Introduction: Lasers

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Courtesy Electron Physics Group, NIST
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Atoms and Light
Introduction Lasers 6.1/ Characteristics of
Atoms 6.2/ Characteristics of Light 6.3/
Absorption and Emission Spectra 6.4/ Properties
of Electrons 6.5/ Quantization and Quantum
Numbers 6.6/ Shapes of Atomic Orbitals 6.7/
Sunlight and the Earth
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6.1 Characteristics of Atoms
1 mole sample of Li
Atoms possess mass Atoms contain positive
nuclei Atoms contain electrons Atoms occupy
volume
Courtesy John Olmsted
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6.1 Characteristics of Atoms
Fig 6-1
Atoms possess mass Atoms contain positive
nuclei Atoms contain electrons Atoms occupy
volume
Atoms have various properties Atoms attract one
another Atoms combine with one another
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6.2 Characteristics of Light
Fig 6-2

• Light has Wave Aspects
• Frequency, ?
• Wavelength, l
• Amplitude

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6.2 Characteristics of Light
Fig 6-3

• Light has Wave Aspects
• Frequency, ?
• Wavelength, l
• Amplitude
• Intensity

l? c 2.9979 x 108 m/s (6-1)
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6.2 Characteristics of Light
Fig 6-4
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6.2 Characteristics of Light

• Light is bent when it interacts with a medium.
• Each wavelength interacting with a medium will
  bend to a different degree.
• If the white light is all the wavelengths of
  visible light, then a rainbow (all the visible
  colors) will be emitted from the prism.

Fig 6-5
Courtesy Bausch Lomb
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What we perceive as white light is actually a
mixture of ls of visible light.
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Bulk Interactions
wave properties
Atomic/Molecular
particle properties
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6.2 Characteristics of Light
The Photoelectric Effect

• Below a characteristic threshold, ?o, no e-s are
  observed.
• 2. Above ?o, KEmax of ejected e- increases
  linearly with ? of light.
• 3. Above ?o, higher light intensity increases
  of ejected e- (not their KE)
• 4. All metals exhibit same pattern, each having a
  different ?o.

Fig 6-6
Fig 6-7
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6.2 Characteristics of Light

• The Photoelectric Effect
• Below a characteristic threshold, ?o, no e-s are
  observed.
• 2. Above ?o, KEmax of ejected e- increases
  linearly with ? of light.
• 3. Above ?o, higher light intensity increases
  of ejected e- (not their KE)
• 4. All metals exhibit same pattern, each having a
  different ?o.

Conclusions

• Light comes in packets called photons.
• Each photon has an energy proportional to its
  frequency
• (6-2)

Ephoton h? photon

• Plancks constant, h 6.62606876 x 10-34 J/s

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6.2 Characteristics of Light
Fig 6-8
Ekinetic(electron)h?photon-h?0
(6-3)

• Below a characteristic threshold, ?o, no
  e-s are observed.
• h?photon
• Above ?o, KEmax of ejected e- increases linearly
  with ? of light.
• h?photon h?0
• Above ?o,higher light intensity increases of
  ejected e- (not their KE)
• Higher intensity means more photons

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6.2 Characteristics of Light
Fig. 6-9

• Light Has Particle Aspects
• Photons have discrete energy described by E h?
• Light and Atoms
• Excited state a higher energy state
• Ground state lowest energy state
• Energy level diagram diagrams depicting atomic
  energy transformations

(6-4)
?Eatom ?h?photon
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Emission and Absorption
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Atomic spectra

• specific energies correspond to specific orbitals
• can be used to identify an atom
• emission occurs when an electron relaxes to a
  lower orbit
• energy must match the transition

photon energy
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6.3 Absorption Emission Spectra

• Emission Spectra

Fig 6-11
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6.3 Absorption Emission Spectra

• Absorption Spectra

Fig 6-10
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6.3 Absorption Emission Spectra

• Quantized Energy

Fig 6-13
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6.3 Absorption Emission Spectra

• Energy Level Diagrams
• Hydrogen

Fig 6-14
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6.3 Absorption Emission Spectra

• Energy Level Diagrams
• Mercury

Fig 6-15
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Bulk Interactions
wave properties
Atomic/Molecular
particle properties
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Types of energy transfer
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1885-1962 Neils BohrPost-Doc of Rutherford

• Why are the negative electrons
  not pulled toward the positive nucleus?
• Electrons are in motion around the nucleus
• string - rock
• Electrons exist in defined orbits
• These orbits are quantized

e
e
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Bohr ModelEnergy Level ModelCloud Model
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Bohr Model

• Defined the quantized energy states of atoms
• Could predict the spectrum of hydrogen
• Could not be extended to other atoms
• fell apart when more than one electron present
• Could not explain WHY energy was quantized
• Could not calculate the intensities

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1923 Prince Louis de Broglie

• Light behaves as particles (photoelectric effect)
  and waves (diffraction)
• Proposed
• Matter also has particle and wave properties
• Matter is quantized due to wave nature
• Energy associated with a particle follows a
  similar equation as a photon

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de Broglies Model

• Each orbital is (must be) a whole number times
  the wavelength of the electron
• Destructive interference demands this
• Concept of electrons having a wave nature was
  profound
• His description was to simplistic

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1924 Erwin Schrödinger

• Agreed with de Broglies concept
• Over New Years wrote the wave eqn. for matter
• Solution to Schrödinger eqn. is a function
• The function2 is the probability of finding an
  electron in a small volume

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1925 Werner Heisenberg"Heisenberg MAY have
slept here."

• We see by the interaction of light with an object
  
• The wavelength must be at least as large as the
  size of the object
• How do we see an electron? IT IS TOO SMALL
• small wavelengths high energy

The smaller m, is the greater the uncertainty
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6.4 Properties of Electrons
Heisenbergs Uncertainty Principle
The position of an electron cannot be precisely
defined. The more accurately we know position of
a particle, the more uncertain we are about its
motion, and visa versa Uncertainty is a feature
of all objects, but becomes noticeable only for
very small objects.
Fig 6-16
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