Positronium is a bound quantum-mechanical system ( exotic atom ) consisting of an electron and a positron . Depending on the mutual direction of the spins of the electron and positron, orthopositronium (spins are codirectional, total spin S = 1 ) and parapositronium (spins are oppositely directed, total spin S = 0 ) are distinguished. Positronium, like a hydrogen atom , is a two-body system , and its behavior and properties are accurately described in quantum mechanics . It was first experimentally identified in 1951 by Martin Deutsch  .
Since the reduced positronium mass is almost half the reduced electron mass  , the radius of the positronium atom in the ground state is 0.106 nm (twice the hydrogen atom), and its ionization potential from the ground state is 6.77 eV (half the hydrogen ionization potential).
Positronium rapidly annihilates , its lifetime depends on the spin: resting parapositronium in vacuum annihilates on average in 0.125 ns (in two gamma quanta with energy of 511 keV and opposite pulses ), while orthopositronium lives three orders of magnitude longer (143 ns) and decays into three gamma rays, due to the conservation of charge parity . In an environment, the positronium lifetime decreases (for orthopositronium in a solid, it becomes less than 1 ns), and the relative probability of annihilation in 2 gamma rays increases. It is possible to annihilate positronium into a larger number of gamma rays, but the likelihood of this is very small. In any case, the total energy of annihilation gamma rays in the positronium center of inertia system is 1022 keV (corresponding to twice the mass of the electron).
The mass of the ground state of orthopositronium ( term 3 S 1 ) is 8.4⋅10 −4 eV greater than the ground state of parapositronium ( term 1 S 0 ), transitions between these two states are possible. When a positronium atom is formed from unpolarized particles, orthopositronium occurs three times more often, since its statistical weight g = 2 S + 1 is three times more than that of parapositronium. Although the positronium lifetime is short, it manages to enter into chemical reactions. The chemistry of positronium is quite well studied (as a rule, it is considered in the framework of meson chemistry , although the electron and positron do not belong to mesons ). The chemical symbol of positronium is Ps . Chemically, positronium is close to hydrogen; its interactions are used to study the kinetics of chemical reactions , diffusion , phase transitions, and other physicochemical processes in gases and condensed matter.
Positronium (like muonium ) is a purely leptonic atom; therefore, its spectroscopy and precision measurement of the lifetime are of particular interest in verifying the predictions of quantum electrodynamics . The negative positronium ion Ps - , consisting of two electrons and a positron, is also being studied.
Molecular positronium , dipositronium , Ps 2 is a molecule consisting of two positronium atoms (that is, a bound system of two electrons and two positrons ).
In 1946, J. A. Wheeler suggested  that two positronium atoms can combine into a molecule with a binding energy of about 0.4 eV (dipositronium). In 2005, there were reports of a possible observation of molecular positronium Ps 2 , confirmed in September 2007   . Ps 2 molecules were detected by irradiating a thin film of porous quartz with a powerful flux of positrons.
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- Molecules of Positronium Observed in the Laboratory for the First Time . Press release. September 12, 2007.