Positronium hydride, or hydrogen positride is an exotic

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 ...

consisting of a

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...

atom bound to an

exotic atom An exotic atom is an otherwise normal atom in which one or more sub-atomic particles have been replaced by other particles of the same charge. For example, electrons may be replaced by other negatively charged particles such as muons (muonic atoms) ...

positronium Positronium (Ps) is a system consisting of an electron and its antimatter, anti-particle, a positron, bound together into an exotic atom, specifically an onium. Unlike hydrogen, the system has no protons. The system is unstable: the two parti ...

(that is a combination of an electron and a positron). Its formula is PsH. It was predicted to exist in 1951 by A Ore, and subsequently studied theoretically, but was not observed until 1990. R. Pareja, R. Gonzalez from Madrid trapped positronium in hydrogen laden magnesia crystals. The trap was prepared by Yok Chen from the

Oak Ridge National Laboratory Oak Ridge National Laboratory (ORNL) is a U.S. multiprogram science and technology national laboratory sponsored by the U.S. Department of Energy (DOE) and administered, managed, and operated by UT–Battelle as a federally funded research and ...

. In this experiment the positrons were thermalized so that they were not traveling at high speed, and they then reacted with H⁻ ions in the crystal. In 1992 it was created in an experiment done by David M. Schrader and F.M. Jacobsen and others at the

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Denmark ) , song = ( en, "King Christian stood by the lofty mast") , song_type = National and royal anthem , image_map = EU-Denmark.svg , map_caption = , subdivision_type = Sovereign state , subdivision_name = Danish Realm, Kingdom of Denmark ...

. The researchers made the positronium hydride molecules by firing intense bursts of

positron The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. It has an electric charge of +1 '' e'', a spin of 1/2 (the same as the electron), and the same mass as an electron. When a positron collides ...

s into

methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...

, which has the highest density of hydrogen atoms. Upon slowing down, the positrons were captured by ordinary electrons to form positronium atoms which then reacted with hydrogen atoms from the methane.

Decay

PsH is constructed from one proton, two electrons, and one positron. The

binding energy In physics and chemistry, binding energy is the smallest amount of energy required to remove a particle from a system of particles or to disassemble a system of particles into individual parts. In the former meaning the term is predominantly use ...

is . The lifetime of the molecule is 0.65

nanosecond A nanosecond (ns) is a unit of time in the International System of Units (SI) equal to one billionth of a second, that is, of a second, or 10 seconds. The term combines the SI prefix ''nano-'' indicating a 1 billionth submultiple of an SI unit ( ...

s. The lifetime of positronium deuteride is indistinguishable from the hydride. The decay of positronium is easily observed by detecting the two 511 keV gamma ray photons emitted in the decay. The energy of the photons from positronium should differ slightly by the binding energy of the molecule. However, this has not yet been detected.

Properties

The structure of PsH is as a diatomic molecule, with a chemical bond between the two positively charged centres. The electrons are more concentrated around the proton. Predicting the properties of PsH is a four body Coulomb problem. Calculated using the stochastic variational method, the size of the molecule is larger than

dihydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, a ...

, which has a bond length of 0.7413 Å. In PsH the positron and proton are separated on average by 3.66 a₀ (1.94 Å). The positronium in the molecule is swollen compared to the positronium atom, increasing to 3.48 a₀ compared to 3 a₀. Average distance of the electrons from the proton is larger than the dihydrogen molecule, at 2.31 a₀ with the maximum density at 2.8 au.

Formation

Due to its short lifetime, establishing the chemistry of positronium hydride poses difficulties. Theoretical calculations can predict outcomes. One method of formation is through alkali metal hydrides reacting with positrons. Molecules with dipole moments greater than 1.625

debye The debye (symbol: D) (; ) is a CGS unit (a non- SI metric unit) of electric dipole momentTwo equal and opposite charges separated by some distance constitute an electric dipole. This dipole possesses an electric dipole moment whose value is give ...

are predicted to attract and hold positrons in a bound state. Crawford's model predicts this positron capture. In the case of

lithium hydride Lithium hydride is an inorganic compound with the formula Li H. This alkali metal hydride is a colorless solid, although commercial samples are grey. Characteristic of a salt-like (ionic) hydride, it has a high melting point, and it is not solub ...

sodium hydride Sodium hydride is the chemical compound with the empirical formula Na H. This alkali metal hydride is primarily used as a strong yet combustible base in organic synthesis. NaH is a saline (salt-like) hydride, composed of Na+ and H− ions, in co ...

and

potassium hydride Potassium hydride, KH, is the inorganic compound of potassium and hydrogen. It is an alkali metal hydride. It is a white solid, although commercial samples appear gray. It is a powerful superbase that is useful in organic synthesis. It is sold c ...

molecules, this adduct decomposes and positronium hydride and the alkali positive ion form.

Similar compounds

PsH is a simple exotic compound. Other compounds of positronium are possible by the reactions e⁺ + AB → PsA + B⁺. Other substances that contain positronium are

di-positronium Di-positronium, or dipositronium, is an exotic molecule consisting of two atoms of positronium. It was predicted to exist in 1946 by John Archibald Wheeler, and subsequently studied theoretically, but was not observed until 2007 in an experiment p ...

and the ion Ps⁻ with two electrons. Molecules of Ps with normal matter include halides and cyanide.

References

{{DEFAULTSORT:Positronium Hydride Antimatter Molecular physics Quantum electrodynamics Exotic atoms Substances discovered in the 1990s