Spectroscopy 2: Electronic Transitions CHAPTER 14

Spectroscopy 2:Spectroscopy 2:Electronic TransitionsElectronic Transitions

CHAPTER 14CHAPTER 14

• 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)

LasersLasers

Fig 14.28 Transitions involved in one kind of three-level laser

100

51

49

Many ground statemolecules must be

excited

Fig 14.29 Transitions involved in a four-level laser

100

1

0

Only one ground statemolecule must be

excited for populationinversion!!

Fig 14.30 Schematic of steps leading to laser action

Active laser medium

Pumping creates population inversion

Each photon emittedstimulates anotheratom to emit a photon

coherent radiation

Laser medium confinedto a cavity

Fig 14.42 Summary of features needed for efficient laser action

Fig 14.30 Principle of Q-switching

Active medium ispumped while cavity

is nonresonant

Resonance is suddenlyrestored resulting in agiant pulse of photons

Fig 14.32 The Pockels cell(When cell is “off” cavity isresonant)

(a) When “on”, plane-polarizedray is circularly polarized

(b) Upon reflection from endmirror, it re-enters Pockelscell

(c) Ray emerges for cell plane-polarized by 90o

Fig 14.33 Mode-locking for producing ultrashort pulses

Inte

nsity

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 whenfocused 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 LasersTypes 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

Laser medium:

Al2O3 doped with Cr3+ ions

Output: cw at ~ 20kW

Disadvantage: >50% of

population must be pumped

to 2E metastable state

10-7 s

3 ms

103 W

/m2

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/m

2

Fig 14.43 Transitions involved in a helium-neon laser

Electric

discharg

e

5 mol:1 mol

Fig 14.44 Transitions involved in a argon-ion laser

Electric

discharg

e

Blue-green

Fig 14.45 Transitions involved in a carbon dioxide laser

Electric

discharg

e

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