1. Lasers and effects of magnetic field
Physics 123
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Lecture XXI

2. Light emission: classical case
Light bulb: current heats up atoms, they collide with each other and emit EM waves – light
Incoherent source of light – a continuous spectrum, isotropic in direction, no correlation in phase
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3. Light emission: quantum transitions
Energy transition  light emission  fixed photon energy (frequency and wave length)
Atoms are usually in the ground state (no emission)
When excited (e.g. from temperature collisions) they go back to the ground state through spontaneous incoherent light emission (frequency=color).
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Lecture XXI

4. Fluorescence, phosphorescence
Three level system, middle state is metastable (DL(E2-E0)=2)
Transition between energy levels  frequency of emitted light is the same
Phase and direction are still uncorrelated Ne
E’3
E’2
Electrons from
current collide
with atoms and
excite them
Two photons
emitted
E’0
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5. How lasers work Metastable level Electrons sit there Until emission is
stimulated
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6. How lasers work The laser in its non-lasing state
The flash tube fires and injects light into the ruby rod. The light excites atoms in the ruby.
Some of these atoms emit photons.
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7. How lasers work
Some of these photons run in a direction parallel to the ruby's axis, so they bounce back and forth off the mirrors. As they pass through the crystal, they stimulate emission in other atoms. Photons are bosons  they want to be emitted with the same
Energy (monochromatic),
Phase (single-phase),
Direction (collimated)
Light leaves the ruby through the half-silvered mirror -- laser light!
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Lecture XXI

8. He-Ne laser Lasing material is gas: 15% He +85%Ne
He is excited to second highest energy level (E1) by electric discharge
Ne atoms get excited by collisions with He atoms to E3
Ne goes down to E2 state which is metastable (DL(E2-E0)=2) He Ne
E’3 E1
E’2
1.96eV
20.61eV
20.66eV
18.70eV E0
E’0
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9. Lasers
Angular spread is very small, determined by diffraction on end mirror:
Q~l/D
Main advantage of lasers – energy is concentrated in one spot  precision. Applications
– laser surgery;
- optical alignment;
- drilling precision holes D
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10. Effects of
Magnetic field
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11. Electron quantum state
Orbital quantum number is a vector length l
Orbital angular momentum:
Its projection on z axis is Lz =mlh another q.n. – magnetic quantum number ml
ml can be only integer z
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12. Atom in magnetic field
Magnetic dipole moment associated with orbital angular momentum
Since ml is quantized, z-projection of magnetic dipole moment is quantized as well
Bohr’s magneton
Potential energy of the magnetic dipole in magnetic field (in z-direction
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13. Zeeman effect
Potential energy of the magnetic dipole in magnetic field splits into several levels
Transitions between these levels – Zeeman effect
Zeeman effect is due to interaction of external magnetic field with orbital angular momentum of electrons in atoms
Electrons also have internal “angular momentum” - spin
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14. Spin All electrons have spin=1/2 It is a vector
Its projection on z axis is another q.n. – spin ms
ms can be only
Similar to orbital angular momentum we expect
But experimentally measured value turned out to be different z
Gyromagnetic ratio, g-factor
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15. G-2 experiment in BNL
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16. G-2 experiment in BNL
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17. Fine structure
In the absence of magnetic field some small splitting of levels in atoms was still observed – interaction of magnetic field created by orbital angular momentum (current loop=nucleus around electron) with electron’s spin
Constant of fine structure (spin-orbit interaction)
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18. Spins in magnetic field
Imagine free spin (electron or nucleus) placed inside magnetic field directed along z-axis
Energy levels split by
This system can absorb photons of the corresponding frequency z E0 DE
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19. Nuclear magnetic resonance
Transition between the two states – photons of only a certain energy (frequency) will be absorbed – resonance absorption (NMR)
For H f=42.58MHz for B=1.0T
This frequency varies slightly for bound (trapped in molecules) H atoms
By mapping f, we can map chemical composition of human body
Precision of NMR – 0.5 – 1.0 mm
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