Ionization

Ionization is the process where an atom or molecule becomes electrically charged—either positively or negatively—by gaining or losing electrons, usually as part of other chemical reactions. The charged particle that results is called an ion. This process can be triggered by collisions with subatomic particles, other atoms, molecules, electrons, positrons, protons, antiprotons, or ions, as well as by exposure to electromagnetic radiation. It can also happen through radioactive decay via internal conversion, where an excited nucleus transfers its energy to an inner-shell electron, ejecting it.

Gas ionization appears in everyday devices like fluorescent lamps and other electrical discharge lamps. It is also key in radiation detectors such as Geiger-Müller counters and ionization chambers. In fundamental science, ionization is used in equipment like mass spectrometers, and in medicine for radiation therapy. It is also applied in air purification, though studies have linked this use to harmful effects.

Negative ions form when a free electron collides with an atom and gets trapped inside its electric potential barrier, releasing excess energy—a process called electron capture ionization. Positive ions form when enough energy is transferred to a bound electron during a collision with charged particles (like ions, electrons, or positrons) or with photons. The minimum energy needed for this is the ionization energy. Studying these collisions is crucial for understanding the few-body problem, a major unsolved challenge in physics. Experiments that measure the complete momentum vectors of all collision fragments—the scattered projectile, the recoiling target-ion, and the ejected electron—have greatly advanced theoretical understanding of this problem in recent years.

Adiabatic ionization occurs when an electron is removed from or added to an atom or molecule in its lowest energy state, producing an ion also in its lowest energy state. The Townsend discharge is a classic example of positive ions and free electrons created by ion impact. It is a cascade reaction in a gas, like air, within a strong electric field. An initial ionization event—from, say, ionizing radiation—causes a positive ion to drift toward the cathode and a free electron toward the anode. If the electric field is strong enough, the free electron gains enough energy to knock another electron loose when it collides with a molecule. These two free electrons then accelerate toward the anode, gaining energy to cause further impact ionization, and so on. This chain reaction of electron generation depends on the electrons gaining sufficient energy between collisions to sustain the avalanche. Ionization efficiency is the ratio of ions formed to the number of electrons or photons used.

The ionization energy of atoms shows a periodic trend with atomic number, as seen in Mendeleev’s table. This pattern helps reveal the ordering of electrons in atomic orbitals without needing wave functions or detailed ionization mechanics. For example, the sharp drop in ionization potential after rare gas atoms signals the start of a new shell in alkali metals. Within a row, local peaks in ionization energy correspond to s, p, d, and f sub-shells.

Classical physics and the Bohr model can explain photoionization and collision-mediated ionization in a basic way: the electron’s energy exceeds the potential barrier it must cross. However, classical theory cannot describe tunnel ionization, where the electron passes through a classically forbidden barrier.

When atoms or molecules interact with very strong laser pulses or other charged particles, they can become singly or multiply charged ions. The ionization rate—the probability of ionization per unit time—can be calculated using quantum mechanics. (Classical methods like the Classical Trajectory Monte Carlo Method exist, but they are not widely accepted and are often criticized.) Two quantum mechanical approaches are perturbative and non-perturbative. Non-perturbative methods include time-dependent coupled-channel or time-independent close coupling, where the wave function is expanded in a finite basis set (e.g., B-splines, generalized Sturmians, or Coulomb wave packets). Another non-perturbative method solves the Schrödinger equation fully numerically on a lattice. In general, analytic solutions are not possible, and the approximations needed for practical numerical calculations often lack acceptable accuracy. However, at very high laser intensities, the detailed structure of the atom or molecule becomes less important.