase difference between two points in space remains the same, we say the em wave has spa8al coherence; If at a point P, the em wave at t and t+dt has same phase difference, temporal coherence exists. How does it work? •  The pump source provides energy to the laser system, that is it excites the gain medium –  A helium ­neon (HeNe) laser uses an electrical discharge in the helium ­neon gas mixture –  A Nd:YAG laser uses either light focused from a xenon flash lamp or diode lasers –  Excimer lasers use a chemical reac8on. The gain medium amplifies light at the desired wavelength –  Gases, such as carbon dioxide, argon, krypton and mixtures such as helium ­neon –  Solids, such as crystals and glasses. The solid host materials are usually doped with an impurity such as chromium, neodymium, erbium . Typical hosts include YAG (yFrium aluminium garnet), YLF (yFrium lithium fluoride), sapphire (aluminium oxide) and various glasses. For example: Nd:YAG, Ti:sapphire, Cr:sapphire (usually known as ruby), Cr:LiSAF (chromium ­doped lithium stron8um aluminium fluoride), Er:YLF, Nd:glass, and Er:glass. –  Semiconductors. Semiconductor lasers are typically very small, and can be pumped with a simple electric current, enabling them to be used in consumer devices. Op.cal resonators: Light from the medium, produced by spontaneous emission, is reflected by the mirrors back into the medium, where it may be amplified by s8mulated emission. The light may reflect from the mirrors and thus pass through the gain medium many hundreds of 8mes before exi8ng the cavity. •  •  Nd: YAG •  Nd:YAG (neodymium ­doped y2rium aluminium garnet; Nd:Y3Al5O12) lasers are op8cally pumped using a flashtube or laser diodes. They typically emit light with a wavelength of 1064 nm, in the infrared. However, there are also transi8ons near 940, 1120, 1320, and 1440 nm. The biggest laser: NIF •  Na8onal Igni8on Facility at LLNL, CA: 192 laser beams to deliver energy to a 8ny D2 capsule The smallest laser •  The world's smallest laser is contained in a silica sphere just 44 nanometres across 'spaser'. •  Whereas a laser amplifies light, using a mirrored cavity to intensify it, a spaser amplifies surface plasmons — 8ny oscilla8ons in the density of free electrons on the surface of metals, which, in turn, produce light waves. •  The spaser opens the door to nanoscale circuits that could process informa8on thousands of 8mes faster than the microelectronic chips inside today's computers. Rates of excita8ons u radiant energy density = radiant energy per unit volume (units J m ­3) uν radiant energy density per unit frequency = du/dν (units: J m ­3 s) A and B are Einstein coefficients Thermodynamic equilibrium Planck black body radia8on law Irradia?on of a 2 ­level system can never put more atoms or molecules in the higher level than in the lower level. Laser ac8on cannot be achieved with a 2 ­level system. Laser ac8on can only be obtained when N2>N1. This is called popula?on inversion. Quantum Theory: Blackbody Radia8on •  A black body: absorbs all electromagne8c radia8on black! •  A black body emits all frequencies and its spectrum does depend on temperature T ν=c / λ: Einstein coefficients Einstein's coefficients are propor8onal to the square of the transi8on dipole moments, as follows: g1 and g2 are the degeneracies of the states. High Resolu8on Spectroscopy Photochemistry Pump ­probe experiments and laser induced fluorescence ...
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