SCEN103 Class 21

Excited States and Quantum Leaps

Excited States

• State of equilibrium, state of lowest energy, is known as the ground state
• Higher energy states are known as excited states.
  • with orbitals that are larger, with different shapes.
• Electrons can be promoted to higher orbits by:
  • collisions with other atoms
  • bombardment with electrons or other particles
  • absorption of light...
• Light is emitted when atom relaxes to ground state from excited state.
  • Light is electromagnetic radiation
  • "pure" energy -- photons
  • The electromagnetic spectrum
  • The visible spectrum
• Demonstrations of discharge lamp (with neon, hydrogen, and oxygen), falling kg mass

Quantum Leaps

• On careful examination, we find that light emitted by excited atoms has discrete values of energy.
• Deduce that electrons in an atom have discrete values of energy.
• Energy levels of electrons in an atom are quantized.
  • Energy level diagrams -- sample of emission spectrum: hydrogen; (from bluegiant.phys.ksu.edu)
  • Quantum physics

Atomic units of energy

• In our study of electricity we measured energy in units of kW-hr (kilowatt-hour).
  
• In atomic physics and semiconductor physics, we find that the eV (electron-volt) is a convenient unit for measuring energy of photons and electrons.
• Relationship between kW-hr and eV; eV and joule?
  • A watt-second is also an acceptable unit for energy, having the same dimensions as kilowatt-hour. The watt-second is more commonly known as a joule (J).
  • A 1 kg (kilogram) mass held overhead has about 20 J of potential energy relative to the floor; that is, when it hits my toe, it will be carrying 20 J of kinetic energy.
  • The electron-volt is the change of energy experienced by an electron when it is relocated from one point to another, when the two points have a 1.0 V difference.
  • The electron-volt is a small quantity compared to the joule:
    1.0 eV = 1.60 x 10^-19 J

The Visible Spectrum

The Tech

Photon "color":   Wavelength:   Energy:   (nm) (eV) Blue 430 2.9 Yellow 530 2.3 Red 650 1.9

From Marshall Space Flight Center, NASA

Additional Online References

Visual Quantum Mechanics -- Laser Adventure

Bohr Model of the Atom

Energy States (Levels)

Photons and the Energy Diagrams

Spontaneous Emission of Electromagnetic Radiation