Major Concepts in Physics Lecture 19.

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Major Concepts in Physics Lecture 19.

• Prof Simon Catterall
• Office 309 Physics, x 5978
• smc_at_physics.syr.edu
• http//physics/courses/PHY102.08Spring

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Announcements

• Exam 3 Monday April 14 in class
• Material
• everything since exam 2. eg temperature, heat,
  work, laws of thermodynamics.
• Photon. Photoelectric effect. Wave-particle
  duality. Electron volts. Bohr model of H atom
  basic ideas.
• Electron as a wave. Diffraction, interference.
  Uncertainty principle. Pauli exclusion principle
  Lasers, chemical bonding

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Recap

• At atomic level classical distinctions of wave
  and particle blur
• Light as photons Ehf, electron as wave lh/p
• Uncertainty principle DxDpgth important for
  reconciling these differing concepts
• Understand discrete energy levels/spectra using
  ideas of wave theory
• Wave function tells us where the particle is most
  likely to be found quantum mechanics predicts
  not definite outcomes but only probabilties .

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Iron atoms confine electron (waves)
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Java simulations of quantum systems

• Electron in potential well. Represented by spread
  out wave packet which oscillates back and forth
• Motion of wave packet determined by the
  Schrodinger equation
• www.falstad.com/mathphysics.html

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Atomic structure

• For a H atom at low temperature electron occupies
  lowest allowed energy state ground state
• To completely remove the electron from the atom
  requires at least 13.6 eV energy called
  ionization energy
• What is the ground state structure of next
  lightest atom helium 2 electrons ?

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Helium energy level diagram
E0
energy
electron
n1 quantum number
ground state
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Pauli exclusion principle

• Experiment tells us that the lowest energy state
  of a multi-electron atom does not consist of all
  electrons inhabiting the lowest energy level
• In fact each new electron must occupy a new
  energy level no two electrons can exist in the
  same state
• Pauli Exclusion Principle

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General atomic structure

• Fill up energy levels one electron at a time
• Typically levels with larger E correspond to
  electron states that are further from nucleus
• These outer electrons can easily be excited or
  transferred to other atoms
• Responsible for chemical properties of that
  particular atom
• Allow us to understand the periodic table

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Absorption / Emission of Photons

• Photon energy
• Ephoton Efinal - Einitial
• The frequency f of the emitted photon is
  determined by
• Ephoton hf

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Fig. 28.23
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Three types of photon-electron interaction

• Absorption photon with correct energy is
  absorbed
• Spontaneous emission. Electron drops to lower
  available energy state emitting photon whose
  energy is difference
• Stimulated emission. Presence of a photon can
  encourage electron to drop producing an
  identical photon

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Lasers

• If cascade of stimulated emissions can take place
  obtain a large number of photons with same
  energy, direction of propagation and are in phase
• Chain reaction each new emission increases the
  number of photons which in turn stimulates
  further photon emission
• Basis of laser

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How it works

• Need stimulated emission to be more likely than
  absorption more of the atoms must be in an
  excited state not the ground state.
• Called population inversion
• Need a long lived excited state metastable
  state.
• If atoms can be pumped up to metastable state
  fast enough a population inversion can occur.

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Ruby laser

• Uses optical pumping incident light of correct
  wavelength is absorbed causing atoms to make
  transitions to short-lived excited state.
• This state then decays to a metastable state.

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Fig. 28.24
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How it works II

• Ruby rod has ends polished and silvered to become
  mirrors
• High intensity flash lamp wraps rod.
• Spontaneous emission from metastable state
• Leads to cascade of stimulated emissions
• Only photons bouncing back and forth along ends
  participate
• Some escape from one end laser beam

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Helium-neon laser

• Gas discharge tube contains low pressure mixture
  of helium and neon.
• Electrically pumped discharge excites helium
  atoms to metastable state.
• Collisions of helium with neon populate a similar
  state in neon decays by stimulated emission
  yielding a 2 eV photon.

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Fig. 28.25
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Semiconductor lasers

• Small, inexpensive and efficient.
• Use in CD/DVD players, bar code readers and laser
  pointers
• Electrically pumped electrons are promoted to a
  higher state through passage of electrical
  current. Decay back to ground state (band) via
  stimulated emission.
• Wavelength depends on energy (band) gap.

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Problems. An electron and a neutron have the same
de Broglie wavelength. Which is true ?

• AThe electron has more kinetic energy and a
  higher speed
• B The electron has less kinetic energy but a
  higher speed.
• C electron and neutron have same kinetic energy
  but electron has higher speed.
• D electron has less kinetic energy and less
  speed.

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A bullet is fired from a rifle. The end of barrel
is a circular aperture. Is diffraction a
measureable effect?

• A No, because only charged particles have a de
  Brogle wavelength
• B No, because a circular aperture never causes
  diffraction
• C No, because the de Broglie wavelength is too
  large
• D No, because the de Brogle wavelength is too
  small

23
What is the minimum kinetic energy for an
electron confined to a region the size of an
atomic nucleus (1.0 fm) ?
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What is the wavelength of the light usually
emitted by a helium-neon laser.