Spectroscopy 2: Electronic Transitions CHAPTER 14

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

23. Fig 14.48 Dye laser configuration