: Astronomy: molecular hydrogen

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molecular hydrogen

A molecule consisting of two hydrogen atoms (H₂) which is the most abundant molecule in the Universe. Molecular hydrogen plays a fundamental role in many astrophysical contexts. It is found in all regions where self-shielding against the ultraviolet photons, responsible for its photodissociation, is sufficiently large. Containing two identical hydrogen atoms, H₂ is highly symmetric. Due to this property, the molecule has no dipole moment and all rotation-vibrational transitions within the electronic ground state are quadrupolar with low spontaneous emission Einstein coefficient values. The molecule exists in two almost independent states, namely orthohydrogen and parahydrogen. H₂ may be excited through several mechanisms, including: 1) → far ultraviolet (FUV) induced optical pumping and collisional excitation in photodissociation regions (PDRs) associated with star formation; 2) hard X-rays penetrating and heating regions within molecular clouds, which in turn excite H₂ via collisions with electrons or hydrogen atoms; and 3) collisional excitation of H₂ due to acceleration produced by interstellar shock waves. H₂ is thought to be chiefly produced via surface reactions on interstellar grains, but the exact formation mechanism is not fully understood. (An Etymological Dictionary of Astronomy and Astrophysics, by M. Heydari-Malayeri, https://dictionary.obspm.fr/index.php?formSearchTextfield=molecular+hydrogen&formSubmit=Search&showAll=1)

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