| 11. | When these excited atoms or ions revert to their ground state, they will emit a photon.
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| 12. | Subsequent evolution of the laser pulse did not ionize completely these states leaving behind some highly excited atoms.
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| 13. | It excites atoms to create light.
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| 14. | As the photons entered the cloud, their energy excited atoms along their path, causing them to lose speed.
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| 15. | Specifically, an excited atom will act like a small electric dipole which will oscillate with the external field provided.
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| 16. | This is because the probabilities of transition cannot be affected by the presence or absence of other excited atoms.
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| 17. | Such irreversible time evolution of the atom-vacuum system is responsible for the apparent spontaneous decay of an excited atom.
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| 18. | To have a medium suitable for laser operation, it is necessary that these excited atoms quickly decay to level 2.
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| 19. | A very well known example of a two-state system is the stimulated and spontaneous emission of photons from excited atoms.
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| 20. | The excited atoms continue expanding until the increase in temperature can no longer provide the energy necessary for further expansion.
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