Electron excitation is the discrete energy storage in an orbital electron. The excitation energy results from the absorption of a photon (photoexcitation) or from the absorption of another electron (electrical excitation). The absorbed energy lifts the electron to a higher energy level. This process ends with electron relaxation.

See also Electron Relaxation.

A lower orbited electron leaves the atom - the reoccupation of this vacancy by a higher orbited electron leads to the emission of energy which in turn leads to the emission of a second electron, the Auger electron.

See also Auger Electron, Electron Excitation, Megaelectron Volt and Auger Pierre Victor.

Electron relaxation is the release of a short time stored discrete energy by an orbital electron which will then falls back to a lower energy level. The energy difference is emitted by the electron as a photon or given to another particle.

See also Electron Excitation.

Radioactive decay is the change of instable atoms to a more stable state. This change to a different nuclide by the spontaneous emission of radiation such as alpha or beta particles, gamma rays, or by electron capture follows an element-specific decay chain. Each step in the decay chain has a definite half-life.
Sometimes also the reduction of excitation energy of the nucleus by e.g. internal conversion is mentioned as radioactive decay.

See also Decay Chain, Radioisotope.