1 / 23

Spectroscopy 2: Electronic Transitions CHAPTER 14

Spectroscopy 2: Electronic Transitions CHAPTER 14. Lasers. L ight A mplification by S timulated E mission of R adiation Requirements for laser action Laser-active medium (e.g., gas, dye, crystal, etc) Metastable excited state (i.e., fairly long-lived)

Download Presentation

Spectroscopy 2: Electronic Transitions CHAPTER 14

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Spectroscopy 2: Electronic Transitions CHAPTER 14

  2. 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)

  3. Fig 14.28 Transitions involved in one kind of three-level laser 51 Many ground state molecules must be excited 100 49

  4. Fig 14.29 Transitions involved in a four-level laser 1 Only one ground state molecule must be excited for population inversion!! 0 100

  5. Fig 14.30 Schematic of steps leading to laser action Laser medium confined to a cavity Active laser medium Pumping creates population inversion Each photon emitted stimulatesanother atom to emit a photon coherent radiation

  6. Fig 14.42 Summary of features needed for efficient laser action

  7. 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

  8. Fig 14.32 The Pockels cell (When cell is “off” cavity is resonant) • When “on”, plane-polarized • ray is circularly polarized • Upon reflection from end • mirror, it re-enters Pockels • cell • Ray emerges for cell plane- • polarized by 90o

  9. Fig 14.33 Mode-locking for producing ultrashort pulses Intensity

  10. Fig 14.34 Mode-locking for producing ultrashort pulses

  11. Table 14.4 Characteristics of laser radiation • High power – enormous number of photons/time

  12. 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 !

  13. 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

  14. Types of Practical Lasers • Solid-state lasers • e.g., Ruby, Nd-YAG, diode • Gas lasers • e.g., He-Ne, Ar-ion, CO2, N2 • Chemical and exiplex (eximer) lasers • e.g., HCl, HF, XeCl, KrF • Dye lasers • e.g., Rhodamine 6G, coumarin

  15. 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 103 W/m2

  16. Transitions involved in a Nd-YAG laser Laser medium: 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 0.23 ms 65 W/m2

  17. Fig 14.43 Transitions involved in a helium-neon laser 5 mol:1 mol Electric discharge

  18. Fig 14.44 Transitions involved in a argon-ion laser Blue-green Electric discharge

  19. Fig 14.45 Transitions involved in a carbon dioxide laser Electric discharge

  20. Fig 14.46 Molecular potential energy curves for an exiplex laser Population is always zero

  21. Fig 14.47 Optical absorption spectrum of Rhodamine 6G

  22. Fig 14.48 Dye laser configuration

More Related