Ionization (or ionisation, see American and British English spelling differences) is the process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or ions. In the case of ionization of a gas, ion pairs are created consisting of a free electron and a positive ion.
Types of ionization 
The process of ionization works slightly differently depending on whether an ion with a positive or a negative electric charge is being produced. A positively charged ion is produced when an electron bonded to an atom (or molecule) absorbs the proper amount of energy to escape from the electric potential barrier that originally confined it, thus breaking the bond and freeing it to move. The amount of energy required is called the ionization energy. A negatively charged ion is produced when a free electron collides with an atom and is subsequently caught inside the electric potential barrier, releasing any excess energy.
In general, ionization can be broken down into two types: sequential ionization and non-sequential ionization. In classical physics, only sequential ionization can take place; refer to the Classical ionization section for more information. Non-sequential ionization violates several laws of classical physics; refer to the Quantum ionization section.
Classical ionization 
Applying only classical physics and the Bohr model of the atom makes both atomic and molecular ionization entirely deterministic; that is, every problem will always have a definite and computable answer. According to classical physics, it is absolutely necessary that the energy of the electron exceeds the energy difference of the potential barrier it is trying to pass. In concept, this idea should make sense: The same way a person cannot jump over a one-meter wall without jumping at least one meter off the ground, an electron cannot get over a 13.6-eV potential barrier without at least 13.6 eV of energy.
Applying to positive ionization 
According to these two principles, the energy required to release an electron is strictly greater than or equal to the potential difference between the current bound atomic or molecular orbital and the highest possible orbital. If the energy absorbed exceeds this potential, then the electron is emitted as a free electron. Otherwise, the electron briefly enters an excited state until the energy absorbed is radiated out and the electron re-enters the lowest available state.
Applying to negative ionization 
Due to the shape of the potential barrier, according to these principles, a free electron must have an energy greater than or equal to that of the potential barrier in order to make it over. If a free electron has enough energy to do so, it will be bound to the lowest available energy state, and the remaining energy will be radiated away. If the electron does not have enough energy to surpass the potential barrier, then it is forced away by the electrostatic force, described by Coulombs Law, associated with the electric potential barrier.
Sequential ionization 
Sequential ionization is a description of how the ionization of an atom or molecule takes place. For example, an ion with a +2 charge can be created only from an ion with a +1 charge or a +3 charge. That is, the numerical charge of an atom or molecule must change sequentially, always moving from one number to an adjacent, or sequential, number.
Quantum ionization 
In quantum mechanics, ionization can still happen classically, whereby the electron has enough energy to make it over the potential barrier, but there is the additional possibility of tunnel ionization.
Tunnel ionization 
Tunnel ionization is ionization due to quantum tunneling. In classical ionization, an electron must have enough energy to make it over the potential barrier, but quantum tunneling allows the electron simply to go through the potential barrier instead of going all the way over it because of the wave nature of the electron. The probability of an electron's tunneling through the barrier drops off exponentially with the width of the potential barrier. Therefore, an electron with a higher energy can make it further up the potential barrier, leaving a much thinner barrier to tunnel through and, thus, a greater chance to do so.
Non-sequential ionization 
When the fact that the electric field of light is an alternating electric field is combined with tunnel ionization, the phenomenon of non-sequential ionization emerges. An electron that tunnels out from an atom or molecule may be sent right back in by the alternating field, at which point it can either recombine with the atom or molecule and release any excess energy or have the chance to further ionize the atom or molecule through high-energy collisions. This additional ionization is referred to as non-sequential ionization for two reasons: One, there is no order to how the second electron is removed, and, two, an atom or molecule with a +2 charge can be created straight from an atom or molecule with a neutral charge, so the integer charges are not sequential. Non-sequential ionization is often studied at lower laser-field intensities, since most ionization events are sequential when the ionization rate is high.
The non-sequential ionization can be readily understood with one-dimensional models of atoms which until recently were the only to handle numerically. It happens when the angular momentum for both electrons remains so low that they effectively move in one dimensional space and may be true for linear polarization but is not for the circular. One may then look at two electrons as the two dimensional atom where the simultaneous ionization of both is just an ionization of one two-dimensional electron which results in jets of probability in 45° direction on the two-electron plane emitted from the multiply charged nucleus or the square center. The sequential ionization on the other hand is represented by the emissions from the axes x and y when the two dimensional hyper-electron is first guided by the Coulomb potential channels from the hyper-nucleus and then ionized by the hyper-electric field in 45° direction.
Atomic stabilization 
According to a general theory of Cook et al., the electron in the external rapidly oscillating potential
will experience the time average effective potential
It means that within this approximation the effective potential for the electric field in the dipole approximation whose time-dependent potential is linear is constant and the atom does not ionize at all. In practice, because atoms ionize, it results in lower counter-intuitive ionization rates for fields higher than for lower field values, even in ultra-strong field limit and atomic stabilization of ionization occurs. More precise theory shows that during the action of the laser field, the time-averaged effective potential of H2+ is formed which binds the electron even if very strong electromagnetic field is present in similarity to the hydrogen molecule ion and prevents ionization.
Dissociation – distinction 
A substance may dissociate without necessarily producing ions. As an example, the molecules of table sugar dissociate in water (sugar is dissolved) but exist as intact neutral entities. Another subtle event is the dissociation of sodium chloride (table salt) into sodium and chlorine ions. Although it may seem as a case of ionization, in reality the ions already exist within the crystal lattice. When salt is dissociated, its constituent ions are simply surrounded by water molecules and their effects are visible (e.g. the solution becomes electrolytic). However, no transfer or displacement of electrons occurs. Actually, the chemical synthesis of salt involves ionization. This is a chemical reaction.
See also 
|Look up ionization in Wiktionary, the free dictionary.|
- Above threshold ionization
- Adiabatic ionization
- Ion source
- Photoionization and photoionization mode
- Thermal ionization
- Ionization chamber
- Campbell, E. E. B.; Hoffmann, K.; Rottke, H.; Hertel, I. V. (2001). "Sequential ionization of C60 with femtosecond laser pulses". The Journal of Chemical Physics 114 (4): 1716. Bibcode:2001JChPh.114.1716C. doi:10.1063/1.1336573.
- Sanpera, A; Watson, J B; Shaw, S E J; Knight, P L; Burnett, K; Lewenstein, M (1998). "Can harmonic generation cause non-sequential ionization?". Journal of Physics B: Atomic, Molecular and Optical Physics 31 (19): L841. Bibcode:1998JPhB...31L.841S. doi:10.1088/0953-4075/31/19/012.
- Wood, J.; English, E. M. L.; Stebbings, S. L.; Bryan, W. A.; Newell, W. R.; McKenna, J.; Suresh, M.; Srigengan, B.; Williams, I. D.; Turcu, I. C. E.; Smith, J. M.; Ertel, K. G.; Divall, E. J.; Hooker, C. J.; Langley, A. J. (2005). "Probing atomic ionization mechanisms in intense laser fields by calculating geometry and diffraction independent ionization probabilities". Central Laser Facility Annual Report 2004/2005.[dead link]
- Becker, Wilhelm; Liu, Xiaojun; Ho, Phay; Eberly, Joseph (2012). "Theories of photoelectron correlation in laser-driven multiple atomic ionization". Reviews of Modern Physics 84 (3): 1011. Bibcode:2012RvMP...84.1011B. doi:10.1103/RevModPhys.84.1011.
- Cook, Richard; Shankland, Donn; Wells, Ann (1985). "Quantum theory of particle motion in a rapidly oscillating field". Physical Review A 31 (2): 564–567. Bibcode:1985PhRvA..31..564C. doi:10.1103/PhysRevA.31.564. PMID 9895523.