Ionization
Ionization is the process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This is often confused with dissociation. 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 whatsoever. Actually, the chemical synthesis of salt involves ionization. This is a chemical reaction.
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.
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.
See also
- Above Threshold Ionization (ATI)
- Adiabatic ionization
- Holstein–Herring method
- Ion source
- Phase diagram
- Phase transition
- Photoionization and Photoionization mode
- Quantum tunneling for detailed treatment of how tunneling works.
- Thermal ionization
References
 | This article includes a list of references, related reading, or external links, but its sources remain unclear because it lacks inline citations. (April 2009) |
- Sequential ionization of C60 with femtosecond laser pulses. The Journal of Chemical Physics—January 22, 2001—Volume 114, Issue 4, pp. 1716–1719.
- Can harmonic generation cause non-sequential ionization? J. Phys. B: At. Mol. Opt. Phys. 31 No 19 (14 October 1998) L841-L848.
- Probing atomic ionization mechanisms in intense laser fields by calculating geometry and diffraction independent ionization probabilities. J Wood, E M L English, S L Stebbings, W A Bryan, W R *Newell, J McKenna, M Suresh, B Srigengan, I D Williams, I C E Turcu, J M Smith, K G Ertel, E J Divall, C J Hooker, A J Langley. Template:PDFlink