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.

In the internal conversion process the multipole electric field of the nucleus of an atom, in an electromagnetically excited state, react with an orbit electron. With enough energy the electron is ejected (internal conversion electron). The energy of the conversion electron depends on the energy transferred from the nucleus reduced by the shell specific binding energy. This process competes with gamma emission. The refilling for the vacancy left by the internal conversion electron occurs through the Auger effect, a higher orbit electron take place and x-ray or an Auger electron will be emitted.
The atomic number of the atom gets not changed by internal conversion.

See also Conversion Electron, Auger Effect and Auger Electron.

If a nucleus still has excess energy after attempts of stabilization, it can emit energy without changing the number of protons or neutrons. This process is named isomeric transition. One way of isomeric transition is the emission of a gamma rays, the other competing way is internal conversion, where the excess energy of the nucleus must exceed the binding energy of an electron, which then will be ejected from the atom.

See also Decay, Gamma Radiation and Internal Conversion.

The process of pair production is the creation of a particle and its antiparticle. If a photon with a minimum energy of the rest mass of an electron and a positron hits a nucleus an electron and a positron is emitted. The surplus energy appears as the kinetic energy of the two created particles. Conservation of energy and momentum makes pair production possible or not.

Quantum is a discrete and the smallest natural unit of energy and momentum. Planck makes the assumption that every energy transfer on a sub-atomic level consist of small units, called quanta. The view of electromagnetic energy as photons reflects this quantization.
E = h x v
E = energy
h = Planck quantum of action = 6.6261 x 10^-27 erg sec
v = frequency.