Addition of a discrete amount of energy to a system that changes it usually from a state of lowest energy (ground state) to one of higher energy (excited state).
For example, in a hydrogen atom, an excitation energy of 10.2 electron volts is required to move the lone electron from its ground state to its first excited state. The excitation energy stored in excited atoms and nuclei is usually emitted as ultraviolet radiation from atoms and as gamma radiation (&see; gamma ray) from nuclei as they return to their ground states.
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