PHYS 17200 Lecture Notes - Lecture 15: Stimulated Emission, Spontaneous Emission, Photon

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PHYS 17200 - Modern Mechanics - Lecture 15_Energy Quantization
Photons
discrete “packets” of electromagnetic energy and momentum that travel at the speed of
light
E_photon = h*v_light = h*c/ lambda_light
E_photon = Energy of a photon
h = Planck’s constant
v_light = speed of photon
c = speed of light
lambda_light = wavelength of photon
Hydrogen Atom
Internal Energy States
There is a wave-particle character to the explanation of the discrete internal
energy states of atoms as well.
Quantum Model of the Interaction between atom’s charged particles and It’s
Environment (The Electromagnetic Field)
○ Absorption:
photon is absorbed
electron jumps to higher energy level
Spontaneous emission:
photon is emitted
electron jumps to lower energy level
Stimulated emission:
external photon causes electron jump to lower energy level
a photon is emitted
the original photon is not absorbed!
How does a laser work?
Lasers contain atomic “oscillators” that emit identical photons, with the same energy,
phase, direction….
Ordinary emission is “spontaneous.”
Laser emission is “stimulated.”
The excited atom could spontaneously decay and emit a photon, at some random time.
A photon of energy equal to the excited state can “stimulate” the atomic oscillator to emit
its photon in phase with the resonant “driving” photon.
If there are many atoms in the same excited state, there can be a cascade or “chain
reaction” that greatly multiplies the number of stimulated photons….
However, ground state absorption can quench this effect. Must instead have a
“population inversion”, not the exponential “thermal” distribution of populated states
mentioned earlier.
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