Autoionization is a process by which an

atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, and ...

or a

molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...

in an

excited state In quantum mechanics, an excited state of a system (such as an atom, molecule or nucleus) is any quantum state of the system that has a higher energy than the ground state (that is, more energy than the absolute minimum). Excitation refers to a ...

spontaneously emits one of the outer-shell

electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no kn ...

s, thus going from a state with charge to a state with charge , for example from an electrically neutral state to a singly ionized state. Autoionizing states are usually short- lived, and thus can be described as

Fano resonance In physics, a Fano resonance is a type of resonant scattering phenomenon that gives rise to an asymmetric line-shape. Interference between a background and a resonant scattering process produces the asymmetric line-shape. It is named after Italian ...

s rather than normal bound states. They can be observed as variations in the ionization cross sections of atoms and molecules, by photoionization,

electron ionization Electron ionization (EI, formerly known as electron impact ionization and electron bombardment ionization) is an ionization method in which energetic electrons interact with solid or gas phase atoms or molecules to produce ions. EI was one of th ...

and other methods.

Examples

As examples, several Fano resonances in the extreme ultraviolet photoionization spectrum of

neon Neon is a chemical element with the symbol Ne and atomic number 10. It is a noble gas. Neon is a colorless, odorless, inert monatomic gas under standard conditions, with about two-thirds the density of air. It was discovered (along with krypton ...

are attributed to autoionizing states.Codling, K., Madden, R.P. and Ederer, D.L. (1967), ''Resonances in the Photoionization Continuum of Ne I (20-150 eV)'',

Phys. Rev. ''Physical Review'' is a peer-reviewed scientific journal established in 1893 by Edward Nichols. It publishes original research as well as scientific and literature reviews on all aspects of physics. It is published by the American Physical Socie ...

''155'', 26-37 DOI: https://doi.org/10.1103/PhysRev.155.26 Some are due to one-electron excitations, such as a series of three strong similarly shaped peaks at energies of 45.546, 47.121 and 47.692 eV which are interpreted as 1s² 2s¹ 2p⁶ ''n''p (¹P) states for ''n'' = 3, 4 and 5. These states of neutral neon lie beyond the first ionization energy because it takes more energy to excite a 2s electron than to remove a 2p electron. When autoionization occurs, the ''n''p → 2s de-excitation provides the energy needed to remove one 2p electron and form the Ne⁺ ground state. Other resonances are attributed to two-electron excitations. The same neon photoionization spectrum considered above contains a fourth strong resonance in the same region at 44.979 eV but with a very different shape, which is interpreted as the 1s² 2s² 2p⁴ 3s 3p (¹P) state. For autoionization, the 3s → 2p transition provides the energy to remove the 3p electron. Electron ionization allows the observation of some states which cannot be excited by photons due to selection rules. In neon for example again, the excitation of triplet states is forbidden by the spin selection rule ΔS = 0, but the 1s² 2s² 2p⁴ 3s 3p (³P) has been observed by electron ionization at 42.04 eV. If a

core electron Core electrons are the electrons in an atom that are not valence electrons and do not participate in chemical bonding. The nucleus and the core electrons of an atom form the atomic core. Core electrons are tightly bound to the nucleus. Therefore, un ...

is missing, a positive ion can autoionize further and lose a second electron in the

Auger effect The Auger effect or Auger−Meitner effect is a physical phenomenon in which the filling of an inner-shell vacancy of an atom is accompanied by the emission of an electron from the same atom. When a core electron is removed, leaving a vacancy, an ...

. In neon, X-ray excitation can remove a 1s electron, producing an excited Ne⁺ ion with configuration 1s¹ 2s² 2p⁶. In the subsequent Auger process a 2s → 1s transition and simultaneous emission of a second electron from 2p leads to the Ne²⁺ 1s² 2s¹ 2p⁵ ionic state. Molecules, in addition, can have vibrationally autoionizing Rydberg states, in which the small amount of energy necessary to ionize a Rydberg state is provided by vibrational excitation.

Autodetachment

When the excited state of the atom or molecule consists of a compound state of a neutral particle and a resonantly attached electron, autoionization is referred to as autodetachment. In this case the compound state begins with a net negative charge before the autoionization process, and ends with a neutral charge. The ending state will often be vibrationally or rotationally excited state as a result of excess energy from the resonant attachment process.

References

{{reflist Atomic physics Molecular physics Quantum chemistry