Spectroscopy 2: Electronic Transitions CHAPTER 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) Lasers

Fig Transitions involved in one kind of three-level laser Many ground state molecules must be excited

Fig Transitions involved in a four-level laser Only one ground state molecule must be excited for population inversion!!

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

Fig Summary of features needed for efficient laser action

Fig Principle of Q-switching Active medium is pumped while cavity is nonresonant Resonance is suddenly restored resulting in a giant pulse of photons

Fig 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 plane- polarized by 90 o

Fig Mode-locking for producing ultrashort pulses Intensity

Fig 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 ,000,000 W/m 2 !

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, CO 2, N 2 (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: Al 2 O 3 doped with Cr 3+ ions Output: cw at ~ 20kW Disadvantage: >50% of population must be pumped to 2 E metastable state s 3 ms 10 3 W/m 2

Transitions involved in a Nd-YAG laser Laser medium: YAG doped with Nd 3+ ions Output: ~ 10 TW in sub-ns pulses Advantage: Only one ion in population must be pumped to 4 F metastable state 0.23 ms 65 W/m 2

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

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

Fig Transitions involved in a carbon dioxide laser Electric discharge

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

Fig Optical absorption spectrum of Rhodamine 6G

Fig Dye laser configuration