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Spectroscopy 2: Electronic Transitions CHAPTER 14 Lasers • Light Amplification by Stimulated Emission of Radiation • Requirements for laser action • Laser-active medium (e.g., gas, dye, crystal, etc) • Metastable excited state (i.e., fairly long-lived) • Population inversion (i.e., more in excited state) • Cavity (for positive feedback or gain) Fig 14.28 Transitions involved in one kind of three-level laser 51 Many ground state molecules must be excited 100 49 Fig 14.29 Transitions involved in a four-level laser 1 Only one ground state molecule must be excited for population inversion!! 0 100 Fig 14.30 Schematic of steps leading to laser action Active laser medium Laser medium confined to a cavity Pumping creates population inversion coherent radiation Each photon emitted stimulates another atom to emit a photon Fig 14.42 Summary of features needed for efficient laser action Fig 14.30 Principle of Q-switching Active medium is pumped while cavity is nonresonant Resonance is suddenly restored resulting in a giant pulse of photons Fig 14.32 The Pockels cell (When cell is “off” cavity is resonant) (a) When “on”, plane-polarized ray is circularly polarized (b) Upon reflection from end mirror, it re-enters Pockels cell (c) Ray emerges for cell planepolarized by 90o Intensity Fig 14.33 Mode-locking for producing ultrashort pulses Fig 14.34 Mode-locking for producing ultrashort pulses Table 14.4 Characteristics of laser radiation • High power – enormous number of photons/time • The power density of a 1 mW laser pointer when focused to a spot of around 2 um (which isn't difficult with a simple convex lens) is around... 250,000,000 W/m2 ! Table 17.4 Characteristics of laser radiation • High power – enormous number of photons/time • Monchromatic – essentially one wavelength • Collimated beam – parallel rays • Coherent – all em waves in phase • Polarized –electric field oscillates in one plane Types of Practical Lasers (a) Solid-state lasers e.g., Ruby, Nd-YAG, diode (b) Gas lasers e.g., He-Ne, Ar-ion, CO2, N2 (c) Chemical and exiplex (eximer) lasers e.g., HCl, HF, XeCl, KrF (d) Dye lasers e.g., Rhodamine 6G, coumarin Transitions involved in a ruby laser 10-7 s 3 ms Laser medium: Al2O3 doped with Cr3+ ions Output: cw at ~ 20kW Disadvantage: >50% of population must be pumped to 2E metastable state Transitions involved in a Nd-YAG laser Laser medium: 0.23 ms YAG doped with Nd3+ ions Output: ~ 10 TW in sub-ns pulses Advantage: Only one ion in population must be pumped to 4F metastable state Fig 14.43 Transitions involved in a helium-neon laser 5 mol:1 mol Fig 14.44 Transitions involved in a argon-ion laser Blue-green Fig 14.45 Transitions involved in a carbon dioxide laser Fig 14.46 Molecular potential energy curves for an exiplex laser Population is always zero Fig 14.47 Optical absorption spectrum of Rhodamine 6G Fig 14.48 Dye laser configuration