Ionization (or ionisation in British English) is the process by which an atom or molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes.

Ionization is the formation of one or more ions due to the removal or addition of electrons from a neutral molecular entity (i.e., atoms or molecules), which can be caused by collisions between particles or by the absorption of radiation (as in the case of solar flares). Atoms or molecules with an electron number less than the atomic number remain positively charged and are called “cations”; those with an electron number greater than the atomic number remain negatively charged and are called “anions”.

To remove an electron from an atom and thus make it an ion, a certain amount of energy, called the ionization energy, can be applied to it, which varies according to the atomic species. Of course, the closer the electrons are to the nucleus (as in nonmetals), the greater the amount of energy that must be applied to remove the electrons from the atom.

“Normally” an atom has a total electric charge of zero; there will be a certain number of protons and neutrons in its nucleus surrounded by electrons, and the number of electrons will equal the number of protons. Under certain circumstances, however, one or more electrons may be torn away from the atom; this phenomenon is called ionization. Some energy is required to cause this separation; this can happen when the atom absorbs a high-energy photon (for example, in the ultraviolet or even higher energy), or sometimes when the atom collides with another atom or ion in a very hot gas. Eventually, however, the atom and its lost electron will tend to recombine; when this happens, a photon is emitted.

Semiclassical description of ionization

Classical physics and the Bohr model of the atom can qualitatively explain photoionization and collision-mediated ionization. In these cases, during the ionization process, the energy of the electron exceeds the energy difference of the potential barrier it is trying to cross. However, the classical description cannot describe tunnel ionization because the process involves the passage of an electron through a classically forbidden potential barrier.

Quantum description of ionization

The interaction of atoms and molecules with sufficiently strong laser pulses leads to ionization to singly or multiply charged ions. The ionization rate, i.e. the ionization probability per unit time, can only be calculated using quantum mechanics. In general, the analytical solutions are not available, and the approximations required for tractable numerical calculations do not provide sufficiently accurate results. However, if the laser intensity is high enough, the detailed structure of the atom or molecule can be ignored and an analytical solution for the ionization rate is possible.

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