Ionization

Ionization is the formation of one or more ions from neutral matter or the production of more highly charged ions from existing ions. In atomic physics, it commonly means removing an electron from an atom or molecule, leaving a positively charged species. More broadly, it includes electron attachment and chemical processes that generate ions without releasing free electrons. Ionization underlies the formation of plasmas, the electrical behavior of gases, and many analytical techniques. Its definition therefore extends beyond the familiar example of radiation knocking electrons out of atoms. (old.goldbook.iupac.org)

Charge formation and related processes

A simple electron-removal process can be represented as

The residual species becomes positive because it has lost negative electric charge. Conversely, attachment of an electron to a neutral species can produce a negative ion. Ionization redistributes charge rather than creating an imbalance in the total charge of an isolated system: the charge of the residual ion and released electron together equals the initial charge. Further electron removal produces successively higher positive charge states. (old.goldbook.iupac.org)

Ionization differs from excitation. During excitation, an electron occupies a higher-energy bound state but remains attached to the atom; during electron-removal ionization, it becomes unbound. Both processes can result from photon absorption or collisions. Ionization also differs from molecular dissociation, which concerns separation into smaller constituents: dissociation may produce neutral fragments, whereas a process producing charged fragments can also constitute ionization. (openstax.org)

Energy requirements

The ionization energy is the minimum energy needed to remove an electron from a specified species. First ionization energy conventionally refers to electron removal from a neutral ground-state atom or molecule. For hydrogen, the ground-state threshold is approximately 13.5984 electronvolts. Removing another electron from a singly charged ion is second ionization, with its own energy requirement; successive removal generally requires progressively greater energies. (goldbook.iupac.org)

For molecules, the threshold depends on how nuclear geometry changes. Adiabatic ionization energy refers to formation of the ion in its lowest vibrational state, allowing its structure to relax. Vertical ionization energy refers to electron removal without a change in molecular geometry. These distinctions matter when interpreting measurements of molecular electronic structure. (goldbook.iupac.org)

Physical mechanisms

Photoionization occurs when a photon interacts with matter and ejects an electron. Photon energy is $h\nu$, where $h$ is the Planck constant and $\nu$ is frequency. In a simple single-photon process, the photon must supply at least the relevant ionization energy; excess energy can appear as the emitted electron’s kinetic energy. Energetic ultraviolet radiation and X-rays are important photoionizing agents. (goldbook.iupac.org)

Collisional ionization transfers energy through interactions with moving particles. For electron-induced ionization, a representative reaction is

The incoming electron is not consumed: it emerges alongside the electron removed from the target. Electron ionization is the preferred name for this process in mass spectrometry; IUPAC discourages the older expression “electron impact” in that context. At sufficiently high temperatures, energetic collisions can ionize a gas and contribute to plasma formation. (old.goldbook.iupac.org)

Field ionization results from a very strong electric field, including fields concentrated near surfaces. Laboratories can also produce ionized matter using high-voltage discharges, lasers, or electromagnetic fields. These methods differ in how energy reaches the particles and in the charge states they produce. (publications.iupac.org)

Ionization in solutions

In chemistry, ionization often describes ion formation through reactions with a solvent. An acid reacting with water, for example, transfers a proton to a water molecule:

This acid–base reaction forms oppositely charged species without ejecting a free electron. Strong acids undergo essentially complete ionization in dilute aqueous solution, whereas weak acids establish a chemical equilibrium containing both ionized and nonionized forms. (openstax.org)

The acid-ionization constant quantifies this equilibrium. Percent ionization describes the fraction of the initial acid that has reacted, but unlike the equilibrium constant at a specified temperature, it depends on initial concentration. A weak acid generally has a greater percentage ionization when diluted. Acid strength and concentration are therefore distinct properties: a concentrated weak acid is not thereby a strong acid. (openstax.org)

Plasmas and natural environments

A plasma contains mobile electrons and ions and exhibits collective electromagnetic behavior. Ionization can arise from high temperatures, radiation, or electrical energy; consequently, ionized matter is not restricted to gases heated uniformly to extreme temperatures. Plasmas occur in space and can also be generated experimentally. (energy.gov)

The ionosphere is a partially ionized region of Earth’s upper atmosphere. Extreme ultraviolet radiation and X-rays from the Sun remove electrons from atmospheric atoms and molecules. Electron–ion recombination counteracts this production, and the balance changes between day and night. The resulting charged population affects radio propagation and satellite communication signals. Ionization and subsequent electron capture also help explain the radiation emitted by gases near hot stars. (helio.data.nasa.gov)

Measurement and applications

An ionization chamber detects ionizing radiation through the electrical current produced when radiation ionizes a gas. Applied voltage collects the liberated electrons and positive ions at opposite electrodes. The measured signal depends on charge production and collection, providing a practical means of detecting radiation. (nrc.gov)

In mass spectrometry, ionization supplies charged particles that instruments can manipulate and analyze. Electron ionization, photoionization, field ionization, and chemical ionization are distinct ways of producing these particles. Their terminology reflects the mechanism of ion formation, rather than simply whether the original sample was solid, liquid, or gaseous. (old.goldbook.iupac.org)