Quasiatom

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 Z_ua = Z₁ + Z₂. See also 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, E_1s > 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 E_e⁺ = |E_1s| − 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, Z_cr = 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 Z_ua, 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 also Antimatter; Electron-positron pair production; Gauge theory; Phase transitions; Positron; Quantum electrodynamics; Supercritical fields.