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Lyman-Werner photon

(ultraviolet photon with proper energy to excite molecular hydrogen)

A Lyman-Werner photon is an ultraviolet (UV) photon in the H2 Lyman band (aka Werner band, 11.2-13.6 eV), which can be absorbed by an H2 molecule, pushing the molecule to an excited state, possibility subsequently resulting in the molecule's dissociation, i.e., splitting into atomic hydrogen. UV in this band is called Lyman-Werner radiation (LW radiation) or Lyman-Werner flux (LW flux). In molecular clouds, this is the most common means of photodissociation. It is a means by which early stars counteract cooling of molecular clouds, suppressing star formation (star formation feedback) and has been of particular interest in the modeling the formation of Population III stars, given that metallicity was very low, giving clouds a lower cooling rate, making them less likely to reach the point of star formation.

(EMR,molecular hydrogen,ultraviolet)
Further reading:
91nm3.29PHz14eVbeginLyman-Werner photon
111nm2.69PHz11eVendLyman-Werner photon