Also found in: Wikipedia.
A structure in which the nuclei of two atoms approach each other closely and their electrons are arranged in atomic orbitals characteristic of a single atom of atomic number equal to the sum of the nuclear charges. Quasiatoms can be formed for short times in atom-atom and ion-atom collisions when the nuclei are much closer than the mean orbital radius of the innermost K-shell electrons. The electrons are then bound in the electric field of both nuclear charges Z1 and Z2, which resembles the spherically symmetric 1/r2 Coulomb field of a single united atom having charge Zua = Z1 + Z2. See Atomic structure and spectra
An interesting effect is associated with quasiatoms with Z > 173, in which the 1s binding energy is more than twice the electron rest mass, E1s > 2mc2. If a vacancy exists in this orbital, it is energetically favorable to create an electron-positron pair with the electron bound in this state. The positron would be repelled from the nucleus with kinetic energy equal to Ee+ = |E1s| - 2mc2. In the Dirac hole picture, in which the vacuum consists of a negative energy continuum (E < -mc2) filled with electrons, the 1s level is said to fall into the negative-energy Dirac sea as Z increases above the critical value, Zcr = 173. A 1s hole (vacancy) becomes embedded in the negative continuum as an unstable resonance state that decays in a time of ∼10-19 s to a bound electron and a spontaneously emitted monoenergetic positron.
The quantum electrodynamic vacuum in the presence of a bare supercritical nuclear charge is therefore unstable and decays to a fundamentally new charged vacuum, which consists of the nucleus with two 1s electrons (from the two spin orientations). At higher values of Zua, as additional quasiatomic levels enter the negative continuum, the charge of the quantum electrodynamic vacuum increases accordingly. If detected, spontaneous positron emission would represent the first observation of a phase transition in a gauge field theory. See Antimatter, Electron-positron pair production, Phase transitions, Positron, Quantum electrodynamics, Supercritical fields