# SCEN103 Class 13

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## 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.
• "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.

### 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 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

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