Excitation in radiobiology and medical physics refers to excitation of an outer orbital (valence) electron to a higher energy level. By absorbing some energy, but insufficient to cause ionization, the valence electron overcomes the weak attractive force of the nucleus, causing it to move further away from the nucleus to an orbit that is not normally occupied (optical orbit).
This excitation of outer orbital electrons alters the chemical force that binds atoms in molecules which leads to a regrouping of the affected atoms into a different molecular structure.
Excitation is one means by which ionizing radiations dissipate their energy in matter. It is an indirect method of inducing chemical change through disruption of atomic bonds. This can damage important molecules such as DNA, inducing detrimental biological effects, which may only become apparent after a period of time (latent period) that maybe as short as minutes, but as long as a lifetime.
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