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Published online by Cambridge University Press: 30 March 2016
Molecular hydrogen may be excited by collisions with other particles (as in shock waves), by ultraviolet pumping (as in photon-dominated regions), through its formation on the surfaces of dust grains (in interstellar clouds), or by absorption of the cosmic background radiation (in the primordial gas). We consider the adequacy of our knowledge of H2, from the viewpoint of being able to reliably predict the spectrum arising from these excitation mechanisms.